1 66 Elements of Water Bacteriology. 



is a well-defined species of this group, which differs from 

 B. coli in lacking motility, in possessing a capsule, in 

 forming a somewhat heavier growth on media than B. 

 coli, and in failing to form indol. This and other allied 

 forms are sometimes called "atypical B. coli," or "para- 

 colon bacilli," and Vincent gives them the picturesque 

 name, "microbic satellites of B. coli." 



Sometimes, as pointed out in Chapter VI, these organ- 

 isms are merely weakened strains of B. coli which have 

 lost certain powers through exposure to unfavorable envi- 

 ronment. The results obtained by Peckham (1897) suggest 

 that the indol reaction in particular is highly variable. 

 By successive daily transfers in peptone broth she was 

 able to increase the amount of indol produced by normal 

 B. coli, and by a longer continuance of the same process 

 to again weaken and abolish the power of forming it. 

 Gas formation too was slackened in the cultures grown 

 for too many transfers in the same medium. Horrocks 

 (1903) found that B. coli kept in imsterilized well waters 

 and tap waters and in sterilized sewage and Thames 

 water for two to three months, showed only a feeble indol 

 production and a delayed action on milk and neutral 

 red. Even the fermentative powers which distinguish 

 the B coli and the B. enteritidis groups may be modified 

 by environmental conditions. Twort (1907) reports 

 that by continued cultivation in sugar media h^ was 

 able to develop fermentative power in certain members 

 of the Gartner group which lacked such powers before. 



