562 REUBEN L. KAHN 



combination is sufficiently toxic to prevent their growth even in 

 the butt. 



This lack of growth in Endo tubes after inoculations from lac- 

 tose broth fermentation tubes showing gas, has helped us to 

 differentiate anaerobic lactose fermenting organisms from B. 

 coll, in a large number of cases. 



Frequently, however, when gas production in lactose broth 

 is due to an anaerobe, other organisms capable of aerobic growth 

 are found in the medium. Under these conditions, after trans- 

 plantations from fermentation tubes to Endo tubes, a growth 

 would occasionally appear on the slant, but as a rule, would be 

 recognized without difficulty to be of non-colon type. 



As a further means for differentiating lactose fermenting 

 anaerobes from B. coli the following procedure may be employed: 



A loop from the lactose fermentation tube showing gas, is 

 smeared on the bottom of a Petri dish. A tube of Endo agar is 

 melted in a water bath, cooled to about 40°C. and poured over 

 the surface of the plate. After the Endo agar is congealed, 

 another loop from the same fermentation tube is now spread 

 over the surface of the agar. We thus have, after incubation, 

 an aerobic growth on the surface and a practically anaerobic 

 growth below the surface of the Endo agar. 



If B. coli be present, typical or semi-typical colonies will 

 appear on the surface; also red colonies, and as a rule, gas 

 bubbles will appear below the surface of the medium. If on the 

 other hand, the gas produced in lactose broth is due to an 

 anaerobe, typical colon colonies are of course, never present; 

 the colonies below the surface of the agar are, as a rule, uncol- 

 ored; neither are gas bubbles observed below the surface of the 

 medium. 



It is well to keep in mind that while B. coli represent a definite 

 group of organisms with well defined characteristics, the ana- 

 erobes found in water vary widely both in traits and morphology. 

 It is likely that geographic and climatic conditions will favor the 

 growth of certain types of anaerobes in one area, and of different 

 typeseof anaerobes in another area of the country. A procedure, 

 therefore, which would help differentiate anaerobes from B. 



