EVALUATION OF VARIOUS MEDIA FOR RECOVERY OF THERMALLY-INJURED ESCHERICHIA-COLI O157-H7

Efficacies of plating media for recovering heated Escherichia coli O15 7:H7 were determined and compared. To compare populations of recovered cells, suspensions of cells (three isolates, four replications/isolat e) were heated at 50, 55, or 60 degrees C, and then inoculated onto ei ght media: PCA-PA (plate count agar with 1% pyruvic acid [PA]), MSA (M acConkey sorbitol agar), MSA-Mg (MSA with 0.025% MgSO4), MSA-PA (MSA w ith 1% PA), MSA-MUG (MSA with 0.005% 4-methylumbelliferyl-beta-d-glucu ronide (MUG), PRSA-MUG (phenol red sorbitol agar [PSRA] with 0.005% MU G), 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 degrees C, mean numbers (log(10) CFU/ml) of reco vered cells on PRSA-MUG were 4.42, 4.62, and 3.32, respectively, as co mpared to 2.78, 2.08, and 1.63, respectively, on MSA. PCA-PA and TSA-P A were less effective than PRSA media, but better than MSA media. Thus , PRSA with MUG or PA was an effective medium for recovering heated ce lls of E. coli O157:H7; whereas MSA failed to detect sublethally injur ed cells. Furthermore, addition of Mg, PA, or MUG to MSA further compr omised this medium.