BACTERIA DEVELOPING AT DIFFERENT TEMPERATURES 255 



The numbers of bacteria and of acid-producing organisms deter- 

 mined on litmus-lactose agar at 30 C. after 24 hours' incubation 

 are smaller than the numbers determined at 20 C. after two to four 

 days' incubation; but even with these smaller numbers the distinction 

 between the polluted waters and the waters of good quality is more 

 sharply marked than is the case with the numbers determined at the 

 lower temperatures. Determinations at this temperature appear to 

 be especially applicable to the control of water niters, since the 

 relative purity of the raw and filtered waters, and the expression of 

 the hygienic efficiency of such filters as the percentage removal 

 of bacteria, are practically identical with similar determinations 

 made at 20, while the advantage obtained by having the results 

 available within 24 hours would prove invaluable in controlling the 

 operations of the filters and preventing any serious change in the 

 character of the filtered water. 



The numbers of bacteria determined at 40 C. are of great inter- 

 est, since in this class of bacteria must be included the disease-pro- 

 ducing organisms. The distinction between waters of different 

 kinds and between waters of the same kind representing different 

 degrees of pollution is well marked by counts at this temperature. 

 The significance of the numbers of acid-producing bacteria deter- 

 mined at 40 C. i. e., bacteria of the colon type is well known. 

 It may be said, however, that the usual practice of making qualita- 

 tive determinations of the presence or absence of B. coli should be 

 supplemented by quantitative determinations. It is the belief of the 

 writer, based on experience gained in the present study, that con- 

 siderable numbers of bacteria of the colon type may occur in waters 

 which are supposedly quite pure, judging from their total bacterial 

 content, while on other occasions a positive test for B. coli may be 

 caused by an isolated organism of that type. In the one case the 

 water would be open to suspicion, in the other case it would prob- 

 bably be relatively harmless, although no such distinction could 

 be made from the results of the qualitative tests. 



The results of determinations of bacteria and of . acid-producing 

 organisms which are able to develop at a temperature of 50 C. 

 are quite interesting. We see that relatively large numbers of bac- 

 teria of this type occur in sewages and in the effluents from sewage 



