292 BACTERIAL VARIATION 



or avirulent ; so miglit asporogenous strains. In Bordet and Renaux's experi- 

 ments the actively sporogenous strains obtained by cultivation on oxalated media 

 were highly virulent, while the asporogenous strains obtained by subculturing from 

 the daughter colonies on media with a high calcium content were completely 

 avirulent. It would seem, then, that loss of ability to form spores is sometimes, 

 but not always, associated with loss of virulence. The most reasonable hypothesis 

 would seem to be, either that asporogenous strains may be produced by different 

 genetic mechanisms, or that the presence in a medium of excess of calcium stimulates 

 some genetic change in addition to that on which the loss of spore-bearing capacity 

 depends. We should not, it may be noted, on the basis of these divergent results, 

 be justified in regarding this loss and the loss of virulence as correlated variations 

 in the usually accepted sense. The use of precisely defined media will in all proba- 

 bility throw light on a number of morphological variations, though there are 

 at present only a few isolated observations on this point. For example, in an 

 amino-acid glucose salt medium, and sub-optimal amounts of an unidentified 

 growth factor present in liver extract, gonococcal strains produced large, distorted, 

 swollen, " vacuolated," and dumb-bell forms ; in the presence of ample growth 

 factor, the cells grew in characteristic diplococcal forms (Lankford, Scott, Cox 

 and Cooke 1943). Badger (1944) records an interesting variation in a Type III 

 pneumococcus for which choline was an essential nutrient. The choline could 

 be replaced by ethanolamine, and in the presence of the latter, the pneumococcus 

 grew in characteristic long chains. Again, Pappenheimer and Shaskan (1944) 

 found that CI. welchii grew as regular rods in a medium containing enough iron 

 to ensure maximum growth, toxin production and breakdown of carbohydrate. 

 With a reduction of iron content only, there was a depression of these activities, 

 and the Clostridia grew as curved, elongated and entirely atypical bacilli. 



There are many other ways in which bacterial variants may depart from the 

 normal morphological type of the species to which they belong. A capsulated 

 organism, such as the pneumococcus, may, for instance, give rise to a non-capsiilated 

 variant. This particular variation is, however, associated with other important 

 changes in behaviour, and it will be more convenient to consider it in the section 

 dealing with antigenic variation. 

 Variations in Biochemical Reactions. 



SLuce the earliest days of bacteriology, differences in fermentation reactions have 

 been extensively utilized in differentiating bacterial species or types that belong 

 to the same genus or group, as judged by morphological or other criteria. To be 

 of use from this point of view the fermentation reactions of any given species, or 

 type, must, of course, be constant. This has been found to be the case, to the 

 extent that it is usually possible to select empirically a number of substrates that a 

 given organism, in its normal form, will consistently alter or leave unaltered. It 

 has, however, been found that for any given bacterial species there are other sub- 

 strates that are sometimes fermented, sometimes not ; from the systematic point 

 of view we should say that those particular substrates, in relation to that particular 

 organism, had little differential value. Similarly, some bacterial species vary more 

 than others in their fermentative abilities ; and the differential value of fermentation 

 tests therefore varies from one bacterial group to another. 



In trying to assess the significance of the numberless recorded instances of 

 variants that differ in their fermentation reactions from the parent strain from 

 which they were derived, we must keep these facts constantly in mind. We may 



