VARIETIES OF COLON BACILLI 185 



generally associated with forms fresh from the intes- 

 tine and may therefore be called typical. Such " typ- 

 ical" B. coli are understood as a rule to be motile, 

 to clot milk, produce indol, reduce nitrate and neutral 

 red and to fail to liquefy gelatin. It seems clear that 

 forms having these characters predominate in the 

 intestine itself while differing or " atypical" forms 

 bear to them somewhat the relation implied in Vincent's 

 term, satellites. Houston (1903*) examined in detail 

 10 1 cultures of coli-like microbes isolated from faeces and 

 found that 72 per cent of the cultures were typical in 

 all respects, while n per cent more differed only in 

 being non-motile. The remaining 17 per cent were 

 atypical, reacting abnormally to milk, indol, neutral 

 red, litmus whey or Capaldi and Proskauer's medium. 

 In a later investigation, Houston (1904) made a careful 

 study of the distribution of the atypical forms in faeces, 

 sewage, polluted water, and the filtered water-supplies 

 of London. According to his ingenious system of 

 nomenclature, " fl " indicates an organism which pro- 

 duces green fluorescence in neutral red broth; " ag," 

 one which forms acid and gas in lactose media; " in," 

 one which produces indol; and " ac " one which acidifies 

 and clots litmus milk. The combination of all these 

 properties gives " Flaginac," or typical B. coli; " aginac" 

 is a form which fails to reduce neutral red; " flagac," 

 one which fails to form indol, etc. " Flaginac " B. coli 

 form the great majority of coli-like microbes in faeces, 

 but Houston found that in filtered water they are 

 outnumbered by atypical forms, of which he recognized 

 thirty-five distinct types. 



