200 BACTERIOLOGY 



finer differences notably, those that took into considera- 

 tion the zymogenic powers of bacteria it was soon possible 

 to speak of groups, strains, or types among the species, one 

 strain or group differing from another in its ability to ferment 

 certain carbohydrates, with recognizable end-products, while 

 other strains were devoid of this power, though in all other 

 particulars the two strains may have been identical. By 

 the application of such tests many species have been sepa- 

 rated into groups and some groups into sub-groups; some 

 fermenting all sugars, others fermenting only particular 

 sugars. Some fermenting with free gases as an end-product 

 others with no gas but only acids as end-products. These 

 functions are subject to quantitative variations and in a 

 few cases they may temporarily disappear, and occasionally 

 by experimental methods, may be made to disappear, but 

 as yet it has not been possible by any known method to 

 endow a species, by nature devoid of the power to ferment 

 sugar, with such power. 



The ability of a pathogenic species to cause in animals 

 pathological lesion identical to those from which the species 

 was obtained was held for a long time as the test par excel- 

 lence for the identification of pathogenic species. Accord- 

 ing to the standards then in common use two cultures from 

 different cases of the same disease may have been identical 

 in all other particulars, yet if one was capable of reproducing 

 in animals lesions similar to those in man from which that 

 culture was obtained and the other was devoid of that 

 power, they were generally regarded as two distinct species. 



To illustrate we have only to recall the confusion that 

 existed in connection with the diphtheria bacillus and the 

 several "pseudo" diphtheria bacilli that were described. 

 We now know that variations in pathogenic power is 



