214 ELEMENTS OP WATER BACTERIOLOGY 



agar each colony will be marked by one or more gas 

 bubbles surrounded by a delicate whitish fringe. The 

 organism is strongly pathogenic for guinea pigs, by 

 which character it is distinguished from the B. butyricus 

 of Botkin. B. welchii differs from B. sporogenes chiefly 

 in lacking motility and in forming spores with less read- 

 iness (Klotz and Holman, 191 1). 



The researches of Klein and Houston (Klein and 

 Houston, 1898, 1899) have shown that the B. sporogenes 

 occurs in EngHsh sewage in numbers varying from 30 to 

 2200 per c.c. and that it is often absent in considerable 

 volumes of pure water. In Boston sewage it may 

 usually be isolated from .01 or .001 of a c.c. (Winslow 

 and Belcher, 1904). Since the spores of an anaerobic 

 bacillus may persist for an indefinite period in polluted 

 waters, their presence need not necessarily indicate 

 recent or dangerous pollution. 



Vincent (1907) and other French observers consider the 

 determination of the total number of anaerobic bacteria 

 as significant, since the decomposition of organic matter 

 is accompanied by anaerobic growth. It is not claimed, 

 however, that bacteria of this type are characteristic of 

 animal more than of vegetable decompositions, and the 

 total anaerobic count apparently adds nothing of impor- 

 tance to the information gained by the ordinary gelatin 

 plate method. The property of liquefaction was for- 

 merly believed to be of significance, inasmuch as the 

 liquefying bacteria were regarded as indicative of pollu- 

 tion. This position is, however, no longer tenable, 

 since many bacteria, typical of the purest waters, may 

 cause liquefaction. 



