Note 18 — Chapter II

The methods adopted for making a quantitative and qualitative examination of sewage are precisely analogous to those used in milk research. Dilution with sterilised water previous to plating out on gelatine in Petri dishes is essential (1 cc. to 10,000 cc. of sterile water, or some equally considerable dilution), otherwise the large numbers of germs would rapidly liquefy and destroy the film. Special methods must be used for the isolation of special organisms; phenol-gelatine, Elsner medium, indol reaction, "shake" cultures, Parietti broth, etc., must often be resorted to for special bacteria. Spores of bacteria may always be numerically estimated by adding the suspected water or sewage to gelatine, and then heating to 80° C. for ten minutes before plating out. This temperature removes the bacilli, but leaves the spores untouched.