SEWAGE BACTERIA 157 



shows no lateral offshoots, and (c) possesses different pathological 

 characters on inoculation. If 1 c.c. of whey from a milk culture be 

 inoculated into a guinea-pig (200-300 grammes) a swelling appears 

 in the groin after six hours, which extends to the abdomen and thigh. 

 The animal is usually dead in eighteen to twenty-four hours with 

 gangrene of the subcutaneous tissue and offensive sanguineous exuda- 

 tion. These characteristics, coupled with the morphological and bio- 

 logical features, are sufficient for differentiation purposes (see also 

 p. 307). 



Houston has shown that the cholera bacillus, B. pyocaneus, and 

 Staphylococcus pyogenes aureus are capable of retaining their vitality 

 in crude sewage in competition with the very numerous bacteria 

 normally present.* The bacillus of anthrax, and still more so its 

 spores, can also live in sewage and sewage effluents (Houston).f 



Whilst we cannot here enter more fully into an account of the 

 bacteria found in sewage or their functions, it is necessary to remark 

 upon one important feature. A large number of these organisms 

 which we have named as normal inhabitants of sewage fulfil as their 

 main function the process of decomposition and denitrification, that 

 is to say, their role is to break down, by means of putrefaction, the 

 organic compounds constituting sewage. For example, urea which 

 is abundantly present in sewage is thus transformed with extra- 

 ordinary rapidity by several different forms of bacteria. 



By way of summary we may quote Houston's account of the 

 " standard " of crude sewage. Crude sewage usually contains, at least, 

 in one cubic centimetre — 



(a) 1-10 million bacteria. 



(&) 100,000 B. coli or closely allied forms. 



(c) 100 spores of B. enteritidis sporogenes. 



(d) 1000 streptococci. 



(e) -.^ c.c. is usually sufficient to produce "gas" in gelatine 

 shake cultures in twenty-four hours at 20° C. 



(f) The inoculation of animals with crude sewage always leads 

 to a local reaction, and not uncommonly results in death.j: 



As we have already said, when dealing with the Bacteria of the 

 Soil, Nature is dependent upon the services of the "economic" 

 organisms. Dead organic matter is broken down as the result of 

 the vital activity of putrefactive bacteria (decomposing and denitri- 

 fying). The ammonia which is thus hberated becomes oxidised first 

 to nitrous and then to nitric acid by the agency of the nitrifying 

 bacteria, and the acids by their action upon bases, always present, 

 produce nitrites and then nitrates. It is upon these substances that 



* Bact. Treatment of Crude Sewage, Third Report, 1900, p. 75. 



t Royal Commission on. Sewage Disposal, Second Report, 1902, p. 31. 



X Ibid., 1902, p. 126. 



