The Reliability of Bromthymol-Blue Lactose Agar and Bacto Phenol-Red Mannitol Agar for the Isolation of Pathogenic Staphylococci.

Since 1937 more than 7,000 samples have been tested routinely in this laboratory on Bacto bromthymol-blue lactose agar (Chapman et at., 1937) and Bacto phenol-red mannitol agar (Chapman, Lieb, and Curcio, 1938) for the isolation of plasma-coagulating staphylococci. Althpugh our impressions of the results obtained have been favorable, the publication of an adverse (but unsubstantiated) opinion by an investigator of recognized ability (Blair, 1939) made it imperative to undertake a critical and extensive analysis of the methods advocated. The efficiency of an isolation method is determined by (1) the proportion of samples that give positive results, (2) the relative numbers of colonies of the organism in different samples, and (3) the proportion of transplants that prove to be pure cultures of the organism sought. Using bromthymol-blue lactose agar prepared in this laboratory, we (Chapman et al., 1937) found that 6.2 per cent of the transplants were nonpathogenic staphylococci and that 1.5 per cent of the pathogens had failed to grow on the medium. The Difco product has proved satisfactory also, as the foliowing analysis will show. In making this analysis it must be remembered that, although they are highly efficient, neither bromthymol-blue lactose agar nor Bacto phenol-red mannitol agar alone provides maximum recovery of pathogenic staphylococci in every instance but that best results are obtained by plating the sample on both media. Moreover, the incubation period is important-20 to 24 hours for Bacto phenol-red mannitol agar, and 40 to 48 hours for bromthymol-blue lactose agar. The results to be reported were obtained with the Difco products. Other formulae and other brands of ingredients may give rather different results, and their performance should be checked before they are used routinely. Bromthymolblue is the more variable chemical.