BACTERIA IN SEWAGE 153 



per c.c. He obtained higher figures during the summer months 

 than at other times ; but in any case his average was extremely low. 

 Laws and Andrewes * found that London crude sewage varied 

 from 2,781,650 to 11,216,666 micro-organisms per c.c. "It will 

 thus be seen," they conclude, " that very wide variations exist in the 

 total number of micro-organisms present in sewage at different times 

 and in different places. Temperature is one important factor in 

 determining the rapidity of their reproduction, and hence their 

 increase in numbers; dilution of the sewage by rainfall must also 

 exert a marked influence." Houston ( has also examined the sewage 

 of London, and found, in 1898, that the Barking crude sewage con- 

 tained an average of nearly four millions of organisms per c.c. and 

 the Crossness crude sewage three and a half millions per c.c. In 

 1899 the same observer J reported 7,357,692 bacteria per c.c. as the 

 average in the sewage at the Crossness outfall. On one occasion he 

 records 19,500,000 micro-organisms as present in one cubic centi- 

 metre. In 1900, Houston reported similar figures, and on occasion 

 as many as 1,900,000 B. coli per c.c. in crude sewage. He further 

 added some records as to the number of bacteria from crude sewage 

 growing at blood-heat and room temperature. In the former 

 case he found as many as 6,830,000, and in the latter 11,170,000 

 per c.c. 



Not only are the numbers incredibly large, but we also find an 

 extensive representation of species, including both saprophytes and 

 parasites, non-pathogenic and pathogenic. Many of these are known 

 as "liquefying" bacteria (from the power which they possess of 

 liquefying or peptonising nutrient gelatine used as a culture medium), 

 and this is one of the features of putrefactive bacteria. Bacilli pre- 

 ponderate over micrococci in actual numbers, and in numbers of 

 species present. There are also many spores. Dr Houston has 

 tabulated these results in his Third Keport (1900) from which it 

 appears that there are about 340 spores per c.c., and 1,076,923 lique- 

 fying bacteria per c.c. Moulds are but rarely found in sewage, 

 though common in sewer air. 



It is probable that the investigations made into the contained 

 bacteria of sewage have up to the present, excellent though they 

 have been, only revealed those species of bacteria which occur in 

 considerable abundance. So though it is impossible to make any 

 very complete record as regards the species of bacteria present in 



* Report on the Result of Investigations on the Micro-organisms of Sewage, London 

 County Council, 1894. 



t " Filtration of Sewage," Report on the Bacteriological Examination of London 

 Crude Sewage (First Report), London County Council, 1898. 



J " Bacterial Treatment of Crude Sewage " (Second Report), London County 

 Council, 1899. 



Ibid. (Third Report), 1900, p. 59. 



