Monensin-based medium for determination of total gram-negative bacteria and Escherichia coli

Plate count-monensin-KCl (PMK) agar, for enumeration of both gram-negative bacteria and Escherichia coli, is composed of (per liter) 23.5 g of plate count agar, 35 mg of monensin, 7.5 g of KCl, and 75 mg of 4-methylumbelliferyl-beta-D-glucuronide (MUG). Monensin was added after the medium was sterilized. The diluent of choice for use with PMK agar was 0.1% peptone (pH 6.8); other diluents were unsatisfactory. Gram-negative bacteria (selected for by the ionophore monensin) can be used to judge the general quality or sanitary history of a commodity. E. coli (differentiated by its ability to hydrolyze the fluorogenic compound MUG) can be used to assess the safety of a commodity in regard to the possible presence of enteric pathogens. Pure-culture studies demonstrated that monensin completely inhibited gram-positive bacteria and had little or no effect on gram-negative bacteria. When gram-negative bacteria were injured by one of several methods, a few species (including E. coli) became sensitive to monensin; this sensitivity was completely reversed in most instances by the inclusion of KCl in the medium. When PMK agar was tested with food and environmental samples, 96% of 535 isolates were gram negative; approximately 68% of colonies from nonselective medium were gram negative. PMK agar was more selective than two other media against gram-positive bacteria and was less inhibitory for gram-negative bacteria. However, with water samples, KCl had an inhibitory effect on gram-negative bacteria, and it should therefore be deleted from monensin-containing medium for water analysis.(ABSTRACT TRUNCATED AT 250 WORDS)