Some Differential Reactions in the Colon-Aerogenes Group of Bacteria

Abstract

Two distinct subgroups are generally recognized in the colon-aerogenes group of bacteria: the genus Escherichia which grows poorly if at all on citric acid, is acid to methyl red (M.R.+), does not produce acetyl-methyl-carbinol from glucose, (V.P.—), and is generally of fecal origin, and the genus Aerobacter which grows luxuriantly on citric acid, is alkaline to methyl red (M.R.—), produces acetyl-methyl-carbinol, (V.P.+) and is generally found in soil or on grains. Recently a number of strains have been reported which grow luxuriantly on citric acid but are negative as respects the V.P. reaction and frequently neutral or acid to methyl-red. The systematic position of these strains is in doubt.

Werkman and Gillen 1 have recently suggested the generic name, Citrobacter for bacteria producing trimethylene-glycol from glycerol, which can utilize citric acid but are V.P. negative and M.R. neutral or acid. Whether all of the strains falling in this category on the citric acid, V.P. and M.R. tests are capable of producing trimethylene-glycol is not known. The lack of a suitable, convenient, rapid and reliable test for trimethylene-glycol together with the questionable M.R. and V.P. reactions of these strains, renders the discovery of a simple differential test which will distinguish these cultures from typical Escherichia and Aerobacter highly desirable. The production of H₂S from Difco proteose peptone seems to serve this purpose admirably.

In the accompanying table are shown the results of a study of 401 strains of members of the colon-aerogenes group isolated from eggs. The percentages are considered tentative, as several additional cultures are under observation, and it is possible that some impure cultures might be detected before completion of the study. Only those characteristics which show distinct and clear-cut differential value are considered here.