A Modified Lactose Broth for use in the Presumptive Test

Abstract

A substitute for or modification of the standard lactose broth used in sanitary water analysis which will reduce the number of false positive presumptive tests and yet exert no inhibition on the development of colif orm bacteria is much to be desired. Numerous preparations have been suggested and tried. Of these some have given initial promise, but for one reason or another have failed to fulfill all requirements. The medium described herein may well have a similar fate, but tentative tests have suggested that it has distinct possibilities, and a description of it is given in the hope that it will be tried out and its merits and defects determined. Standard lactose broth is used for the basic medium, and to this is added a small amount of an alkyl sulfate. The alkyl sulfates seem to prevent, by and large, the growth of Gram-positive organisms, including the lactose-fermenting aerobes and anaerobes, while the Gram-negatives are, in general, able to develop in the presence of relatively high concentrations of these substances. Studies of this problem will appear in a later paper. Sodium lauryl sulfate is the most readily obtained pure compound, and most supply houses can furnish it for about $1.50 per pound. To each liter of standard lactose broth, 0.2 g. of the sulfate are added and the pH adjusted to 7.2 before tubing and autoclaving. Ordinary Drene shampoo can be substituted in an amount of 1.0 cc. per liter, adjusted to pH 7.4, and there is some evidence, in fact, that results with Drene are better. The finished medium looks exactly like the standard broth, but can easily be distinguished from it by the