PARASITIC BACTERIA AND THEIR RELATION TO DISEASE.
Perhaps the most universally known fact in regard to bacteria is
that they are the cause of disease. It is this fact that has made
them objects of such wide interest. This is the side of the
subject that first attracted attention, has been most studied, and
in regard to which there has been the greatest accumulation of
evidence. So persistently has the relation of bacteria to disease
been discussed and emphasized that the majority of readers are
hardly able to disassociate the two. To most people the very word
bacteria is almost equivalent to disease, and the thought of
swallowing microbes in drinking water or milk is decidedly
repugnant and alarming. In the public mind it is only necessary to
demonstrate that an article holds bacteria to throw it under
condemnation.
We have already seen that bacteria are to be regarded as agents
for good, and that from their fundamental relation to plant life
they must be looked upon as our friends rather than as our
enemies. It is true that there is another side to the story which
relates to the parasitic species. These parasitic forms may do us
direct or indirect injury. But the species of bacteria which are
capable of doing us any injury, the pathogenic bacteria, are
really very few compared to the great host of species which are
harmless. A small number of species, perhaps a score or two, are
pathogenic, while a much larger number, amounting to hundreds and
perhaps thousands of species, are perfectly harmless. This latter
class do no injury even though swallowed by man in thousands. They
are not parasitic, and are unable to grow in the body of man.
Their presence is entirely consistent with the most perfect
health, and, indeed, there are some reasons for believing that
they are sometimes directly beneficial to health. It is entirely
unjust to condemn all bacteria because a few chance to produce
mischief. Bacteria in general are agents for good rather than ill.
There are, however, some species which cause mankind much trouble
by interfering in one way or another with the normal processes of
life. These pathogenic bacteria, or disease germs, do not all act
alike, but bring about injury to man in a number of different
ways. We may recognise two different classes among them, which,
however, we shall see are connected by intermediate types. These
two classes are, first, the pathogenic bacteria, which are not
strictly parasitic but live free in Nature; and, second, those
which live as true parasites in the bodies of man or other
animals. To understand the real relation of these two classes, we
must first notice the method by which bacteria in general produce
disease.
METHOD BY WHICH BACTERIA PRODUCE DISEASE.
Since it was first clearly recognised that certain species of
bacteria have the power of producing disease, the question as to
how they do so has ever been a prominent one Even if they do grow
in the body, why should their presence give rise to the symptoms
characterizing disease? Various answers to this question have been
given in the past It has been suggested that in their growth they
consume the food of the body and thus exhaust it, that they
produce an oxidation of the body tissues, or that they produce a
reduction of these tissues, or that they mechanically interfere
with the circulation None of these suggestions have proved of much
value Another view was early advanced, and has stood the test of
time. This claim is that the bacteria while growing in the body
produce poisons, and these poisons then have a direct action on
the body We have already noticed that bacteria during their growth
in any medium produce a large number of biproducts of
decomposition. We noticed also that among these biproducts there
are some which have a poisonous nature; so poisonous are they that
when inoculated into the body of an animal they may produce
poisoning and death. We have only to suppose that the pathogenic
bacteria, when growing as parasites in man, produce such poisons,
and we have at once an explanation of the method by which they
give rise to disease.
This explanation of germ disease is more than simple theory. It
has been in many cases clearly demonstrated. It has been found
that the bacteria which cause diphtheria, tetanus, typhoid,
tuberculosis, and many other diseases, produce, even when growing
in common culture media, poisons which are of a very violent
nature. These poisons when inoculated into the bodies of animals
give rise to much the same symptoms as the bacteria do themselves
when growing as parasites in the animals. The chief difference in
the results from inoculating an animal with the poison and with
the living bacteria is in the rapidity of the action. When the
poison is injected the poisoning symptoms are almost immediately
seen, but when the living bacteria are inoculated the effect is
only seen after several days or longer, not, in short, until the
inoculated bacteria have had time enough to grow in the body and
produce the poison in quantity. It has not by any means been shown
that all pathogenic germs produce their effect in this way, but it
has been proved to be the real method in quite a number of cases,
and is extremely probable in others. While some bacteria perhaps
produce results by a different method, we must recognise the
production of poisons as at all events the common direct cause of
the symptoms of disease. This explanation will enable us more
clearly to understand the relation of different bacteria to
disease.
PATHOGENIC GERMS WHICH ARE NOT STRICTLY PARASITIC
Recognising that bacteria may produce poisons, we readily see that
it is not always necessary that they should be parasites in order
to produce trouble. In their ordinary growth in Nature such
bacteria will produce no trouble The poisons will be produced in
decaying material but will seldom be taken into the human body.
