This wonderful power of growth is chiefly due to the fact that bacteria feed upon food which is highly organized and already in condition for absorption.  Most plants must manufacture their own foods out of simpler substances, like carbonic dioxide (Co2) and water, but bacteria, as a rule, feed upon complex organic material already prepared by the previous life of plants or animals.  For this reason they can grow faster than other plants.  Not being obliged to make their own foods like most plants, nor to search for it like animals, but living in its midst, their rapidity of growth and multiplication is limited only by their power to seize and assimilate this food.  As they grow in such masses of food, they cause certain chemical changes to take place in it, changes doubtless directly connected with their use of the material as food.  Recognising that they do cause chemical changes in food material, and remembering this marvellous power of growth, we are prepared to believe them capable of producing changes wherever they get a foothold and begin to grow.  Their power of feeding upon complex organic 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.