MAX LEVINE 



DIFFERENTIATION OF B. COLI AND B. AEROGENES ON EOSIN-METHYLENE 



BLUE AGAR 



Size: 



Confluence : 



Elevation : 



Appearance by 

 mitted Light : 



Trans- 



Appearance by Reflected 

 Light : 



B. coli 



Well isolated colonies 

 are 3-4 mm. in di- 

 ameter. 



Neighboring colonies 

 show little tendency to 

 run together. 



Colonies slightly raised; 

 surface flat or slightly 

 concave, rarely convex. 



Dark almost black cen- 

 ters which extend 

 more than % across 

 the diameter of col- 

 ony ; internal struc- 

 ture of central dark 

 portion difficult to dis- 

 cern. 



Colonies dark, button- 

 like, often concentric- 

 ally ringed with a 

 greenish metallic sheen. 



B. aerogenes. 

 Well isolated colonies 

 are larger than coli ; 

 usually 4-6 mm. in di- 

 ameter or more. 



Neighboring colonies run 

 together quickly. 



Colonies considerably 

 raised and markedly 

 convex ; occasionally 

 the center drops pre- 

 cipitately. 



Centers deep brown ; not 

 as dark as B. coli, and 

 smaller in proportion 

 to the rest of the 

 colony. Striated inter- 

 nal structure often ob- 

 served in young col- 

 onies. 



Much lighter than B. 

 coli.. Metallic sheen 

 not observed except 

 occasionally i n de- 

 pressed center when 

 such is present. 



RESULTS WITH PURE CULTURES 



A number of pure cultures were employed to test the value of this 

 medium for the differentiation of B. coli, B. aerogenes, and members 

 of the typhoid and paratyphoid groups. 



Of 22 cultures of B. aerogenes all but 3 gave the characteristic 

 reactions. Of these 3 cultures, 1 resembled B. coli on the eosin- 

 methylene blue agar, another failed to produce a black center, and the 

 3rd showed a slight metallic lustre, but did not resemble B. coll closely. 



Of 35 cultures of B. coli tested, 29 were typical. Six did not show 

 a distinct metallic lustre, but were typical in other respects. 



There were 23 strains of B. cholerasuis, B. paratyphosus, and 

 B. typhosus tested. One strain of B. paratyphosus A did not grow. 

 All other strains of the intermediate group developed typical trans- 

 parent colonies. 



RESULTS OBTAINED WITH WATER SAMPLES 



The differentiation of pure strains seemed to be so marked and 

 distinct that it was thought the medium might be employed for con- 

 firmation of the presumptive test for B. coli, and that it might be pos- 



