From the brilliant overthrow by Pasteur, in 1861, and by Koch and Cohn,
in 1876, of the theory of spontaneous generation, we may date its
modern growth. Wrapped up in this theory of spontaneous generation,
upon which speculation raged centuries before the invention of the
microscope, lies the history of bacteriology.
The ancient Greek and Roman philosophers wrestled with the question,
and very interesting views of the relation of germ life to disease
are preserved to us in their manuscripts. With the invention of the
microscope we can mark the first positive step towards the goal of
to-day. A Jesuit priest, Kircher, in 1671, was the first to investigate
putrefying meat, milk, and cheese with the crude microscope of his
day, and left us indefinite remarks concerning “very minute living
worms” found therein. Four years after Kircher a Dutch linen merchant,
Antonius von Leeuwenhoek, by improving the lenses of the microscope,
saw in rain-water, putrefying fluids, intestinal contents, and saliva,
minute, moving, living particles, which he called “animalculæ.” In
medical circles of his day these observations aroused the keenest
interest, and the theory that these “animalculæ” might be the cause
of all disease was eagerly discussed. Pleincz, of Vienna, after much
observation of various fluids, putrefying and otherwise, wrote in
1762 that it was his firm belief that the phenomena of diseases and
the decomposition of animal fluids were wholly caused by these minute
living things.
Notwithstanding such assertions, from his day on until Pasteur, Koch,
and Cohn finally proved its misconceptions in 1876, the theory of
spontaneous generation held the upper hand in all discussions upon the
question.
The stimulus to research as to the causes of disease along the line
of bacterial origin did not entirely cease to be felt, and the names
of Pollender and Davaine are linked together in the first undoubted
discovery of micro-organisms in disease, when the cause of anthrax, a
disease of cattle, was solved in 1863. Following closely upon Davaine’s
researches, the primary causes of wound infection were worked out, and
to the efforts of the British surgeon Lister are due the great advances
of modern surgery.
In rapid succession the presence of bacteria was clearly demonstrated
in relapsing fever, leprosy, and typhoid fever; but far eclipsing all
former discoveries, on account of the magnitude of the difficulties
encountered and overcome, were the brilliant demonstrations of the
cause of consumption and allied diseases, and that of Asiatic cholera,
by Dr. Robert Koch in 1882 and in 1884 respectively.
From that time onward innumerable workers have satisfied the critical
scientific world as to the causes of pneumonia, diphtheria, tetanus,
influenza, and bubonic plague, besides many diseases of cattle, horses,
sheep, and other animals and insects.
Having glanced hastily at the history of bacteriology, we may next
consider some facts concerning the germs themselves. What are they? To
the lay mind the words germ, microbe, bacterium, and bacillus often
convey confused ideas of invisible, wriggling, worm-like creatures,
enemies of mankind, ever on the watch to gain a stealthy entrance
into our bodies, where they wreak harm and death. Scientifically
considered, however, they are the smallest of living things yet known.
They are not animals, but are members of the vegetable kingdom, and are
possessed of definite yet varying shapes. They consist of a jelly-like
substance called protoplasm, which is covered in and held in place by a
well-formed membrane of a relatively hard and dense character, exactly
similar in composition to the woody fibre of trees.
According to their shape the bacteria are divided into three chief
groups, called respectively cocci, bacilli, and spirilla. The cocci
are spherical bodies and may exist singly or in pairs, in fours, in
clusters, or in chains. In this group we find the smallest bacteria
known, many of them not over 1-150,000 of an inch in diameter. The
bacilli are rod-like bodies, varying much in size in different species
and in members of the same species. They are larger than the cocci,
measuring in length from 1-25,000 of an inch to 1-4000, and in breadth
from 1-125,000 to 1-16,000 of an inch. Many varieties are possessed of
organs of locomotion called flagella.
The spirilla resemble the bacilli, except that they are twisted into
corkscrew shapes, or have gently undulating outlines. Upon an average
they are much longer than the bacilli, one species being very long,
measuring about 1-600 of an inch. As seen in the natural state bacteria
are found to be colorless, but it is by the application of various
aniline dyes that they are usually studied. These minute plants
increase by a simple method of division into two equal parts, or by
a more complex process of forming a seed—the so-called spore—which
later on develops into the adult form. Under favorable conditions they
are able to multiply at an enormous rate; for instance, it has been
calculated that a bacillus dividing once every hour would at the end
of twenty-four hours have increased to seventeen millions; and if the
division continued at the same rate we should find at the end of the
third day an incalculable number of billions, whose weight would be
nearly seven thousand five hundred tons!
But, fortunately for our welfare, nature by various means renders the
possibility of such a happening entirely beyond the slightest chance of
realization, her greatest barrier being the lack of an adequate food
supply.
The distribution in nature of bacteria is wellnigh universal,
occurring as they do in the air we breathe, the water and milk we
drink, upon the exposed surfaces of man and animals, and in their
intestinal tracts, and in the soil to a depth of about nine feet. But
it has been noted that at very high altitudes and in glacier ice none
exist, while in the Arctic regions and at sea far from land their
numbers are very few.
The conditions governing their growth involve many complex problems,
but a few of the chief factors concerned are moisture, air, food,
temperature, and light. All bacteria must have moisture, else they
die sooner or later, depending upon the hardness of the species, and
none can multiply without it. A supply of air is by no means essential
to all germs. To some it is absolutely necessary, and such germs
are called aerobes. To others air is wholly detrimental, and they
constitute the anaerobes, while to the majority of bacteria air supply
is a matter of indifference, and in consequence they are grouped under
the term facultative anaerobes.
