352 APPLIED BACTERIOLOGY 



Sewage begins to alter in composition almost immediately, 

 and this alteration is due almost entirely to the action of 

 bacteria, as if sewage is sterilised by heat and kept free 

 from bacteria it will remain unchanged for any length of 

 time. This rapid change in the composition of sewage 

 necessitates the performance of any analytical operations 

 on the spot or within the shortest possible time after the 

 collection of the samples. 



If it is desired to estimate the number of bacteria present, 

 gelatine plate cultures should be made on the spot after 

 diluting the sewage with sterile water. Besides plate 

 cultures, anaerobic cultures should be made, as sewage will 

 always contain some anaerobes which will not grow in plate 

 cultures exposed to the air. 



Such bacteria as the typhoid bacillus, the cholera bacillus, 

 or the bacillus of diphtheria, might, of course, be identi- 

 fied in sewage taken immediately below a hospital, but 

 researches distinctly point to the probability that no 

 pathogenic bacteria can survive long in sewage, probably 

 because they are crowded out by other organisms that 

 are more readily able to flourish in that particular medium. 



Laws and Andrews, for example, found that not merely 

 did typhoid not flourish in sewage, but that it could not 

 remain alive in it for anything like the length of time that 

 it survives in ordinary drinking water, and that its dis- 

 appearance from sewage is hastened by the presence of 

 certain sarcinse. 



The bacteriology of sewage is receiving attention at the 

 present time on account of the recent development of 

 so - called bacteriological methods of sewage treatment, 

 which seem likely to supplant the earlier methods of 

 sewage treatment in which chemical precipitation was 

 relied on. 



The intention of these processes is to bring about the 