These poisons, produced in the first stages of putrefaction, are
oxidized by further stages of decomposition into harmless
products. But should it happen that some of these bacteria
obtained a chance to grow vigorously for a while in organic
products that are subsequently swallowed as man's food, it is
plain that evil results might follow. If such food is swallowed by
man after the bacteria have produced their poisonous bodies, it
will tend to produce an immediate poisoning of his system. The
effect may be sudden and severe if considerable quantity of the
poisonous material is swallowed, or slight but protracted if small
quantities are repeatedly consumed in food. Such instances are not
uncommon. Well-known examples are cases of ice-cream poisoning,
poisoning from eating cheese or from drinking milk, or in not a
few instances from eating fish or meats within which bacteria have
had opportunity for growth. In all these cases the poison is
swallowed in quantity sufficient to give rise quickly to severe
symptoms, sometimes resulting fatally, and at other times passing
off as soon as the body succeeds in throwing off the poisons. In
other cases still, however, the amount of poison swallowed may be
very slight, too slight to produce much effect unless the same be
consumed repeatedly. All such trouble may be attributed to
fermented or partly decayed food. It is difficult to distinguish
such instances from others produced in a slightly different way,
as follows:
It may happen that the bacteria which grow in food products
continue to grow in the food even after it is swallowed and has
passed into the stomach or intestines. This appears particularly
true of milk bacteria. Under these conditions the bacteria are not
in any proper sense parasitic, since they are simply living in and
feeding upon the same food which they consume outside the body,
and are not feeding upon the tissues of man. The poisons which
they produce will continue to be developed as long as the bacteria
continue to grow, whether in a milk pail or a human stomach. If
now the poisons are absorbed by the body, they may produce a mild
or severe disease which will be more or less lasting, continuing
perhaps as long as the same food and the same bacteria are
supplied to the individual. The most important disease of this
class appears to be the dreaded cholera infantum, so common among
infants who feed upon cow's milk in warm weather. It is easy to
understand the nature of this disease when we remember the great
number of bacteria in milk, especially in hot weather, and when we
remember that the delicate organism of the infant will be thrown
at once into disorder by slight amounts of poison which would have
no appreciable effect upon the stronger adult. We can easily
understand, further, how the disease readily yields to treatment
if care is taken to sterilize the milk given to the patient.
We do not know to-day the extent of the troubles which are
produced by bacteria of this sort. They will, of course, be
chiefly connected with our food products, and commonly, though not
always, will affect the digestive functions. It is probable that
many of the cases of summer diarrhoea are produced by some such
cause, and if they could be traced to their source would be found
to be produced by bacterial poisons swallowed with food or drink,
or by similar poisons produced by bacteria growing in such food
after it is swallowed by the individual. In hot weather, when
bacteria are so abundant everywhere and growing so rapidly, it is
impossible to avoid such dangers completely without exercising
over all food a guard which would be decidedly oppressive. It is
well to bear in mind, however, that the most common and most
dangerous source of such poisons is milk or its products, and for
this reason one should hesitate to drink milk in hot weather
unless it is either quite fresh or has been boiled to destroy its
bacteria.
PATHOGENIC BACTERIA WHICH ARE TRUE PARASITES.
This class of pathogenic bacteria includes those which actually
invade the body and feed upon its tissues instead of living simply
upon swallowed food. It is difficult, however, to draw any sharp
line separating the two classes. The bacteria which cause
diphtheria (Fig. 28), for instance, do not really invade the body.
They grow in the throat, attached to its walls, and are confined
to this external location or to the superficial tissues. This
bacillus is, in short, only found in the mouth and throat, and is
practically confined to the so-called false membranes. It never
enters any of the tissues of the body, although attached to the
mucous membrane. It grows vigorously in this membrane, and there
secretes or in some way produces extremely violent poisons. These
poisons are then absorbed by the body and give rise to the general
symptoms of the disease. Much the same is true of the bacillus
which causes tetanus or lockjaw (Fig. 29). This bacillus is
commonly inoculated into the flesh of the victim by a wound made
with some object which has been lying upon the earth where the
bacillus lives. The bacillus grows readily after being inoculated,
but it is localized at the point of the wound, without invading
the tissue to any extent. It produces, however, during its growth
several poisons which have been separated and studied. Among them
are some of the most violent poisons of which we have any
knowledge. While the bacillus grows in the tissues around the
wound it secretes these poisons, which are then absorbed by the
body generally. Their poisoning effects produce the violent
symptoms of the disease. Of much the same nature is Asiatic
cholera. This is caused by a bacillus which is able to grow
rapidly in the intestines, feeding perhaps in part on the food in
the intestines and perhaps in part upon the body secretions. To a
slight extent also it appears to be able to invade the tissues of
the body, for the bacilli are found in the walls of the
intestines. But it is not a proper parasite, and the fatal disease
it produces is the result of the absorption of the poisons
secreted in the intestines.
It is but a step from this to the true parasites. Typhoid fever,
for example, is a disease produced by bacteria which grow in the
intestines, but which also invade the tissues more extensively
than the cholera germs (Fig. 30). They do not invade the body
generally, however, but become somewhat localized in special
glands like the liver, the spleen, etc. Even here they do not
appear to find a very favourable condition, for they do not grow
extensively in these places. They are likely to be found in the
spleen in small groups or centres, but not generally distributed
through it. Wherever they grow they produce poison, which has been
called typhotoxine, and it is this poison chiefly which gives rise
to the fever.
Quite a considerable number of the pathogenic germs are, like the
typhoid bacillus, more or less confined to special places. Instead
of distributing themselves through the body after they find
entrance, they are restricted to special organs. The most common
example of a parasite of this sort is the tuberculosis bacillus,
the cause of consumption, scrofula, white swelling, lupus, etc.
(Fig. 31). Although this bacillus is very common and is able to
attack almost any organ in the body, it is usually very restricted
in growth. It may become localized in a small gland, a single
joint, a small spot in the lungs, or in the glands of the
mesentery, the other parts of the body remaining free from
infection. Not infrequently the whole trouble is thus confined to
such a small locality that nothing serious results. But in other
instances the bacilli may after a time slowly or rapidly
distribute themselves from these centres, attacking more and more
of the body until perhaps fatal results follow in the end. This
disease is therefore commonly of very slow progress.
Again, we have still other parasites which are not thus confined,
but which, as soon as they enter the body, produce a general
infection, attacking the blood and perhaps nearly all tissues
simultaneously. The most typical example of this sort is anthrax
or malignant pustule, a disease fortunately rare in man (Fig. 32).
