BACTERIA AND PROTOZOA
BACTERIA
On the border line between the plant and the animal worlds are many
forms which possess some of the characteristics of both. Indeed when an
attempt is made to separate all known organisms into two groups one is
immediately confronted with difficulties. In looking over the text-books
of Botany we will find that certain low forms are discussed there as
belonging with the plants, and on turning to the manuals of Zoölogy we
will find that the same organisms are placed among the lowest forms of
animals. The question is of course of little actual importance from
certain points of view. It serves, however, to show the close relation
of all forms of life, and from a medical standpoint it may be of very
great importance owing to the difference in the life-habits, methods of
reproduction and methods of transmission of many of the forms that cause
disease. We have already seen that none of the diseases that are caused
by animal parasites is contagious, while many of those caused by
bacteria are both contagious and infectious.
Just over on the plant side of this indefinite border line are the
minute organisms known as bacteria. Their numbers are infinite and they
are found everywhere. The majority of them are beneficial to mankind in
one way or another, but some of them cause certain of the diseases that
we will have to discuss later so attention may be called here to a few
of the important facts in regard to their organization and life-history
in order that we may better understand how they may be so easily
transferred from one host to another.
Although these bacilli are so extremely minute (Fig. 7), some of them so
small that they cannot be seen with the most powerful microscopes, they
differ in size, shape, methods of division and spore-formation. Each
species makes a characteristic growth on gelatin, agar or other media
upon which it may be cultivated. In this way as well as by the
inoculation of animals the presence of the ultramicroscopic kinds may be
demonstrated.
The method of reproduction is very simple. They increase to a certain
point in size, then divide. This growth and division takes place very
rapidly. Twenty to thirty minutes is sufficient time in some cases for
a just-divided cell to attain full size and divide again. Thus in a few
days time the number of bacteria resulting from a single individual
would be inconceivable if they should all develop.
Fortunately for us, however, they do not all multiply so rapidly as this
and besides there are natural checks, not the least of which are the
substances given off by the bacteria themselves in their growth and
development. Such excretions often serve to inhibit further
multiplication. Sometimes, though not often, they form spores which not
only provide for a more rapid multiplication, but enable the organism to
live under conditions that would otherwise prove fatal to it.
Bacteria may be conveniently grouped under two heads: those that live
upon dead organic matter, known as the saprophytic forms, and those that
are found in living plants or animals, the true parasites. Such a
grouping is not always entirely satisfactory, for many of the kinds that
live saprophytically under normal conditions may become parasitic if
opportunity offers, and also many of those that are usually regarded as
parasitic may be grown in cultures of agar or other media, under which
conditions they may be regarded as living saprophytically.
It is this power of easily adapting themselves to different conditions
that makes many of the kinds dangerous. The bacillus which causes
tetanus or lockjaw will illustrate this. It is a rather common bacillus
in soil in many localities. As long as it remains there it is of no
special importance, but if it is introduced into the body through a
scratch or any other wound it becomes a very serious matter.
We may say, then, that the effect the bacillus has on the host depends
largely on the host. Not only does it depend on what the host is, but
the particular condition of the host at the time of infection is of
importance. Children are subject to many diseases that adults seldom
have. Hunger, thirst, fatigue, exposure and other factors may make a
person susceptible to the actions of certain bacteria that would be
harmless under other conditions.
The minute size and great numbers of the bacteria make their
dissemination a comparatively simple matter. They may be carried in the
air as minute particles of dust; they may be carried in water or milk;
they may be carried on the clothing or on the person from one host to
another, or they may be disseminated in scores of other ways. In other
chapters, particularly the one dealing with the house-fly and typhoid,
we shall see how it is that insects are often important factors in
spreading some of the most dreaded of the bacterial diseases.
THE PROTOZOA
The Protozoa, or one-celled animals, belonged to an unknown world before
the invention of the microscope. The first of these instruments enabled
the early observers to see some of the larger and more conspicuous
members of the group and each improvement of the microscope has enabled
us to see more and more of them and to study in detail not only the
structure but to follow the life-history of many of them.
_The Amoeba._ With some, as the common amoeba (Fig. 8), a minute
little form that is to be found in the slime at the bottom of almost any
body of water, the life-history is extremely simple. The organism itself
consists of a minute particle of protoplasm, a single cell with no
definite shape or body-wall and no specialized organs or apparatus for
carrying on the life-functions. It lives in the slime or ooze in fresh
or salt water, takes its food by simply flowing over the particle that
is to be ingested, grows to a certain limit of size, then divides into
two more or less equal parts, each part becoming a new animal that goes
on with its development as did the parent form. This process of growth
and division may go on for many generations, but cannot continue
indefinitely unless there is a conjugation of two separate individuals.
This process of conjugation is just the opposite to that of division.
Two amoeba flow together and become one. It seems to rejuvenate the
organism so that it is able to go on with its division and thus fulfil
its life-mission which is the same for these lowly animals as with the
higher, that of perpetuating the species.
_Classes of Protozoa._ The group or Phylum Protozoa is divided into four
smaller groups or classes. The amoeba belongs to the lowest of these,
the Rhizopoda. Rhizopoda means "root-footed," and the name is applied to
these animals because most of them move about by means of root-like
processes known as pseudopodia or "false feet." This is by far the
largest class and contains thousands of forms, mostly living in salt
water but there are many fresh-water species. They are non-parasitic,
but some of them by their presence in the body may cause such diseases
as dysentery, etc.
