The refraction index of the bacterial cell is so slight that the microscopic study is facilitated or made possible by staining them with various aniline dyes.

Owing to differences in the cell material the different species of bacteria show differences in the facility with which they take the color and the tenacity with which they retain it, and this also forms a means of species differentiation.
The interrelation of science is well shown in this, for it was the discovery of the aniline dyes in the latter half of the nineteenth century which made the fruitful study of bacteria possible.
From the simplicity of structure it is not improbable that the bacteria are among the oldest forms of life, and all life has become adapted to their presence.

They are of universal distribution; they play such an important part in the inter-relations of living things that it is probable life could not continue without them, at least not in the present way.

They form important food for other unicellular organisms which are important links in the chain; they are the agents of decomposition, by which the complex substances of living things are reduced to elementary substances and made available for use; without them plant life would be impossible, for it is by their instrumentality that material in the soil is so changed as to be available as plant food; by their action many of the important foods of man, often those especially delectable, are produced; they are constantly with us on all the surfaces of the body; masses live on the intestinal surfaces and the excrement is largely composed of bacteria.
It has been said that life would be impossible without bacteria, for the accumulation of the carcasses of all animals which have died would so encumber the earth as to prevent its use; but the folly of such speculation is shown by the fact that animals would not have been there without bacteria.

It has been shown, however, that the presence of bacteria in the intestine of the higher animals is not essential for life.