THERMOTOLERANCE OF ESCHERICHIA-COLI O157-H7 ATCC43894, ESCHERICHIA-COLI-B, AND AN RPOS-DEFICIENT MUTANT OF ESCHERICHIA-COLI O157-H7 ATCC43895 FOLLOWING EXPOSURE TO 1.5-PERCENT ACETIC-ACID

On a beef carcass, Escherichia coli may sequentially encounter acid- a nd heat-intervention steps. This study tested whether acid stress (1.5 % [vol/vol] acetic acid, pH 4.0, 37 degrees C, 15 min) would enhance s ubsequent heat resistance of E. coli. Initially, cells (E. coli O157:H 7 ATCC 43894, nonpathogenic E. coli B [strain FRIK-124], and rpoS-defi cient mutant 813-6 [derived from E. coli O157:H7 ATCC 43895]) were aci d stressed and transferred to 54 degrees C trypticase soy broth (TSB), and survivors were immediately enumerated after at least three interv als of 12, 2, and 6 min, respectively, by plating. The ATCC 43894 and 813-6 strains survived the acid stress but strain FRIK-124 did not. Ac id-stressed ATCC 43894 had significantly lower D values than the non-a cid-stressed controls. Strain 813-6 had significantly lower D values t han strain ATCC 43894, with no significant difference between acid-str essed and non-acid-stressed cells. In a second experiment, cooling of cells prior to plating resulted in an increased D value for acid-stres sed ATCC 43894 cells, such that it was not significantly different fro m the D value for non-acid-stressed controls. Using this protocol, the re was no significant difference in D values between acid-stressed and non-acid-stressed ATCC 43894 cells in prewarmed TSB (54, 58, and 62 d egrees C), in prewarmed ground beef slurry (GBS; 58 degrees C), or in TSB and GBS inoculated at 5 degrees C and heated to 58 degrees C. The acid stress tested does not enhance subsequent heat resistance of E. c oli.