[Illustration: FIG. 7--Typhoid Fever bacilli. (After Muir and
Ritchie.)]
[Illustration: FIG. 8--_Amoeba_, showing the forms assumed by a single
individual in four successive changes. (From Kellogg's Elementary
Zoöl.)]
[Illustration: FIG. 9--_Euglina virdis._ (After Saville Kent.)]
[Illustration: FIG. 10--_Spirocheta duttoni_, × 4500. (After Breinl and
Carter.)]
The next class which may be known as the whip-bearers (_Mastigophora_)
includes those Protozoa that move by fine undulating processes called
flagella. One of the common representatives of this class is the little
green _Euglena_ (Fig. 9), whose presence in standing ponds and puddles
often imparts a greenish color to the water. Then in the salt water near
the surface there are often myriads of minute _Noctiluca_ whose
wonderfully phosphorescent little bodies glow like coals of fire when
the water is disturbed at night. Although this class contains fewer
forms than the preceding some of these have within recent years been
found to be of great importance because they live as parasites on man
and other animals. The trypanosome whose presence in the blood and
tissues of the patient causes that dreadful disease which ends in
sleeping sickness belongs here as well as do several other similar kinds
that produce serious troubles for various mammals and birds. The
Spirochæta, about which there has been so much recent discussion, also
belong here. These are simple spiral-like forms (Fig. 10), that are
sometimes classed with the simple plants, bacteria, but Nuttall and
others have shown very definitely that they should be classed with the
simplest animals, the Protozoans. These are the cause of relapsing
fevers in man and of several diseases of domestic animals. It is
believed by certain eminent zoölogists that when the germ that causes
yellow fever is discovered it will be found to belong to this group.
The members of the class Infusoria, so called because they were early
found to be abundant in various infusions, are characterized by numerous
fine cilia or hair-like organs by means of which the organism moves
about and procures its food. The well-known "slipper animalcule"
(_Paramoecium_) (Fig. 11), and the "bell-animalcule" (_Vorticella_)
(Fig. 12) are two common representatives. The _Paramoecia_ were the
animals mostly used by Jennings in his wonderfully interesting
experiments on the behavior of these lowly forms of life. He showed that
they always reacted in a certain definite way in response to particular
stimuli, and he was led to believe that he could see "what must be
considered the beginnings of intelligence and of many other qualities
found in the higher animals." A species of _Vorticella_ was probably the
first Protozoan that was ever observed. An old Dutch microscopist, Anton
von Leeuwenhoek, in 1675, while studying with lenses of his own
manufacture, discovered and described forms which undoubtedly belong to
this genus. Few if any of the Infusoria are pathogenic, although some
are said to be associated with certain intestinal diseases both in man
and the lower animals (Fig. 13).
[Illustration: FIG. 11--_Paramoecium._ (From Kellogg's Elementary
Zoöl.)]
[Illustration: FIG. 12--_Vorticella_, one individual with the stalk
coiled, the other with the stalk extended. (From Kellogg's Elementary
Zoöl.)]
[Illustration: FIG. 13--Pathogenic Protozoa; a group of intestinal
parasites. _A_, _B_, _Megastoma entericum_, _C_, _Balantidum entozoon_.
(After Calkins.)]
The last class, the Sporozoa, or the spore-forming animals, while small
in the number of known species, only about three hundred kinds being
known, is extremely important. A number of diseases in man and other
animals are due to the presence of these Sporozoans, for they are all
parasitic. Few if any animals are exempt from their attacks. They even
attack other minute Protozoa. One hundred and fifty-seven species have
been recorded as attacking insects, one hundred species attack birds,
fifty-two reptiles, eighty crustaceans, twenty-two fish, and so through
the list. Ten have been recorded as attacking man. In some instances the
parasite is always present in the host and some hosts may harbor several
different species of Sporozoa.
Very little work had been done on this group of parasites prior to 1900.
Since that time most of the species that we now know have been
discovered, and within the last few years the life-histories of many of
these have been worked out quite completely. No other group of animals
is being studied more to-day by both the physicians and biologists.
The Sporozoa vary greatly in appearance, organization and life-history.
They are so very plastic that they can adapt themselves readily to their
various hosts, hence we have a great variety of forms. But they all
agree in certain characters; all take their food and oxygen and carry on
excretory processes by osmosis, _i.e._, through the body-wall; all are
capable of some kind of locomotion, some have one or more flagella,
others move by a pseudopod movement. Some are capable of moving from
cell to cell in the body as do the white blood-corpuscles. They all
agree in the production of spores--hence the name.
At certain stages in their development the nucleus within the body of
the organism divides again and again until there are a great many
daughter nuclei, each accompanied by a small mass of protoplasm, often
inclosed in a little sac or cyst of its own. This is the process of
spore-formation and we see that from a single individual we may have by
division, not two animals as in the amoeba, but a score or more of
them. The little cysts or capsules that inclose them enable them to
resist without injury many vicissitudes that would otherwise destroy
them. They may dry up or freeze or lie for a long time in the ground or
water until the time comes when they are introduced into another host.
The class Sporozoa is divided into five small groups or orders. Nearly
all of these contain forms that are of more or less importance, but the
ones that live in the blood-cells (_Hæmosporidiida_) are of the most
interest to us because the parasites that cause the malarial fevers and
various other diseases belong here. These are dependent on two hosts for
their existence, the sexual generation usually occuring in an insect or
other invertebrate and the asexual generation in some vertebrate.