BACTERIA IN WATER 85 



petition of micro-organisms. It is clear that these latter are 

 the chief agents in bringing about the change, because, if 

 sewage be placed in hermetically sealed flasks and sterilised 

 by heat, it is found that no change occurs. From facts such 

 as the above it will be apparent that no exact standard of 

 chemical or bacterial contents is possible. Respecting the 

 chemical condition we may shortly say that the chief char- 

 acteristic of sewage is its enormous amount of contained 

 organic matter in suspension or solution; respecting the 

 bacterial content we may say that the chief species of the 

 very numerous organisms are those commonly concerned in 

 fermentative putrefaction. London crude sewage contains 

 on an average about four millions of micro-organisms per 

 cc. Many of these are " liquefying" bacteria; that is to 

 say, they have the power of liquefying gelatine, which is 

 generally one of the features of putrefactive species. In 

 considering the quality of the bacteria present in sewage, a 

 still wider field of research opens before us. For though we 

 can say that, roughly, all sewage will contain probably be- 

 tween four and eight millions of bacteria, we cannot even 

 lay down a rough standard respecting the kinds of bacteria 

 present more than we have done already in stating that a 

 very large number indeed out of the total will belong to 

 putrefactive species. 



We may, however, make a provisional list of normal 

 sewage bacteria 1 as follows : 



1 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 essen- 

 tial (i 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, Eisner 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 tempera- 

 ture removes the bacilli, but leaves the spores untouched. 