Here the bacilli multiply in the blood, and very soon a general
and fatal infection of the whole body arises, resulting from the
abundance of the bacilli everywhere. Some of the obscure diseases
known as blood poisoning appear to be of the same general nature,
these diseases resulting from a very general invasion of the whole
body by certain pathogenic bacteria.
In general, then, we see that the so-called germ diseases result
from the action upon the body of poisons produced by bacterial
growth. Differences in the nature of these poisons produce
differences in the character of the disease, and differences in
the parasitic powers of the different species of bacteria produce
wide differences in the course of the diseases and their relation
to external phenomena.
WHAT DISEASES ARE DUE TO BACTERIA?
It is, of course, an extremely important matter to determine to
what extent human diseases are caused by bacteria. It is not easy,
nor indeed possible, to do this to-day with accuracy. It is no
easy matter to prove that any particular disease is caused by
bacteria. To do this it is necessary to find some particular
bacterium present in all cases of the disease; to find some method
of getting it to grow outside the body in culture media; to
demonstrate its absence in healthy animals, or healthy human
individuals if it be a human disease; and, finally, to reproduce
the disease in healthy animals by inoculating them with the
bacterium. All of these steps of proof present difficulties, but
especially the last one. In the study of animals it is
comparatively easy to reproduce a disease by inoculation. But
experiments upon man are commonly impossible, and in the case of
human diseases it is frequently very difficult or impossible to
obtain the final test of the matter. After finding a specific
bacterium associated with a disease, it is usually possible to
experiment with it further upon animals only. But some human
diseases do not attack animals, and in the case of diseases that
may be given to animals it is frequently uncertain whether the
disease produced in the animal by such inoculation is identical
with the human disease in question, owing to the difference of
symptoms in the different animals. As a consequence, the proof of
the germ nature of different diseases varies all the way from
absolute demonstration to mere suspicion. To give a complete and
correct list of the diseases caused by bacteria, or to give a list
of the bacteria species pathogenic to man, is therefore at present
impossible.
The difficulty of giving such a list is rendered greater from the
fact that we have in recent years learned that the same species of
pathogenic bacterium may produce different results under different
conditions. When the subject of germ disease was first studied and
the connection between bacteria and disease was first
demonstrated, it was thought that each particular species of
pathogenic bacteria produced a single definite disease; and
conversely, each germ disease was supposed to have its own
definite species of bacterium as its cause. Recent study has
shown, however, that this is not wholly true. It is true that some
diseases do have such a definite relation to definite bacteria.
The anthrax germ, for example, will always produce anthrax, no
matter where or how it is inoculated into the body. So, also, in
quite a number of other cases distinct specific bacteria are
associated with distinct diseases. But, on the other hand, there
are some pathogenic bacteria which are not so definite in their
action, and produce different results in accordance with
circumstances, the effect varying both with the organ attacked and
with the condition of the individual. For instance, a considerable
number of different types of blood poisoning, septicaemia,
pyaemia, gangrene, inflammation of wounds, or formation of pus
from slight skin wounds--indeed, a host of miscellaneous troubles,
ranging all the way from a slight pus formation to a violent and
severe blood poisoning--all appear to be caused by bacteria, and
it is impossible to make out any definite species associated with
the different types of these troubles. There are three common
forms of so-called pus cocci, and these are found almost
indiscriminately with various types of inflammatory troubles.
Moreover, these species of bacteria are found with almost absolute
constancy in and around the body, even in health. They are on the
clothing, on the skin, in the mouth and alimentary canal. Here
they exist, commonly doing no harm. They have, however, the power
of doing injury if by chance they get into wounds. But their power
of doing injury varies both with the condition of the individual
and with variations in the bacteria themselves. If the individual
is in a good condition of health these bacteria have little power
of injuring him even when they do get into such wounds, while at
times of feeble vitality they may do much more injury, and take
the occasion of any little cut or bruise to enter under the skin
and give rise to inflammation and pus. Some people will develop
slight abscesses or slight inflammations whenever the skin is
bruised, while with others such bruises or cuts heal at once
without trouble. Both are doubtless subject to the same chance of
infection, but the one resists, while the other does not. In
common parlance, we say that such a tendency to abscesses
indicates a bad condition of the blood--a phrase which means
nothing. Further, we find that the same species of bacterium may
have varying powers of producing disease at different times. Some
species are universal inhabitants of the alimentary canal and are
ordinarily harmless, while under other conditions of unknown
character they invade the tissues and give rise to a serious and
perhaps fatal disease. We may thus recognise some bacteria which
may be compared to foreign invaders, while others are domestic
enemies. The former, like the typhoid bacillus, always produce
trouble when they succeed in entering the body and finding a
foothold. The latter, like the normal intestinal bacilli, are
always present but commonly harmless, only under special
conditions becoming troublesome. All this shows that there are
other factors in determining the course of a disease, or even the
existence of a disease, than the simple presence of a peculiar
species of pathogenic bacterium.
From the facts just stated it will be evident that any list of
germ diseases will be rather uncertain. Still, the studies of the
last twenty years or more have disclosed some definite relations
of bacteria and disease, and a list of the diseases more or less
definitely associated with distinct species of bacteria is of
interest. Such a list, including only well-known diseases, is as
follows:
Name of disease. Name of bacterium producing the disease.
Anthrax (Malignant pustule). Bacillus anthracis.
Cholera. Spirillum cholera: asiaticae
Croupous pneumonia. Micrococcus pneumonia crouposa.
Diphtheria. Bacillus diphtheria.
Glanders. Bacillus mallei.
Gonorrhoea. Micrococcus gonorrhaeae
Influenza. Bacillus of influenza.
Leprosy. Bacillus leprae.
Relapsing fever. Spirillum Obermeieri.
Tetanus (lockjaw). Bacillus tetani.
