RECOVERY OF THERMALLY-STRESSED ESCHERICHIA-COLI O157-H7 BY MEDIA SUPPLEMENTED WITH PYRUVATE

Unheated and heat-stressed (57 degrees C, 50 min and 60 min) cells of Escherichia coli O157:H7, were enumerated using three media supplement ed with 1% sodium pyruvate (NaPyr): plate count agar (PCA), tryptic so y agar (TSA) and phenol red sorbitol agar (PhRSA) using the spread pla te method. The medium recovering the greatest numbers of severely heat ed E. coli O157:H7 was PCA with 1% NaPyr. Recovery of heat stressed E. coli O157:H7 on this medium was significantly higher (P < 0.05) than the two other media with pyruvate: 16.3% (50 min heating) and 0.55% (6 0 min heating) of the total population was recovered with TSA + 1% NaP yr when compared to those numbers found on PCA + 1% NaPyr. The ability of PhRSA + 1% NaPyr to recover heat-stressed E. coli O157:H7 was simi lar to that of TSA + 1% NaPyr. Using PhRSA + 1% NaPyr media, 12.9% (50 min heating) and 0.61% (60 min heating) of the total population were recovered when compared with the cells enumerated on PCA + 1% NaPyr. R ecovery of the heat-stressed cells using the spread plate method was g reater than using pour plate method. Recovery was significantly higher (P < 0.05) on the spread plates for highly stressed E. coil O157:H7 ( 1.2 log) heated for 60 min than on the pour plates. Overall, the popul ations on the TSA spread and pour plates were low compared with the sa me heat-stressed cells recovered on media containing pyruvate. The sod ium pyruvate apparently permitted the stressed cells to repair from th e heat damage. The synergistic action of the pyruvate and the PCA crea ted an excellent medium for recovery of heat-stressed E. coli O157:H7. Enumeration of unheated bacteria was not affected by either the type of medium or the plating method.