Effect of Metabolites on Growth,and Differentiation in the Colon-Group. ∗

Abstract

The subdivision of the colon-group of bacteria into distinct subgroups is still a problem despite many studies. The genera Escher-ichia and Aerobacter can be differentiated from each other by nearly a dozen correlated characters. The genus Aerobacter can be further subdivided on correlated characters into at least 2 distinct subgroups represented by Aerobacter aëro genes and Aërobacter doacce. Aside from these divisions, however, considerable confusion still exists.

There are many strains whose characters are such that they can-not be allocated to either of the genera. In this paper these bacteria will be referred to collectively as “Intermediates”.

Garre 1 and later Eijkman 2 showed that bacterial growth-products exhibit specific inhibitory effects. The purpose of this paper is to ascertain whether antagonism, or antibiosis, offers possibilities for differentiation of bacteria of the colon-group.

The method adopted was as follows: Broth containing 1.0% Bacto proteose-peptone and 0.1% K₂HPO₄ was inoculated with colon-organisms. These broth cultures were then incubated for 10 days at 37°C. An equal volume of a 3.0% agar gel was added to each of these 10-day broth cultures and plates were poured. This constituted what is known as a “staled” agar substratum.

These “staled“ agar plates were then streaked with 24-hour broth cultures of the homologous organism and with 24-hour broth cultures of a number of other test-organisms employed in producing “staled” substrata. The cultures employed were 5 strains of the genus Escherichia, 6 “Intermediates”, 6 Aerobacter aërogenes, and 6 Aerobacter cloaca:. Each of the 23 test-cultures was streaked on each of the 23 “staled” substrata, and the plates incubated for 48 hours at 37°C. Typical results are shown in Table I.

It is evident that the relative vigor of growth of an organism is distinctly better when streaked on a medium “staled” by an organism other than a member of its own genus or species than was the case with “staled” agar of the same genus or species.