SEWAGE AND SEWAGE DISPOSAL. 245 



rods often form chains, and, as the ends of the rods are sharply 

 rounded, a chain gives the impression of a row of cocci. When grown 

 in bouillon, the latter is rendered turbid, and a characteristic slimy 

 film is formed at the top, whilst at the bottom a slimy sediment is 

 produced which can be drawn into long threads. 



Next is a small group clustering round the Bac. enteritidis of 

 Gaertner. This bacillus is in the main not unlike Bac. typhosus, but 

 differs from it in growing more rapidly in gelatine, in possessing fewer 

 cilia, and in being able to ferment lactose, and sometimes dextrose. 

 Like Bac. typhosus, it can produce indol, coagulate milk, and is unable 

 to form endospores. 



Finally, it is important to note the Streptococci that are found in 

 sewage, which in the crude state contains on the average about 1000 

 of these organisms per cub. cm. Their importance to the sewage 

 bacteriologist will be shown in the next paragraph. 



§ 2. DETECTION OF SEWAGE CONTAMINATION. 



It is important to be able to identify the commoner of the sewage 

 bacteria, because the power to do so furnishes us with an extremely 

 delicate method of finding out whether drinking or other water has 

 been contaminated with sewage or faecal matter. The chief organisms 

 for which search is made are Bac. coli communis, Bac. enteritidis 

 sporogenes and Streptococci. These are as stated above very numerous 

 in crude sewage, are absent from pure water, at least in any consider- 

 able numbers, and are easy to identify. Houston's "standard of crude 

 sewage " states that one cub. cm. contains 



1. 1-10 million bacteria. 



2. 100,000 Bac. coli communis or allied forms. 



3. 100 spores of Bac. enteritidis sporogenes. 



4. 1000 Streptococci. 



Bac. coli communis, Bac. enteritidis sporogenes and the Streptococci 

 have consequently been termed the "microbes of identification." Of 

 these three Bac. coli communis gives the most accurate measure of 

 intestinal or of sewage pollution, because it is an intestinal parasite 

 and tends to perish in other media ; so if present in fair numbers in 

 drinking water, it shows that there must be food for it, which should 

 not be the case in drinking water. The presence of Streptococci in 

 water is an indication of recent and dangerous pollution. These 

 organisms are entirely absent, or present only in small quantities in 