Tuberculosis (including
consumption, scrofula, etc.) Bacillus tuberculosis.
Typhoid fever. Bacillus typhi abdominalis.
Various wound infections, including septicaemia, pyaemia, acute
abscesses, ulcers, erysipelas, etc., are produced by a few forms
of micrococci, resembling each other in many points but differing
slightly. They are found almost indiscriminately in any of these
wound infections, and none of them appears to have any definite
relation to any special form of disease unless it be the
micrococcus of erysipelas. The common pus micrococci are grouped
under three species, Staphylococcus pyogenes aureus,
Staphylococcus pyogenes, and Streptococcus pyogenes. These three
are the most common, but others are occasionally found.
In addition to these, which may be regarded as demonstrated, the
following diseases are with more or less certainty regarded as
caused by distinct specific bacteria: Bronchitis, endocarditis,
measles, whooping-cough, peritonitis, pneumonia, syphilis.
Still another list might be given of diseases whose general nature
indicates that they are caused by bacteria, but in connection with
which no distinct bacterium has yet been found. As might be
expected also, a larger list of animal diseases has been
demonstrated to be caused by these organisms. In addition, quite a
number of species of bacteria have been found in such material as
faeces, putrefying blood, etc., which have been shown by
experiment to be capable of producing diseases in animals, but in
regard to which we have no evidence that they ever do produce
actual disease under any normal conditions. These may contribute,
perhaps, to the troubles arising from poisonous foods, but can not
be regarded as disease germs proper.
VARIABILITY OF PATHOGENIC POWERS.
As has already been stated, our ideas of the relation of bacteria
to disease have undergone quite a change since they were first
formulated, and we recognise other factors influencing disease
besides the actual presence of the bacterium. These we may briefly
consider under two heads, viz., variation in the bacterium, and
variation in the susceptibility of the individual. The first will
require only a brief consideration.
That the same species of pathogenic bacteria at different times
varies in its powers to produce disease has long been known.
Various conditions are known to affect thus the virulence of
bacteria. The bacillus which is supposed to give rise to pneumonia
loses its power to produce the disease after having been
cultivated for a short time in ordinary culture media in the
laboratory. This is easily understood upon the suggestion that it
is a parasitic bacillus and does not thrive except under parasitic
conditions. Its pathogenic powers can sometimes be restored by
passing it again through some susceptible animal. One of the most
violent pathogenic bacteria is that which produces anthrax, but
this loses its pathogenic powers if it is cultivated for a
considerable period at a high temperature. The micrococcus which
causes fowl cholera loses its power if it be cultivated in common
culture media, care being taken to allow several days to elapse
between the successive inoculations into new culture flasks. Most
pathogenic bacteria can in some way be so treated as to suffer a
diminution or complete loss of their powers of producing a fatal
disease. On the other hand, other conditions will cause an
increase in the virulence of a pathogenic germ. The virus which
produces hydrophobia is increased in violence if it is inoculated
into a rabbit and subsequently taken from the rabbit for further
inoculation. The fowl cholera micrococcus, which has been weakened
as just mentioned, may be restored to its original violence by
inoculating it into a small bird, like a sparrow, and inoculating
a second bird from this. A few such inoculations will make it as
active as ever. These variations doubtless exist among the species
in Nature as well as in artificial cultures. The bacteria which
produce the various wound infections and abscesses, etc., appear
to vary under normal conditions from a type capable of producing
violent and fatal blood poisoning to a type producing only a
simple abscess, or even to a type that is entirely innocuous. It
is this factor, doubtless, which in a large measure determines the
severity of any epidemic of a bacterial contagious disease.
SUSCEPTIBILITY OF THE INDIVIDUAL.
The very great modification of our early views has affected our
ideas as to the power which individuals have of resisting the
invasion of pathogenic bacteria. It has from the first been
understood that some individuals are more susceptible to disease
than others, and in attempting to determine the significance of
this fact many valuable and interesting discoveries have been
made. After the exposure to the disease there follows a period of
some length in which there are no discernible effects. This is
followed by the onset of the disease and its development to a
crisis, and, if this be passed, by a recovery. The general course
of a germ disease is divided into three stages: the stage of
incubation, the development of the disease, and the recovery. The
susceptibility of the body to a disease may be best considered
under the three heads of Invasion, Resistance, Recovery.
Means of Invasion.--In order that a germ disease should arise in
an individual, it is first necessary that the special bacterium
which causes the disease should get into the body. There are
several channels through which bacteria can thus find entrance;
these are through the mouth, through the nose, through the skin,
and occasionally through excretory ducts. Those which come through
the mouth come with the food or drink which we swallow; those
which enter through the nose must be traced to the air; and those
which enter through the skin come in most cases through contact
with some infected object, such as direct contact with the body of
an infected person or his clothing or some objects he has handled,
etc. Occasionally, perhaps, the bacteria may get into the skin
from the air, but this is certainly uncommon and confined to a few
diseases. There are here two facts of the utmost importance for
every one to understand: first, that the chance of disease
bacteria being carried to us through the air is very slight and
confined to a few diseases, such as smallpox, tuberculosis,
scarlet fever; etc., and, secondly, that the uninjured skin and
the uninjured mucous membrane also is almost a sure protection
against the invasion of the bacteria. If the skin is whole,
without bruises or cuts, bacteria can seldom, if ever, find
passage through it. These two facts are of the utmost importance,
since of all sources of infection we have the least power to guard
against infection through the air, and since of all means 'of
entrance we can guard the skin with the greatest difficulty. We
can easily render food free from pathogenic bacteria by heating
it. The material we drink can similarly be rendered harmless, but
we can not by any known means avoid breathing air, nor is there
any known method of disinfecting the air, and it is impossible for
those who have anything to do with sick persons to avoid entirely
having contact either with the patient or with infected clothing
or utensils.
