Evaluation of Various Media for Recovery of Thermally-Injured Escherichia coli O157:H71

Abstract

Efficacies of plating media for recovering heated Escherichia coli O157:H7 were determined and compared. To compare populations of recovered cells, suspensions of cells (three isolates, four replications/isolate) were heated at 50, 55, or 60°C, and then inoculated onto eight media: PCA-PA (plate count agar with 1% pyruvic acid [PA]), MSA (MacConkey sorbitol agar), MSA-Mg (MSA with 0.025% MgSO4), MSA-PA (MSA with 1% PA), MSA-MUG (MSA with 0.005% 4-methylumbelliferyl-β-d-glucuronide (MUG), PRSA-MUG (phenol red sorbitol agar [PSRA] with 0.005% MUG), PRSA-PA (PRSA with 1% PA), and TSA-PA (tryptic soy agar with 1% PA). Recovery was consistently higher (P < 0.05) with PRSA-MUG and PRSA-PA. At 50, 55, and 60°C, mean numbers (log10 CFU/ml) of recovered cells on PRSA-MUG were 4.42, 4.62, and 3.32, respectively, as compared to 2.78, 2.08, and 1.63, respectively, on MSA. PCA-PA and TSA-PA were less effective than PRSA media, but better than MSA media. Thus, PRSA with MUG or PA was an effective medium for recovering heated cells of E. coli O157:H7; whereas MSA failed to detect sublethally injured cells. Furthermore, addition of Mg, PA, or MUG to MSA further compromised this medium.