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Page 30
Certain species of bacteria are a�robic, that is, they need free
oxygen for their growth; others are ana�robic and will not grow in the
presence of oxygen. Most of the bacteria which produce disease are
facultative, that is, they grow either with or without oxygen; but
certain of them, as the bacillus of tetanus, are ana�robic. There is,
of course, abundance of oxygen in the blood and tissues, but it is so
combined as to be unavailable for the bacteria. Bacteria may further
be divided into those which are saprophytic or which find favorable
conditions for life outside of the body, and the parasitic. Many are
exclusively parasitic or saprophytic, and many are facultative, both
conditions of living being possible. It has been found possible by
varying in many ways the character of the culture medium and
temperature to grow under artificial conditions outside of the body
most, if not all, of the bacteria which cause disease. Thus, such
bacteria as tubercle bacilli and the influenza bacillus can be
cultivated, but they certainly would not find natural conditions which
would make saprophytic growth possible.
Bacteria may be very sensitive to the presence of certain substances
in the fluid in which they are growing. Growth may be inhibited by the
smallest trace of some of the metallic salts, as corrosive sublimate,
although the bacteria themselves are not destroyed. If small pieces of
gold foil be placed on the surface of prepared jelly on which bacteria
have been planted, no growth will take place in the vicinity of the
gold foil.
Variations can easily be produced in bacteria, but they do not tend to
become established. In certain of the bacterial species there are
strains which represent slight variations from the type but which are
not sufficient to constitute new species. If the environment in which
bacteria are living be unusual and to a greater or less degree
unfavorable, those individuals in the mass with the least power of
adaptibility will perish, those more resistant and with greater
adaptability will survive and propagate; and the peculiarity being
transmitted a new strain will arise characterized by this
adaptability. Bacteria with slight adaptability to the environment of
the tissues and fluids of the animal body can, by repeated
inoculations, become so adapted to the new environment as to be in a
high degree pathogenic. In such a process the organisms with the least
power of adaptation are destroyed and new generations are formed from
those of greater power of adaptation. When bacteria are caused to grow
in a new environment they may acquire new characteristics. The anthrax
bacilli find the optimum conditions for growth at the temperature of
the animal body, but they will grow at temperatures both above and
below this. Pasteur found that by gradually increasing the temperature
they could be grown at one hundred and ten degrees. When grown at this
temperature they were no longer so virulent and produced in animals a
mild non-fatal form of anthrax which protected the animal when
inoculated with the virulent strain. The well known variations in the
character of disease, shown in differences in severity and ease of
transmission, seen in different years and in different epidemics, may
be due to many conditions, but probably variation in the infecting
organisms is the most important.
The protozoa, like the bacteria, are unicellular organisms and contain
a nucleus as do all cells. They vary in size from forms seen with
difficulty under the highest power of the microscope to forms readily
seen with the unaided eye. Their structure in general is more complex
than is the structure of bacteria, and many show extreme
differentiation of parts of the single cells, as a firm exterior
surface or cuticle, an internal skeleton, organs of locomotion, mouth
and digestive organs and organs of excretion. They are more widely
distributed than are the bacteria, and found from pole to pole in all
oceans and in all fresh water. There are many modes of multiplication,
and these are often extremely complicated. The most general mode and
one which is common to all is by simple division; a modification of
this is by budding in which projections or buds form on the body and
after separation become new organisms. In other cases spores form
within the cell which become free and develop further into complete
organisms. These simple modes of multiplication often alternate in the
same organism with sexual differentiation and conjugation. There is
never a permanent sexual differentiation, but the sexual forms develop
from a simple and non-sexual organism. Usually the sexual forms
develop only in a special environment; thus the protozoon which in man
is the cause of malaria, multiplies in the human blood by simple
division, but in the body of the mosquito multiplication by sexual
differentiation takes place. Under no conditions is multiplication so
rapid as with the bacteria, and in general the simpler the form of
organism the more rapid is the multiplication. It is common to all of
the protozoa to develop forms which have great powers of resistance,
this being due in some cases to encystment, in which condition a
resistant membrane is formed on the outside, in others to the
production of spores. A fluid environment is essential to the life of
the protozoa, but the resistant forms can endure long periods of
dryness or other unfavorable environmental conditions. The universal
distribution of the protozoa is due to this; the spores or cysts can
be carried long distances by the wind and develop into active forms
when they reach an environment which is favorable. Their distribution
in water depends upon the amount of organic material this contains. In
pure drinking water there may be very few, but in stagnant water they
are very numerous, living not on the organic material in solution in
this, but on the bacteria which find in such fluid favorable
conditions for existence. The food of protozoa consists chiefly of
other organisms, particularly bacteria, and they are classed with the
animals. The protozoa are the most widely distributed and the most
universal of the parasites. The infectious diseases which they produce
in man, although among the most serious are less in number than those
produced by bacteria. So marked is the tendency to parasitism that
they are often parasitic for each other, smaller forms entering into
and living upon the larger. Variation does not seem to be so marked in
the protozoa as in the bacteria, though this is possibly due to our
greater ignorance of them as a class. We are not able, except in rare
instances, to grow them in pure culture, and study innumerable
generations under changes in the environment, as the bacteria have
been studied.
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