Other Intestinal Bacteria. 165 



disease in man. Unless, however, specific pathogenes 

 can be identified by serum reactions, the non-lactose- 

 fermenting forms have no clear significance in water 

 analysis. They occur in the intestine, though not appar- 

 ently in great abundance. Houston (1904) attempted 

 to isolate bacteria fermenting dextrose but not lactose 

 from normal human stools; but out of 257 colonies studied 

 only six failed to form acid and gas in lactose media. 

 Organisms of this character may also be found in unpol- 

 luted water, as shown by the occurrence of positive 

 dextrose-broth tests followed by negative litmus-lactose- 

 agar plates (see Chapter VIII). 



Organisms belonging to the colon group itself, and 

 producing gas and acid in both dextrose and lactose 

 media, are much more clearly related to sewage pollution. 

 Numerous investigations of normal waters have shown 

 that bacteria possessing these two properties are rarely 

 found where pollution is absent, while the commonest 

 intestinal forms exhibit this dual fermentative power. 

 The mass of evidence establishing excretal -origin is 

 stronger in the case of B. coli than for any of its allies. 

 B. communior, which differs in possessing the power of 

 fermenting saccharose, is, however, almost invariably 

 included with B. coli in practical work. The signifi- 

 cance of those forms which differ from B. coli by lacking 

 one or all of the properties of motility, of indol formation, 

 or of nitrate reduction, or by failing to coagulate milk in 

 the standard time, is somewhat less clear. B. aerogenes 



