Medium for inhibition of spreaders and differentiation of B. coli and B. aerogenes

Abstract

Many mediums have been used for the isolation and identification of the colon bacilli 1 following preliminary enrichment in lactose broth. In the United States, litmus lactose agar and Endo medium have been largely used. On the European continent, the Drigalski-Conradi agar 2 has been more often employed, while in England, the MacConkey bile agar 3 has also been used. Besides these, other solid carbohydrate agar mediums are used locally to a limited extent, among which might be mentioned litmus gentian violet agar,4 brom cresol purple agar, litmus mannite agar, and neutral red agar. These mediums are all peptone-sugar agars to which appropriate indicators have been added. In a few cases an agent has been included which inhibits the development of some of the other bacteria likely to be encountered in routine water analysis. There are many objections to the mediums mentioned. 1. The acid formed from the colon bacilli (acid and aldehyde in the case of the Endo agar) diffuses rapidly, and colonies close together cannot be distinguished, the colonies of the colon bacilli often appearing the same as the nonlactose splitters. Frequently the whole plate is discolored, and it is somewhat difficult to differentiate the colonies with any degree of accuracy. 2. Lactose fermenting organisms of the coli-aerogenes group produce alkali after the acid fermentation has taken place. The acids which are formed from the lactose are further attacked and split into neutral or alkaline salts, the acid color of the medium changing thereby to the alkaline color.5 This is especially true with B. aerogenes.