CHAPTER 9 



BACTERIAL VARIATION 



In the earlier bacteriological writings, from the days of Pasteur and Koch onwards, 

 there will be found scattered references to certain bacterial strains that have 

 deviated, in one way or another, from the modal form of the particular species 

 concerned. In the earliest days of all, when the doctrine of spontaneous generation 

 was dying, but not yet dead, it was, indeed, the fixity rather than the variability 

 of bacterial species that was in dispute. It was not, however, until the beginning 

 of the present century that any serious attempt was made to study bacterial varia- 

 tion as a problem sui generis, or to apply to bacteria the concepts that had proved so 

 fruitful in the study of the higher plants and animals. During the first decade or 

 so of the present century these attempts were sporadic ; but during the last twenty- 

 five years an immense impetus has been given to this line of inquiry by a series 

 of converging studies, and the relevant literature has expanded from a trickle 

 to a flood. 



The significance of many of the observations that have been recorded is at the 

 moment exceedingly difficult to assess. Some of them serve to illustrate the wide 

 range of variation that may occur within a single bacterial species, but tell us 

 little or nothing in regard to the relative frequency of the different variants described, 

 or the factors on which the variation depends. Others are concerned, at least in 

 part, with problems that have been described in Chapter 2 — the existence of a 

 complex bacterial life-cycle, of filtrable forms of bacteria, of some form of sexual 

 reproduction and so on. In the present chapter we shall discuss the general problem 

 of bacterial variation, as illustrated by a series of observations the selection of 

 which must of necessity be to some extent arbitrary. A more detailed account of 

 certain variants will be found in the systematic descriptions of the different genera 

 and species given in later chapters. 

 Terminology. 



The application to bacteria of terms that have been coined to express changes 

 in form or function occurring in higher plants or animals is not without its dangers ; 

 and it is possible that there is little real justification for the use of such a term 

 as mutation, in connection with the variations which bacteria may undergo. Some 

 biologists would attach two implications to the use of this term : the suddenness 

 of the change — the variation per saltxim — and the permanency of the change, once 

 it has occurred. Dobell (1913) regards as a mutation any permanent change, 

 which is transmitted to subsequent generations of bacteria, without any implica- 

 tion in regard to the suddenness or gradualness of the change, or the manner 

 of its acquisition. We must, at all events, remember that most of our conceptions 

 with regard to variation and heredity have been built up on data derived from 

 observations and experiments on living things which pass through a sexual cycle ; 



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