The Story of Germ Life | Page 6

food and producing chemical changes therein, together with
their marvellous power of assimilating this material as food, make them
agents in Nature of extreme importance.
DIFFERENCES BETWEEN DIFFERENT SPECIES OF BACTERIA.
While bacteria are thus very simple in form, there are a few other slight
variations in detail which assist in distinguishing them. The rods are
sometimes very blunt at the ends, almost as if cut square across, while
in other species they are more rounded and occasionally slightly
tapering. Sometimes they are surrounded by a thin layer of some
gelatinous substance, which forms what is called a capsule (Fig. 10).

This capsule may connect them and serve as a cement, to prevent the
separate elements of a chain from falling apart.
Sometimes such a gelatinous secretion will unite great masses of
bacteria into clusters, which may float on the surface of the liquid in
which they grow or may sink to the bottom. Such masses are called
zoogloea, and their general appearance serves as one of the characters
for distinguishing different species of bacteria (Fig. 10, a and b). When
growing in solid media, such as a nutritious liquid made stiff with
gelatine, the different species have different methods of spreading from
their central point of origin. A single bacterium in the midst of such a
stiffened mass will feed upon it and produce descendants rapidly; but
these descendants, not being able to move through the gelatine, will
remain clustered together in a mass, which the bacteriologist calls a
colony. But their method of clustering, due to different methods of
growth, is by no means always alike, and these colonies show great
differences in general appearance. The differences appear to be
constant, however, for the same species of bacteria, and hence the
shape and appearance of the colony enable bacteriologists to discern
different species (Fig. II). All these points of difference are of practical
use to the bacteriologist in distinguishing species.
SPORE FORMATION.
In addition to their power of reproduction by simple division, many
species of bacteria have a second method by means of spores. Spores
are special rounded or oval bits of bacteria protoplasm capable of
resisting adverse conditions which would destroy the ordinary bacteria.
They arise among bacteria in two different methods.
Endogenous spores.--These spores arise inside of the rods or the spiral
forms (Fig. 12). They first appear as slight granular masses, or as dark
points which become gradually distinct from the rest of the rod.
Eventually there is thus formed inside the rod a clear, highly refractive,
spherical or oval spore, which may even be of a greater diameter than
the rod producing it, thus causing it to swell out and become spindle
formed [Fig. 12 c]. These spores may form in the middle or at the ends
of the rods (Fig. 12). They may use up all the protoplasm of the rod in

their formation, or they may use only a small part of it, the rod which
forms them continuing its activities in spite of the formation of the
spores within it. They are always clear and highly refractive from
containing little water, and they do not so readily absorb staining
material as the ordinary rods. They appear to be covered with a layer of
some substance which resists the stain, and which also enables them to
resist various external agencies. This protective covering, together with
their small amount of water, enables them to resist almost any amount
of drying, a high degree of heat, and many other adverse conditions.
Commonly the spores break out of the rod, and the rod producing them
dies, although sometimes the rod may continue its activity even after
the spores have been produced.
Arthrogenous spores (?).--Certain species of bacteria do not produce
spores as just described, but may give rise to bodies that are sometimes
called arthrospores. These bodies are formed as short segments of rods.
A long rod may sometimes break up into several short rounded
elements, which are clear and appear to have a somewhat increased
power of resisting adverse conditions. The same may happen among
the spherical forms, which only in rare instances form endogenous
spores. Among the spheres which form a chain of streptococci some
may occasionally be slightly different from the rest. They are a little
larger, and have been thought to have an increased resisting power like
that of true spores (Fig. 13 b). It is quite doubtful, however, whether it
is proper to regard these bodies as spores. There is no good evidence
that they have any special resisting power to heat like endogenous
spores, and bacteriologists in general are inclined to regard them
simply as resting cells. The term arthrospores has been given to them to
indicate that they are formed as joints or segments, and this term may
be a