The Use of Phenol Red and Brom-cresol Purple as Indicators in the Bacteriological Examination of Stools

1. Either phenol red or brom-cresol purple may be used as indicators in the preparation of lactose agar plates for the isolation of members of the typhoid-dysentery group of bacteria from stools. Of the two, brom-cresol purple gives sharper differentiation and is to be preferred. 2. These indicators exercise no restraining influence upon the growth of cultures of the typhoid bacillus or paratyphoid bacillus freshly isolated from the human body, or of laboratory cultures of Bacillus dysenteria. 3. Both indicators may be successfully employed with brilliant green in the isolation of the typhoid-paratyphoid group from stools without sacrificing the restraining activity of the brilliant green upon other bacteria.