VARIABILITY AMONG BACTERIA 23 



duction being observed. That a purely intracellular digestion 

 may take place is illustrated by what has been shown to occur 

 in the case of the micrococcus ureae, which from urea forms 

 ammonium carbonate by adding water to the urea molecule. 

 Here, if after the action has commenced, the bacteria are filtered 

 off, no further production of ammonium carbonate takes place, 

 which shows that no ferment has been dissolved out into the 

 urine. If now the bodies of the bacteria be extracted with 

 absolute alcohol or ether, which of course destroy their vitality, 

 a substance is obtained of the nature of a ferment, which, when 

 added to sterile urine, rapidly causes the production of ammonium 

 carbonate. This ferment has evidently been contained within 

 the bacterial cells. 



In considering the effects of bacteria in nature it must be recognised 

 that some species are capable of building up complex substances out of 

 simple chemical compounds. Examples of these are found in the 

 bacteria which in the soil make nitrogen more available for plant nutri- 

 tion by converting ammonia into nitrites and nitrates. Winogradski, by 

 using media containing non-nitrogenous salts of magnesium, potassium, 

 and ammonium, and free of organic matter, has demonstrated the 

 existence of forms which convert, by oxidation, ammonia into nitrites, 

 and of other forms which convert these nitrites into nitrates. Both can 

 derive their necessary carbon from alkaline carbonates. Other bacteria, 

 or organisms allied to the bacteria, exist which can actually take up and 

 combine into new compounds the free nitrogen of the air. These are 

 found in the tubercles which develop on the rootlets of the leguminosae. 

 Without such organisms the tubercles do not develop, and without the 

 development of the tubercles the plants are poor and stunted. Bacteria 

 thus play an important part in the enrichment and fertilisation of the 

 soil. 



The Occurrence of Variability among Bacteria. The question of the 

 division of the group of bacteria into definite species has given rise to 

 much discussion among vegetable and animal morphologists, and at 

 one time very divergent views were held. Some even thought that the 

 same species might at one time give rise to one disease, at another 

 time to another. There is, however, now practical unanimity that 

 bacteria show as distinct species as the other lower plants and animals, 

 though, of course, the difficulty of defining the concept of a species is as 

 great in them as it is in the latter. Still, we can say that among the 

 bacteria we see exhibited (to use the words of De Bary) "the same 

 periodically repeated course of development within certain empirically 

 determined limits of variation " which justifies, among higher forms of 

 life, a species to be recognised. What at first raised doubts as to the 

 occurrence of species among the bacteria was the observation in certain 

 cases of what is known as pleomorphism. By this is meant that one 

 species may assume at different times different forms, e.g. appear as a 

 coccus, a bacillus, or a leptothrix. Undoubtedly, many of the cases 

 where this was alleged to have been observed occurred before the 

 elaboration of the modern technique for the obtaining of pure cultures, 

 but at the present day there are cases where evidence appears to exist 



