VARIETIES OF COLON BACILLI 183 



Health Association Committee recommends three suc- 

 cessive cultivations in broth at 20 degrees, each of 24 

 hours' duration, inoculation from the last broth tube 

 of a gelatin plate which is incubated for 48 hours at 20 

 degrees, inoculation of an agar streak from one colony 

 on the plate and incubation of this streak for 48 hours 

 at 20 degrees. 



Often, however, the differences between tj-pes of the 

 colon group indicate something much more fimda- 

 mental than temporary weakening due to unfavorable 

 en\'ironment. In particular sudden more or less perma- 

 nent mutations may suddenly appear. Twort (1907) 

 reports that by continued cultivation in sugar media he 

 was able to develop fermentative power in certain mem- 

 bers of the Gartner group which lacked such powers 

 before. 



The work of Massini, Miiller, Sauerbeck, Konrich 

 and others (well svunmarized by Konrich, 1910) has 

 also shown that mutations capable of fermenting sugars 

 may suddenly arise from a parent strain lacking this 

 power. Burri (1910) has contributed to the same 

 problem and has found that the latent power to ferment 

 a given sugar is released by growing the organism on 

 that particular sugar, but that as Konrich and the others 

 show only a certain proportion of the cells develop 

 this power. The most important recent studies of 

 bacterial mutation have been made by Penfold. In 

 his latest communication (Penfold, 191 2) he shows 

 that many bacteria of the colon-tj-phoid group pro- 

 duce a mutant capable of fermenting lactose, that 

 all strains of the t^-phoid bacillus produce dulcite and 



