Thermotolerance of Rifampicin‐Resistant Escherichia coli O157:H7 Derivatives and Their Parental Strains in a Broth System

Abstract

AbstractRifampicin‐resistant (RifR) strains have often been used in studies of Escherichia coli O157:H7 to assure more specific recovery. The present study compared the heat inactivation kinetics of rifampicin‐resistant variants of pathogenic and nonpathogenic E. coli O157 strains with those of the wild‐type parental strains. Spontaneous RifR derivatives of 11 pathogenic and six nonpathogenic E. coli O157 strains were selected. Stationary‐phase cells of each strain in sterile tryptic soy broth were heated at 60C for 0, 30, 60, 90, 120, 150, 180 or 240 s, and aliquots were plated on tryptic soy agar supplemented with 0.1% pyruvate + 100 μg/mL rifampicin. D values of three pathogenic and two nonpathogenic RifR E. coli O157 strains were 35–70% lower (P < 0.05) than those of their wild‐type counterparts. However, the heat resistance of most of the RifR E. coli O157:H7 strains (12 strains) was similar (P ≥ 0.05) to that of their parental counterparts. Therefore, the findings of this study demonstrated that the majority of the rifampicin‐resistant E. coli O157:H7 variants that were evaluated to assure more reliable recovery in microbial inactivation studies are suitable for use in various heat challenge studies.Practical ApplicationsMore reliable media are needed for the isolation or recovery of E. coli O157:H7 in inoculated challenge studies. Prior to use of rifampicin‐resistant E. coli O157:H7 strains in heat challenge studies, it is important that their thermal inactivation kinetics be compared with those of their wild‐type parental strains. The results of this study showed that the thermotolerance at 60C of the majority of the tested rifampicin‐resistant E. coli O157:H7 variants was not different than that of the corresponding wild‐type strains. These rifampicin‐resistant strains can, therefore, be used in heat challenge studies and can be recovered with non‐selective culture media supplemented with rifampicin.