Pfizer selective enterococcus agar overlay method for the enumeration of fecal streptococci by membrane filtration

The use of Pfizer selective enterococcus (PSE) agar with the membrane filter technique for the enumeration of fecal streptococci is limited due to the inability of the characteristic black precipitate, indicative of esculin hydrolysis, to diffuse from the medium through the membrane. A modification of the membrane filter technique that consisted of placing the membrane on PSE agar and overlaying it with tempered PSE agar was evaluated by comparing recovery, selectivity, and other parameters with M-enterococcus and KF-streptococcus agars, two selective media routinely used with the membrane filter technique for the enumeration of fecal streptococci in water and wastewater. No statistically significant differences could be demonstrated in the recovery capabilities of the three media. Inasmuch as the PSE overlay technique requires only 24 h of incubation as opposed to 48 h for the other two media, this modification may have some merit in water pollution monitoring programs.