Elimination of overgrowth in delayed-incubation membrane filter test for total coliforms by m-ST holding medium

Two holding media were compared for their effects on total coliform recovery by the delayed-incubation membrane filter procedure. LES-MF holding medium contains tryptone, m-Endo broth, dipotassium hydrogen phosphate, sodium benzoate, sulfanilamide, para-aminobenzoic acid, and cycloheximide (pH 7.0). m-ST holding medium contains ethanol, sodium monophosphate, dipotassium hydrogen phosphate, sulfanilamide, and Tris (pH 8.6). In tests with natural water and wastewater samples from various sources, recovery with LES-MF and m-ST were similar after a 1-day holding period. With LES-MF, however, after a 2- or 3-day holding period, coliform bacteria frequently were partially or totally overgrown by noncoliforms, causing significant reductions in coliform counts. No significant overgrowth was observed with m-ST. We propose that m-ST be used for all holding periods longer than 1 day.