Bromothymol blue and carbohydrate-sensitive plating media

A new plating medium using bromothymol blue (BTB) indicator is described and compared with eosin-methylene blue (EMB), MacConkey, and Endo media. These media were tested with L-arabinose by plating fermenting and nonfermenting mutant strains of Escherichia coli. The minimum concentrations of L-arabinose that permitted differentiation of these strains were determined. Different concentrations were required for differentiating confluent patches of cells, isolated colonies, and closely spaced or adjacent colonies. L-Arabinose, L-rhamnose, D-lactose, and D-galactose were tested with modified enteric media and with BTB medium, again to determine minimum usable concentrations. BTB media and reformulated conventional media allowed detection of acidification, aerobically, at one-fifth to one-hundredth the (1%, wt/vol) concentration of carbohydrate used in standard indicator plates.