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Page 35
Bacteria cause disease by producing substances called toxines which
are poisonous to the cells, and of which two sorts are distinguished.
One form of toxines is produced by the bacteria as a sort of
secretion, and is formed both in the body and when the bacteria are
growing in cultures. Substances of this character, many of them highly
poisonous, are produced both by animals and plants. They may serve the
purpose both of offence and defence, as in the case of the snake
venom, and in other cases they seem to benefit their producers in no
way whatever, and may even be injurious to them. After the different
cereals have been grown for succeeding years in the same place, growth
finally diminishes not from the exhaustion of the soil, but from the
accumulation in it of substances produced by the plants. Beneath
certain trees, as the Norway maple, grass will not grow, and it has
been shown that the tree produces substances which inhibit the growth
of grass. When bacteria are grown in a culture flask, growth ceases
long before the nutritive material has been consumed, from the
accumulation of waste products in the fluid. The other class of toxic
substances, called endotoxines, are not secretion products, but are
contained in the bacterial substance and become active by the
destruction and disintegration of the bacteria. They can be
artificially produced by grinding up masses of bacteria, and in the
body the destruction and solution of bacteria which is constantly
taking place sets them free. The toxines and the endotoxines are of an
albuminous nature, and act only when they come in contact with the
living cells within the body. When taken into the alimentary canal
they are either not absorbed or so changed by the digestive fluids as
to be innocuous. Many of the ordinary food substances, even a material
apparently so simple as the white of an egg, are highly injurious if
they reach the tissues in an unchanged form.
By means of these substances the bacteria produce such changes in
their environment within the body that this becomes adapted to their
parasitic existence. In symbiosis the bacteria probably undergo
changes by which they become adapted to the environment, and in
parasitism the environment becomes adapted to them. In the same way
man can change his immediate environment by means of clothing,
artificial heating, etc., and adapt it to his needs; or by hardening
his body he can adapt it to the environment. The pathogenic bacterium
finds the living tissue hostile, its cells devour him, the tissue
fluids destroy him, and by means of the toxines he changes the
environment from that of living to dead tissue, or in other ways so
alters it that it is no longer hostile. The parasite has also means of
passive defence comparable to the armor of the warrior in the past. It
may form a protective mantle called a capsule around itself, which
serves to protect it from the action of the body fluids. Such capsule
formation is a very common thing in the pathogenic organisms, and they
are found only when these are growing in the body and do not appear in
cultures (Fig. 17-c).
It is evident that just as the parasite has his weapons of offence and
defence so has the host, otherwise there would be no recovery from
infectious diseases. Although many of the infectious diseases have a
high mortality, which in rare instances reaches one hundred per cent,
the majority do recover. In certain cases the recovery is attended by
immunity, the individual being protected to a greater or less degree
from a recurrence of the same disease. The immunity is never absolute;
it may last for a number of years only, and usually, if the disease be
again acquired, the second attack is milder than the primary. Probably
the most enduring immunity is in smallpox, although cases are known of
two and even three attacks; the immunity is high in scarlet fever,
measles, mumps and typhoid fever. The immunity from diphtheria is
short, and in pneumonia, although there must be a temporary immunity,
future susceptibility to the disease is probably increased. In certain
cases the immunity is only local; the focus of disease heals because
the tissue there has evolved means of protection from the parasite,
but if any other part of the body be infected, the disease pursues the
usual course. A boil, for example, is frequently followed by the
appearance of similar boils in the vicinity due to the infection of
the skin by the micrococci from the first boil, which by dressings,
etc., have become spread over the surface.
The natural methods of defence of the host against the parasites have
formed the main subject in the study of the infectious diseases for
the last twenty years. Speculation in this territory has been rife and
most of it fruitless, but by patient study of disease in man and by
animal experimentation there has been gradually evolved a sum of
knowledge which has been applied in many cases to the treatment of
infectious diseases with immense benefit. Research was naturally
turned to this subject, for it was evident that the processes by which
the protection of the body was brought about must be known before
there could be a really rational method of treatment directed towards
the artificial induction of such processes, or hastening and
strengthening those which were taking place. Previous to knowledge of
the bacteria, their mode of life, their methods of infection and
knowledge of the defences of the body, most of the methods of
prevention and treatment of the infectious diseases was based largely
on conjecture, the one brilliant exception being the discovery of
vaccination by Jenner in 1798.
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