From the facts here given it will be seen that the individual's
susceptibility to disease produced by parasitic bacteria will
depend upon his habits of cleanliness, his care in handling
infectious material, or care in cleansing the hands after such
handling, upon his habit of eating food cooked or raw, and upon
the condition of his skin and mucous membranes, since any kind of
bruises will increase susceptibility. Slight ailments, such as
colds, which inflame the mucous membrane, will decrease its
resisting power and render the individual more susceptible to the
entrance of any pathogenic germs should they happen to be present.
Sores in the mouth or decayed teeth may in the same way be
prominent factors in the individual's susceptibility. Thus quite a
number of purely physical factors may contribute to an
individual's susceptibility.
Resisting Power of the Body.--Even after the bacteria get into the
body it is by no means certain that they will give rise to
disease, for they have now a battle to fight before they can be
sure of holding their own. It is now, indeed, that the actual
conflict between the powers of the body and these microscopic
invaders begins. After they have found entrance into the body the
bacteria have arrayed against them strong resisting forces of the
human organism, endeavouring to destroy and expel them. Many of
them are rapidly killed, and sometimes they are all destroyed
without being able to gain a foothold. In such cases, of course,
no trouble results. In other cases the body fails to overcome the
powers of the invaders and they eventually multiply rapidly. In
this struggle the success of the invaders is not necessarily a
matter of numbers. They are simply struggling to gain a position
in the body, where they can feed and grow. A few individuals may
be entirely sufficient to seize such a foothold, and then these by
multiplying may soon become indefinitely numerous. To protect
itself, therefore, the human body must destroy every individual
bacterium, or at least render them all incapable of growth. Their
marvellous reproductive powers give the bacteria an advantage in
the battle. On the other hand, it takes time even for these
rapidly multiplying beings to become sufficiently numerous to do
injury. There is thus an interval after their penetration into the
body when these invaders are weak in numbers. During this
interval--the period of incubation--the body may organize a
resistance sufficient to expel them.
We do not as yet thoroughly understand the forces which the human
organism is able to array against these invading foes. Some of its
methods of defence are, however, already intelligible to us, and
we know enough, at all events, to give us an idea of the intensity
of the conflict that is going on, and of the vigorous and powerful
forces which the human organism is able to bring against its
invading enemies.
In the first place, we notice that a majority of bacteria are
utterly unable to grow in the human body even if they do find
entrance. There are known to bacteriologists to-day many hundreds,
even thousands of species, but the vast majority of these find in
the human tissues conditions so hostile to their life that they
are utterly unable to grow therein. Human flesh or human blood
will furnish excellent food for them if the individual be dead,
but living human flesh and blood in some way exerts a repressing
influence upon them which is fatal to the growth of a vast
majority of species. Some few species, however, are not thus
destroyed by the hostile agencies of the tissues of the animal,
but are capable of growing and multiplying in the living body.
These alone are what constitute the pathogenic bacteria, since, of
course, these are the only bacteria which can produce disease by
growing in the tissues of an animal. The fact that the vast
majority of bacteria can not grow in the living organism shows
clearly enough that there are some conditions existing in the
living tissue hostile to bacterial life. There can be little
doubt, moreover, that it is these same hostile conditions, which
enable the body to resist the attack of the pathogenic species in
cases where resistance is successfully made.
What are the forces arrayed against these invaders? The essential
nature of the battle appears to be a production of poisons and
counter poisons. It appears to be an undoubted fact that the first
step in repelling these bacteria is to flood them with certain
poisons which check their growth. In the blood and lymph of man
and other animals there are present certain products which have a
direct deleterious influence upon the growth of micro-organisms.
The existence of these poisons is undoubted, many an experiment
having directly attested to their presence in the blood of
animals. Of their nature we know very little, but of their
repressing influence upon bacterial growth we are sure. They have
been named alexines, and they are produced in the living tissue,
although as to the method of their production we are in ignorance.
By the aid of these poisons the body is able to prevent the growth
of the vast majority of bacteria which get into its tissues.
Ordinary micro-organisms are killed at once, for these alexines
act as antiseptics, and common bacteria can no more grow in the
living body than they could in a solution containing other poisons
Thus the body has a perfect protection against the majority of
bacteria. The great host of species which are found in water,
milk, air, in our mouths or clinging to our skin, and which are
almost omnipresent in Nature, are capable of growing well enough
in ordinary lifeless organic foods, but just as soon as they
succeed in finding entrance into living human tissue their growth
is checked at once by these antiseptic agents which are poured
upon them. Such bacteria are therefore not pathogenic germs, and
not sources of trouble to human health.
There are, on the other hand, a few species of bacteria which may
be able to retain their lodgment in the body m spite of this
attempt of the individual to get rid of them. These, of course,
constitute the pathogenic species, or so called "disease germs".
Only such species as can overcome this first resistance can be
disease germs, for they alone can retain their foothold in the
body.
But how do these species overcome the poisons, which kill the
other harmless bacteria? They, as well as the harmless forms, find
these alexines injurious to their growth, but in some way they are
able to counteract the poisons. In this general discussion of
poisons we are dealing with a subject which is somewhat obscure,
but apparently the pathogenic bacteria are able to overcome the
alexines of the body by producing in their turn certain other
products which neutralize the alexines, thus annulling their
action. These pathogenic bacteria, when they get into the body,
give rise at once to a group of bodies which have been named
lysines. These lysines are as mysterious to us as the alexines,
but they neutralize the effect of the alexines and thus overcome
the resistance the body offers to bacterial growth The invaders
can now multiply rapidly enough to get a lasting foothold in the
body and then soon produce the abnormal symptoms which we call
disease Pathogenic bacteria thus differ from the non-pathogenic
bacteria primarily in this power of secreting products which can
neutralize the ordinary effects of the alexines, and so overcome
the body's normal resistance to their parasitic life.
