DIFFERENTIATION OF B. COLI AND B. AEROGENES 5 



sible to differentiate B. coli from B. aerogenes simultaneously with 

 confirming the presumptive test. For this purpose the following 

 experiment was carried out. 



Seven samples of water from different parts of the Iowa river, one of 

 sewage, one of a small creek, and one from a stagnant body of water were 

 plated out directly on litmus lactose agar and inoculated into lactose broth. 

 After 24 hours' incubation 10 acid colonies were fished from the litmus lactose 

 agar plates of each sample for further study. The lactose broth tubes were 

 plated out after 48 hours' incubation onto eosin-methylene blue agar and onto 

 litmus lactose agar. After 24 hours' incubation 10 colonies were fished from 

 these litmus lactose agar plates of cash sample for further observation. From 

 the eosin-methylene blue plates made from the preliminary lactose broth tubes 

 colonies which resembled B. coli or B. aerogenes were fished and tentatively 

 designated as such, with a view to determining the accuracy and reliability of 

 the plate differentiation. 



All colonies fished from litmus lactose agar were reinoculated into lactose 

 broth and after 24 hours' incubation were plated out on eosin-methylene blue 

 agar. From each plate was picked a well isolated colony which was designated 

 as B. coli or B. aerogenes. These designations were then checked by growing 

 the organisms in Clark and Lubs medium and testing with the methyl-red and 

 Voges-Proskauer reactions. 



CULTURES OBTAINED DIRECTLY FROM 'LITMUS LACTOSE AGAR 



Of the 10 water samples examined 1 did not show acid colonies by 

 direct plating on litmus lactose agar. Of the 90 acid colonies fished, 

 3 were found to be lactose nonfermenters. Thirty-three cultures were 

 regarded, from their appearance on eosin-methylene blue agar, as B. 

 aerogenes. Of these, 24 (72%) gave the Voges-Proskauer reaction. 

 Seven cultures were diagnosed tentatively as questionable but probably 

 B. aerogenes but none of these gave a Voges-Proskauer reaction. 



Eight cultures were regarded as questionable, but probably B. coli, 

 of which 6 (75%) did not give the Voges-Proskauer reaction. Thirty- 

 nine cultures were designated from their appearance on eosin-methy- 

 lene blue agar, as B. coli, and all were confirmed, as none gave the 

 Voges-Proskauer reaction. 



Of 40 cultures which were regarded tentatively as B. aerogenes or 

 probably B. aerogenes 24 (60%) were correct. Of 47 cultures regarded 

 as B. coli or probably B. coli, 45 (95.8%) were correct. 



CULTURES OBTAINED FROM LITMUS LACTOSE AGAR AFTER PRELIMINARY 

 ENRICHMENT IN LACTOSE BROTH 



After elimination of a few strains which proved to be other than 

 coli forms, 85 cultures which were isolated from litmus lactose agar 

 plates made from the lactose broth preliminary enrichment tubes were 



