C. DOBELL 327 



A. Physiological Mutations. 



The first series of experiments which I shall describe in this 

 section concerns changes in the powers of fermentation observed in 

 the Bacteria belonging to the coli-typhosus group'. This is a very 

 large group containing mostly gut-inhabiting organisms. It contains 

 a large number of different "species" or "races," ranging from the 

 common "harmless" Bacillus colt to the parasitic Bacillus typhosus 

 of typhoid fever. It is convenient to regard these two organisms as 

 the extreme limits of the group, and to place the dozens of other 

 members in various intermediate positions according to their properties. 

 Morphological differences between the members of the group are so 

 slight and inconstant that a physiological classification is at present 

 the only one possible. For our present purposes it should be remem- 

 bered that — in addition to the difference in pathogenicity — Bacillus 

 coli differs from B. typhosus in the following features among others : 

 it is able to ferment lactose and glucose, to produce indol when grown 

 in broth, and to clot milk. B. typhosus, on the other hand, can do 

 none of these things. Owing to the differences in their powers of 

 splitting sugars, we find that these two organisms shew characteristic 

 differences when grown in certain test media. On the medium of 

 Drigalski and Conradi, B. coli forms red colonies, whilst the colonies 

 of B. typhosus are blue. On Endo's medium, similarly, B. coli forms 



' For those unversed in bacteriology a few additional remarks concerning the naming 

 of these organisms will perhaps be necessary. The common colon bacillus is variously 

 known as Bacillus coli and Bacterium coli commuTte. The typhoid organism is called 

 Bacillus typhosus or Bacterium typhi abdominalis. The other members are in part named 

 on the binominal system, but frequently also on a trinominal or even quadrinominal or 

 quinquenominal system (e.g. Bacillus faecalis alcaligenes liquefaciens). The strings of 

 Latin names thus used are as a rule descriptive terms rather than ordinary specific or 

 varietal names. In part, however, the members of the group are known by the names 

 of their describers (e.g. Gartner's bacillus, Flexner's bacillus). In part also they are 

 designated by numbers or letters of the alphabet (e.g. paratyphoid bacillus B, etc.). 

 Combinations of these methods are also resorted to. The vast numbers of these 

 organisms, and the extreme difficulty of deciding upon the systematic status of the 

 various "species,"' "varieties," "strains," etc. have thrown the nomenclature into a 

 state of well-nigh hopeless chaos. Throughout this paper I shall call the common colon 

 organism Bacillus coli and the typhoid organism Bacillus typhosus. In other cases I 

 shall use the terms employed by the writers whose work I am considering. 