Even if the bacteria do thus overcome the alexines the battle is
not yet over, for the individual has another method of defence
which is now brought into activity to check the growth of the
invading organisms. This second method of resistance is by means
of a series of active cells found in the blood, known as white
blood-corpuscles (Fig. 33 a, b). They are minute bits of
protoplasm present in the blood and lymph in large quantities.
They are active cells, capable of locomotion and able to crawl out
of the blood-vessels Not infrequently they are found to take into
their bodies small objects with which they come in contact. One of
their duties is thus to engulf minute irritating bodies which may
be in the tissues, and to carry them away for excretion. They thus
act as scavengers These corpuscles certainly have some agency in
warding off the attacks of pathogenic bacteria Very commonly they
collect in great numbers in the region of the body where invading
bacteria are found. Such invading bacteria exist upon them a
strong attraction, and the corpuscles leave the blood-vessels and
sometimes form a solid phalanx completely surrounding the invading
germs. Their collection at these points may make itself seen
externally by the phenomenon we call inflammation.
There is no question that the corpuscles engage in conflict with
the bacteria when they thus surround them. There has been not a
little dispute, however, as to the method by which they carry on
the conflict. It has been held by some that the corpuscles
actually take the bacteria into their bodies, swallow them, as it
were, and subsequently digest them (Fig. 33 c, d, e). This idea
gave rise to the theory of phagocytosis, and the corpuscles were
consequently named phagocytes. The study of several years has,
however, made it probable that this is not the ordinary method by
which the corpuscles destroy the bacteria. According to our
present knowledge the method is a chemical one. These cells, when
they thus collect in quantities around the invaders, appear to
secrete from their own bodies certain injurious products which act
upon the bacteria much as do the alexines already mentioned. These
new bodies have a decidedly injurious effect upon the multiplying
bacteria; they rapidly check their growth, and, acting in union
with the alexines, may perhaps entirely destroy them.
After the bacteria are thus killed, the white blood-corpuscles may
load themselves with their dead bodies and carry them away (Fig.
33 d, e). Sometimes they pass back into the blood stream and carry
the bacteria to various parts of the body for elimination. Not
infrequently the white corpuscles die in the contest, and then may
accumulate in the form of pus and make their way through the skin
to be discharged directly. The battle between these phagocytes and
the bacteria goes on vigorously. If in the end the phagocytes
prove too strong for the invaders, the bacteria are gradually all
destroyed, and the attack is repelled. Under these circumstances
the individual commonly knows nothing--of the matter. This
conflict has taken place entirely without any consciousness on his
part, and he may not even know that he has been exposed to the
attack of the bacteria. In other cases the bacteria prove too
strong for the phagocytes. They multiply too rapidly, and
sometimes they produce secretions which actually drive the
phagocytes away. Commonly, as already noticed, the corpuscles are
attracted to the point of invasion, but in some cases, when a
particularly deadly and vigorous species of bacteria invades the
body, the secretions produced by them are so powerful as actually
to drive the corpuscles away. Under these circumstances the
invading hosts have a chance to multiply unimpeded, to distribute
themselves over the body, and the disease rapidly follows as the
result of their poisoning action on the body tissues.
It is plain, then, that the human body is not helpless in the
presence of the bacteria of disease, but that it is supplied with
powerful resistant forces. It must not be supposed, however, that
the outline of the action of these forces just given is anything
like a complete account of the matter; nor must it be inferred
that the resistance is in all respects exactly as outlined. The
subject has only recently been an object of investigation, and we
are as yet in the dark in regard to many of the facts. The future
may require us to modify to some extent even the brief outline
which has been given. But while we recognise this uncertainty in
the details, we may be assured of the general facts. The living
body has some very efficacious resistant forces which prevent most
bacteria from growing within its tissues, and which in large
measure may be relied upon to drive out the true pathogenic
bacteria. These resistant forces are in part associated with the
productions of body poisons, and are in part associated with the
active powers of special cells which have been called phagocytes.
The origin of the poisons and the exact method of action of the
phagocytes we may well leave to the future to explain.
These resisting powers of the body will vary with conditions. It
is evident that they are natural powers, and they will doubtless
vary with the general condition of vigour of the individual.
Robust health, a body whose powers are strong, well nourished, and
vigorous, will plainly furnish the conditions for the greatest
resistance to bacterial diseases. One whose bodily activities are
weakened by poor nutrition can offer less resistance. The question
whether one shall suffer from a germ disease is not simply the
question whether he shall be exposed, or even the question whether
the bacteria shall find entrance into his body. It is equally
dependent upon whether he has the bodily vigour to produce
alexines in proper quantity, or to summon the phagocytes in
sufficient abundance and vigour to ward off the attack. We may do
much to prevent disease by sanitation, which aids in protecting
the individual from attack; but we must not forget that the other
half of the battle is of equal importance, and hence we must do
all we can to strengthen the resisting forces of the organism.
RECOVERY FROM GERM DISEASES.
These resisting forces are not always sufficient to drive off the
invaders. The organisms may retain their hold in the body for a
time and eventually break down the resistance. After this they may
multiply unimpeded and take entire possession of the body. As they
become more numerous their poisonous products increase and begin
to produce direct poisoning effects on the body. The incubation
period is over and the disease comes on. The disease now runs its
course. It becomes commonly more and more severe until a crisis is
reached. Then, unless the poisoning is so severe that death
occurs, the effects pass away and recovery takes place.
