CHAPTER IV 



BACTERIA OCCURRING IN SEWAGE 



Their identification and numbers and morphological characteristics 

 — Bacterial tests for the purity of effluents — Possibility of the 

 survival of pathogenic organisms. 



The enormous number and variety of organisms present causes 

 some difficulty in the bacteriological examination of sewage. 

 In London, for example, in 1898 the number of bacteria per 

 c.c. in the crude sewage at Barking varied between 500,000 and 

 7,000,000, and at the Crossness outfall between 2,500,000 and 

 5,000,000. Obviously millions cannot be dealt with, and there- 

 fore a minute average fraction of the sample, bearing a definite 

 relation to the whole, has to be examined. If we attempted 

 to measure such small quantities directly it would neither be 

 accurate nor representative of the whole ; a systematic dilution 

 of the sewage has therefore to be followed. 



Collection of Samples. — These are collected in previously 

 sterilized glass-stoppered, or, better, rubber-stoppered, bottles 

 holding about 100 c.c, which are filled so as to allow a small 

 air-space, placed in water-tight tins, at once packed in ice, 

 and despatched to the laboratory. If possible, the culture 

 plates for counting the number of bacteria should be started on 

 the spot, and then conveyed to the laboratory for incubation, 

 as organisms multiply exceedingly rapidly in sewage, owing to 

 the quantity of organic matter that is present, though this is to 

 a great extent retarded by the ice. 



Dilution of the Sewage. — To inoculate the different cultures 

 for isolating and counting the various bacteria present, the 

 sewage is diluted in the following manner : 



A number of i-c.c. pipettes and flasks, holding about 150 c.c, 

 are plugged with cotton-wool and sterilized. Ninety-nine c.c. 

 of sterile water are then placed in each of the flasks, and I c.c. 

 of the sewage is then added to No. i flask and well shaken ; 

 with another pipette i c.c. of this dilution (corresponding to 



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