DIFFERENTIATION OF B. COLI AND B. AEROGENES 7 



In this group of strains studied a similar correlation was observed. 

 The relation between the Voges-Proskauer and methyl-red reactions is 

 indicated in the following table : 



Methyl-red 



+ ' N * 



?+ 2 2 84 



> 121 13 2 



* In previous work neutral reactions to methyl-red have been grouped with the acid 

 strains. 



It is seen from the table that there is an excellent correlation 

 between the two reactions. Placing the neutral reacting strains with 

 the acid group as was done in previous studies, we find that 84 

 (97.8%) of 86 methyl-red negative strains give the V. P. reaction; 

 while of 136 strains not giving the V. P. reaction 134 (98.7%) were 

 acid or neutral to methyl-red. 



In this series of cultures the V. P. reactions were more clear-cut 

 than the methyl-red test but we have worked with collections in which 

 the reverse was true. It seems best to employ both the V. P. and 

 methyl-red tests and to repeat if the results do not agree. 



It is interesting to note that of 87 cultures isolated from litmus 

 lactose agar plates made directly, 29.9% proved to be B. aerogenes; 

 whereas of 85 cultures isolated from litmus lactose agar plates made 

 after preliminary enrichment for 48 hours in lactose broth, 40% proved 

 to be B. aerogenes. "This indicates the correctness of the conten- 

 tion of Race and others that preliminary enrichment tends to an over- 

 growth of B. aerogenes types. 



Organisms other than B. coli and B. aerogenes grow quite well on 

 this medium and several have been observed to produce small blue 

 centers ; but the appearance is so distinct from B. coli and B. aerogenes 

 that once having observed the true types there should be no mistake. 

 Just what these other forms are has not been determined but several 

 have been isolated and will be reported on in a future report. They 

 produce very small colonies with pinpoint light blue or delf-blue cen- 

 ters. The color is very different from the brownish black appearance 

 of the B. coli and B. aerogenes types. Perhaps the introduction of 

 some inhibitory dye into the medium will make it even more reliable 

 .for the isolation and differentiation of B. coli and B. aerogenes and 

 confirmation of the presumptive test. 



Jour. Infect. Dis., 1916, 19, p. 647. 

 Jour. Bacteriol., 1917, 2, p. 585. 

 Jour. Biol. Chem., 1917, 30, p. 209. 

 Jour. Infect. Dis., 1916, 19, p. 606. 

 Ibid., 21, p. 162. 