But why should not a germ disease be always fatal? If the bacteria
thus take possession of the body and can grow there, why do they
not always continue to multiply until they produce sufficient
poison to destroy the life of the individual? Such fatal results
do, of course, occur, but in by far the larger proportion of cases
recovery finally takes place.
Plainly, the body must have another set of resisting forces which
is concerned in the final recovery. Although weakened by the
poisoning and suffering from the disease, it does not yield the
battle, but somewhat slowly organizes a new attack upon the
invaders. For a time the multiplying bacteria have an unimpeded
course and grow rapidly; but finally their further increase is
checked, their vigour impaired, and after this they diminish in
numbers and are finally expelled from the body entirely. Of the
nature of this new resistance but little is yet known. We notice,
in the first place, that commonly after such a recovery the
individual has decidedly increased resistance to the disease. This
increased resistance may be very lasting, and may be so
considerable as to give almost complete immunity from the disease
for many years, or for life. One attack of scarlet fever gives the
individual great immunity for the future. On the other hand, the
resistance thus derived may be very temporary, as in the case of
diphtheria. But a certain amount of resistance appears to be
always acquired. This power of resisting the activities of the
parasites seems to be increased during the progress of the
disease, and, if it becomes sufficient, it finally drives off the
bacteria before they have produced death. After this, recovery
takes place. To what this newly acquired resisting power is due is
by no means clear to bacteriologists, although certain factors
are already known. It appears beyond question that in the case of
certain diseases the cells of the body after a time produce
substances which serve as antidotes to the poisons produced by the
bacteria during their growth in the body-antitoxines. In the case
of diphtheria, for instance, the germs growing in the throat
produce poisons which are absorbed by the body and give rise to
the symptoms of the disease; but after a time the body cells
react, and themselves produce a counter toxic body which
neutralizes the poisonous effect of the diphtheria poison. This
substance has been isolated from the blood of animals that have
recovered from an attack of diphtheria, and has been called
diphtheria antitoxine. But even with this knowledge the recovery
is not fully explained. This antitoxine neutralizes the effects of
the diphtheria toxine, and then the body develops strength to
drive off the bacteria which have obtained lodgment in the throat.
How they accomplish this latter achievement we do not know as yet.
The antitoxme developed simply neutralizes the effects of the
toxine. Some other force must be at work to get rid of the
bacteria, a force which can only exert itself after the poisoning
effect of the poison is neutralized. In these cases, then, the
recovery is due, first, to the development in the body of the
natural antidotes to the toxic poisons, and, second, to some other
unknown force which drives off the parasites.
These facts are certainly surprising. If one had been asked to
suggest the least likely theory to explain recovery from disease,
he could hardly have found one more unlikely than that the body
cells developed during the disease an antidote to the poison which
the disease bacteria were producing. Nevertheless, it is beyond
question that such antidotes are formed during the course of the
germ diseases. It has not yet been shown in all diseases, and it
would be entirely too much to claim that this is the method of
recovery in all cases. We may say, however, in regard to bacterial
diseases in general, that after the bacteria enter the body at
some weak point they have first a battle to fight with the
resisting powers of the body, which appear to be partly biological
and partly chemical. These resisting powers are in many cases
entirely sufficient to prevent the bacteria from obtaining a
foothold. If the invading host overcome the resisting powers, then
they begin to multiply rapidly, and take possession of the body or
some part of it. They continue to grow until either the individual
dies or something occurs to check their growth. After the
individual develops the renewed powers of checking their growth,
recovery takes place, and the individual is then, because of these
renewed powers of resistance, immune from a second attack of the
disease for a variable length of time.
This, in the merest outline, represents the relation of bacterial
parasites to the human body But while this is a fair general
expression of the matter, it must be recognised that different
diseases differ much in their relations, and no general outline
will apply to all They differ in their method of attack and in the
point of attack. Not only do they produce different kinds of
poisons giving rise to different symptoms of poisoning; not only
do they produce different results in different animals; not only
do the different pathogenic species differ much in their power
to develop serious disease, but the different species are very
particular as to what species of animal they attack. Some of them
can live as parasites in man alone; some can live as parasites
upon man and the mouse and a few other animals; some can live in
various animals but not in man; some appear to be able to live in
the field mouse, but not in the common mouse; some live in the
horse; some in birds, but not in warm-blooded mammals; while
others, again, can live almost equally well in the tissues of a
long list of animals. Those which can live as parasites upon man
are, of course, especially related to human disease, and are of
particular interest to the physician, while those which live in
animals are in a similar way of interest to veterinarians.
Thus we see that parasitic bacteria show the widest variations.
They differ in point of attack, in method of attack, and in the
part of the body which they seize upon as a nucleus for growth.
They differ in violence and in the character of the poisons they
produce, as well as in their power of overcoming the resisting
powers of the body. They differ at different times in their powers
of producing disease. In short, they show such a large number of
different methods of action that no general statements can be made
which will apply universally, and no one method of guarding
against them or in driving them off can be hoped to apply to any
extended list of diseases.
DISEASES CAUSED BY OTHER ORGANISMS THAN BACTERIA.
Although the purpose of this work is to deal primarily with the
bacterial world, it would hardly be fitting to leave the subject
without some reference to diseases caused by organisms which do
not belong to the group of bacteria. While most of the so-called
germ diseases are caused by the bacteria which we have been
studying in the previous chapters, there are some whose inciting
cause is to be found among organisms belonging to other groups.