The food supply of many consists of dead animal and vegetable
materials, a few require living tissues, while a small number can exist
wholly upon mineral salts, or even the nitrogen of the air. The lowest
temperature at which some bacteria can multiply is the freezing-point
of water, and the highest 170 degrees Fahrenheit. However, the average
range of temperature suitable to the majority lies between 60 and 104
degrees Fahrenheit, 98 2-5 degrees Fahrenheit being the most suitable
for the growth of disease-producing germs. Light, ordinarily diffused
daylight, or its absence, is a matter of no moment to most germs,
whereas direct sunlight is a destroyer of all bacteria.
The study of the life histories of these diminutive plants excites the
wonder of those who make observations upon them. It is truly marvellous
to know that these bacteria can accomplish in their short lives of
possibly a few hours or days feats which would baffle the cleverest of
chemists if given years of a lifetime to work upon. They give to the
farmer the good quality of his crops, to the dairyman superior butter
and cheese; they assist in large measure in freeing our rivers and
lakes from harmful pollutions. Here it should be strongly emphasized
that those bacteria which cause disease are only of a few species, all
others contributing to our welfare in countless ways.
Quite as astonishing is the discovery that within the root-knobs of
pease and beans live bacteria which by splitting up mineral salts
containing nitrogen, and by absorbing nitrogen from the air, give it
over to the plant so that it is enabled to grow luxuriantly, whereas,
without their presence, the tiller of the soil might fertilize the
ground in vain. It is quite possible that not alone pease and beans,
but all grasses and plants and trees depend upon the presence of such
germs for their very existence, which in turn supply man and animals
with their means of existence. Hence we see that these nitrifying
bacteria, as they are called, if swept out of existence, would be the
cause of cessation of all life upon the globe. And arguing backward,
one prominent authority states it as his belief that the first of all
life on this earth were those lowly forms of plants which only required
the nitrogen of air or salts to enable them to multiply.
Limiting observation now to the sphere of medicine, it will be readily
perceived that the presence of bacterial life in a causative relation
to disease is an object of paramount regard. The following paragraphs
will briefly treat of the diseases associated with micro-organisms and
the common modes of infection in each, the chain of events subsequent
to an infection, and the possibilities of protection or cure by means
of substances elaborated in the body of an individual or animal
recently recovered from an infectious disease:
_Anthrax._—A disease chiefly of cattle and sheep, occasionally of
man, is caused by the _Bacillus anthracis_, discovered in 1849–50 by
Pollender and Davaine. It enters the body through abrasions of the
skin, by inhalation of the spores, or seeds, into the lungs, or by
swallowing infected material.
_Leprosy._—This disease is caused by a bacillus known as _Bacillus
leprae_, which was discovered by Hansen in 1879. It is doubtful if it
has been grown outside the body. It is supposed to enter by abrasions
of the skin, but it is very feebly contagious, notwithstanding popular
ideas as to its supposedly highly contagious nature.
_Tuberculosis._—All forms of this disease, among which is ordinary
consumption, are caused by a bacillus closely resembling that of
leprosy. It was discovered by Koch in 1880–82, and named _Bacillus
tuberculosis_. The ways of infection are by inhaling the dried sputum
of consumptives, drinking infected cow’s milk, or eating infected meat.
_Typhoid Fever._—A disease of human beings only. Eberth in 1880
discovered the germ causing it and called it _Bacillus typhosus_. It
gains entrance to our bodies chiefly in the milk and water we drink,
which comes from infected sources; a rarer method is by inhalation of
infected air.
_Diphtheria._—A disease of human beings chiefly. It is caused by a
bacillus which was described in 1883–84 by Klebs and Loeffler, and is
known as _Bacillus diphtheriae_, or Klebs-Loeffler bacillus. Its mode
of entry is by inhaling infected air, or by drinking or eating infected
milk or food.
_Cholera._—This disease is peculiar to human beings. Its native home
is on the banks of the river Ganges in India, where Koch in 1884 was
able to isolate its causative spirillum. Man is infected by drinking
contaminated water or by contact.
_Lockjaw, or Tetanus._—Afflicts man, horses, and dogs. The _Bacillus
tetani_ is the most deadly of all known bacteria. It enters the body by
wounds. It was discovered in 1884 by Nicolaier.
_Influenza, or the Grip._—Caused by one of the smallest-known bacilli;
discovered in 1892 by Canon and Pfeiffer. Infection spreads by the
scattering about by air-currents of the dried nasal and bronchial
secretion of those suffering from the disease, and its portal of entry
is by the nose and bronchial tubes.
_Pneumonia._—Caused by a coccus which grows in pairs and small chains.
It enters the body by means of the respiratory tract. It is present in
the saliva of twenty per cent. of healthy persons. Proved by Frankel in
1886 to be the cause of this disease.
_Bubonic Plague._—In 1894 Kitasato and Yersin isolated a small
bacillus in a large number of cases and proved it to be the cause. It
enters the body by means of wounds of the skin, and through bites of
fleas from infected rats, which are said to be one of the chief factors
in spreading this dread malady.
_Yellow Fever._—The cause of this disease is still under discussion.
Such are a few of the infectious diseases which we can readily
attribute to the presence of definite micro-organisms in respective
cases. But strange as it may seem, the most typical of all infectious
diseases, small-pox, scarlet fever, measles, and hydrophobia, have as
yet not yielded up their secrets. This is possibly due to the minute
size of the micro-organisms concerned, which make it beyond the power
of the best microscope to demonstrate them. In this connection it has
recently been shown by Roux and Nocard that in the case of the disease
known as pleuro-pneumonia of cattle the causative agent is so very
small as just to be barely visible. Again, it is quite possible that
these diseases may be caused by living things we know nothing about,
which may be quite dissimilar from the bacteria.