4 THE BIOLOGY OF BACTERIA 



microscope and bacteriological method and technique have played a 

 large share in this development. The sterilisation of culture fluids 

 by heat, the use of aniline dyes as staining agents, the introduction 

 of solid culture media (such as gelatine and agar), and Koch's 

 " plate " method, have all contributed not a little to the enormous 

 advance of bacteriology. 



The Place of Bacteria in Nature 



As we have seen, for a considerable period of time after their first 

 detection these unicellular organisms were considered to be members 

 of the animal kingdom. As late as 1838, when Ehrenberg and 

 Dujardin drew up their classification, bacteria were placed among the 

 Infusorians. This was in part due to the powers of motion which 

 these observers detected in bacteria. It is now, of course, recognised 

 that animals have no monopoly of motion. But what, after all, 

 are the differences between animals and vegetables so low down in 

 the scale of life ? Chiefly two : there is a difference in life-history 

 (in structure and development), and there is a difference in pabulum. 

 A plant secures its nourishment from much simpler elements than is 

 the case with animals ; for example, it obtains its carbon from the 

 carbonic acid gas in air and water. This it is able to do, as regards 

 the carbon, by means of the green colouring matter known as chloro- 

 phyll, by the aid of which, with sunlight, carbonic acid is decomposed 

 in the chlorophyll corpuscles, the oxygen passing back into the 

 atmosphere, the carbon being stored in the plant in the form of 

 starch or other organic compound. The supply of carbon in the 

 chlorophyll-free plants, amongst which are the bacteria, is obtained 

 by breaking up different forms of carbohydrates. Beside albumen 

 and peptone, they use sugar and similar carbohydrates and glycerine 

 as a source of carbon. Many of them also have the capacity of using 

 organic matters of complex constitution by converting such into 

 water, carbonic acid gas, and ammonia. Their hydrogen comes from 

 water, their nitrogen from the soil, chiefly in the form of nitrates. 

 From the soil, too, they obtain other necessary salts. Now all these 

 substances are in elementary conditions, and as such plants can 

 absorb them. Animals, on the other hand, are only able to utilise 

 compound food products which have been, so to speak, prepared for 

 them, for example albuminoids and proteids. They cannot directly 

 feed upon the elementary substances forming the diet of vegetables. 

 This distinction, however, did not at once clear up the difficult 

 matter of the classification of bacteria. It is true, they possess powers 

 of motion, are free from chlorophyll, and even feed occasionally upon 

 products of decomposition three physiological characters which 



