The Differentiation of the Coli and Aerogenes Groups of Bacteria

Abstract

In recent years far greater importance has been attached to the separation of the coli and aerogenes divisions of the colon groups than heretofore. Rogers (1) (2) (3) and his associates showed many years ago, by a study of the gas ratios and final hydrogen ion concentration of many colon cultures, that the natural habitat of B. coli is the intestinal tract of man and animals. On the other hand, B. aerogenes is rarely found in human and animal excreta, but is predominantly a soil or grain type of organism. Therefore, these two groups of bacteria are no longer believed to have the same sanitary significance in passing on the potability of a water supply. Unfortunately under many conditions a clear-cut differentiation of these two divisions with the identification of the organisms is a difficult and long procedure. In order that this may be more easily achieved, certain disturbing factors must be eliminated or removed as far as possible, such as (a) aerobic and anaerobic spore formers capable of producing acid and gas in lactose broth and (b) bacterial symbiosis or synergism. Michaelis and Marcora (4) and later, Clark (5) showed that the final pH for cultures of B. coli are a physiological constant. This work was, also, extended to include B. aerogenes. By means of a simple medium conditions were established whereby the metabolism of one group was made to diverge very far from that of the other, resulting in a distinct difference in the final hydrogen ion concentration produced by these two main divisions of the colon group. This test is known as the methyl red reaction (6) (7). Other well known methods which have been used for the separation of the two divisions of the colon group are the Voges-Proskauer (V.-P.) (8), uric acid (9) and citrate tests (10) (11).