Some of these are plants of a higher organization than bacteria,
but others are undoubtedly microscopic animals. Their life habits
are somewhat different from those of bacteria, and hence the
course of the diseases is commonly different. Of the diseases thus
produced by microscopic animals or by higher plants, one or two
are of importance enough to deserve special mention here.
Malaria.--The most important of these diseases is malaria in its
various forms, and known under various names--chills and fever,
autumnal fever, etc. This disease, so common almost everywhere,
has been studied by physicians and scientists for a long time, and
many have been the causes assigned to it. At one time it was
thought to be the result of the growth of a bacterium, and a
distinct bacillus was described as producing it. It has finally
been shown, however, to be caused by a microscopic organism
belonging to the group of unicellular animals, and somewhat
closely related to the well-known amoeba. This organism is shown
in Fig. 34. The whole history of the malarial organism is not yet
known. The following statements comprise the most important facts
known in regard to it, and its relation to the disease in man.
Undoubtedly the malarial germ has some home outside the human
body, but it is not yet very definitely known what this external
home is; nor do we know from what source the human parasite is
derived. It appears probable that water serves in some cases as
its means of transference to man, and air in other cases. From
some external source it gains access to man and finds its way into
the blood. Here it attacks the red blood-corpuscles, each malarial
organism making its way into a single one (Fig. 340). Here it now
grows, increasing in size at the expense of the substance of the
corpuscle. As it becomes larger it becomes granular, and soon
shows a tendency to separate into a number of irregular masses.
Finally it breaks up into many minute bodies called spores. These
bodies break out of the corpuscle and for a time live a free life
in the blood. After a time they make their way into other red
blood-corpuscles, develop into new malarial amoeboid parasites,
and repeat the growth and sporulation. This process can apparently
be repeated many times without check.
These organisms are thus to be regarded as parasites of the red
corpuscles. It is, of course, easy to believe that an extensive
parasitism and destruction of the corpuscles would be disastrous
to the health of the individual, and the severity of the disease
will depend upon the extent of the parasitism. Corresponding to
this life history of the organism, the disease malaria is commonly
characterized by a decided intermittency, periods of chill and
fever alternating with periods of intermission in which these
symptoms are abated. The paroxysms of the disease, characterized
by the chill, occur at the time that the spores are escaping from
the blood-corpuscles and floating in the blood. After they have
again found their way into a blood-corpuscle the fever diminishes,
and during their growth in the corpuscle until the next
sporulation the individual has a rest from the more severe
symptoms.
There appears to be more than one variety of the malarial
organism, the different types differing in the length of time it
takes for their growth and sporulation. There is one variety, the
most common one, which requires two days for its growth, thus
giving rise to the paroxysm of the disease about once in forty-
eight hours; another variety appears to require three days for its
growth; while still another variety appears to be decidedly
irregular in its period of growth and sporulation. These facts
readily explain some of the variations in the disease. Certain
other irregularities appear to be due to a different cause. More
than one brood of parasites may be in the blood of the individual
at the same time, one producing sporulation at one time and
another at a different time. Such a simultaneous growth of two
independent broods may plainly produce almost any kind of
modification in the regularity of the disease.
The malarial organism appears to be very sensitive to quinine, a
very small quantity being sufficient to kill it. Upon this point
depends the value of quinine as a medicine. If the drug be present
in the blood at the time when the spores are set free from the
blood-corpuscle, they are rapidly killed by it before they have a
chance to enter another corpuscle. During their growth in the
corpuscle they are far less sensitive to quinine than when they
exist in the free condition as spores, and at this time the drug
has little effect.
The malarial organism is an animal, and can not be cultivated in
the laboratory by any artificial method yet devised. Its whole
history is therefore not known. It doubtless has some home outside
the blood of animals, and very likely it may pass through other
stages of a metamorphosis in the bodies of other animals. Most
parasitic animals have two or more hosts upon which they live,
alternating from one to the other, and that such is the case with
the malarial parasite is at least probable. But as yet
bacteriologists have been unable to discover anything very
definite in regard to the matter. Until we can learn something in
regard to its life outside the blood of man we can do little in
the way of devising methods to avoid it.
Malaria differs from most germ diseases in the fact that the
organisms which produce it are not eliminated from the body in any
way. In most germ diseases the germs are discharged from the
patient by secretions or excretions of some kind, and from these
excretions may readily find their way into other individuals. The
malarial organism is not discharged from the body in any way, and
hence is not contagious. If the parasite does pass part of its
history in some other animal than man, there must be some means by
which it passes from man to its other host. It has been suggested
that some of the insects which feed upon human blood may serve as
the second host and become inoculated when feeding upon such
blood. This has been demonstrated with startling success in regard
to the mosquito (Anopheles), some investigators going so far as to
say that this is the only way in which the disease can be
communicated.
Several other microscopic animals occur as parasites upon man, and
some of them are so definitely associated with certain diseases as
to lead to the belief that they are the cause of these diseases.
The only one of very common occurrence is a species known as
Amaeba coli, which is found in cases of dysentery. In a certain
type of dysentery this organism is so universally found that there
is little doubt that it is in some very intimate way associated
with the cause of the disease. Definite proof of the matter is,
however, as yet wanting.
On the side of plants, we find that several plants of a higher
organization than bacteria may become parasitic upon the body of
man and produce various types of disease. These plants belong
mostly to the same group as the moulds, and they are especially
apt to attack the skin. They grow in the skin, particularly under
the hair, and may send their threadlike branches into some of the
subdermal tissues. This produces irritation and inflammation of
the skin, resulting in trouble, and making sores difficult to
heal. So long as the plant continues to grow, the sores, of
course, can not be healed, and when the organisms get into the
skin under the hair it is frequently difficult to destroy them.
Among the diseases thus caused are ringworm, thrush, alopecia,
etc.