Location: Meat Safety and QualityTitle: Locus of Heat Resistance (LHR) in meat-borne Escherichia coli: Screening and genetic characterization
Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/12/2021
Publication Date: 3/11/2021
Citation: Guragain, M., Brichta-Harhay, D.M., Bono, J.L., Bosilevac, J.M. 2021. Locus of Heat Resistance (LHR) in meat-borne Escherichia coli: Screening and genetic characterization. Applied and Environmental Microbiology. 87:e02343-20. https://doi.org/10.1128/AEM.02343-20.
Interpretive Summary: Some E. coli are extremely heat resistance (XHR), and this resistance is due to a genetic element called the Locus of Heat Resistance (LHR). The LHR helps the bacteria deal with stresses such as those that might occur during meat processing. It is not known if XHR E. coli are more common in certain types of harvest systems like beef, veal, pork, or lamb so 4,123 E. coli in the US MARC collection from these different harvest systems were examined. Most XHR E. coli turned out to be non-pathogens with no specific harvest system source, however they most commonly originated in samples from finished meat, rather than from steps earlier in processing. Five representative heat resistant E. coli were sequenced and it was found that the most resistant isolates possessed multiple copies of the LHR, while the less resistant ones possessed only one copy. Heat resistance is not currently a threat to meat safety because almost all heat resistance E. coli detected thus far are non-pathogenic, but there is the possibility of heat resistance genes transferring to foodborne pathogens.
Technical Abstract: Microbial resistance to processing treatments poses a food safety concern, as treatment tolerant pathogens can emerge. Occasional foodborne outbreaks caused by pathogenic Escherichia coli have led to human and economic losses. Therefore, this study screened for the extreme heat resistance (XHR) phenotype as well as one known genetic marker, the locus of heat resistance (LHR), in 4,123 E. coli isolates from diverse meat animals at different processing stages. The prevalences of XHR and LHR among the meat-borne E. coli were found to be 10.3% and 11.4%, respectively, with 19% agreement between the two. Finished meat products showed the highest LHR prevalence (24.3%) compared to other processing stages (0 to 0.6%). None of the LHR1 E. coli in this study would be considered pathogens based on screening for virulence genes. Four high-quality genomes were generated by whole-genome sequencing of representative LHR1 isolates. Nine horizontally acquired LHRs were identified and characterized, four plasmid-borne and five chromosomal. Nine newly identified LHRs belong to ClpK1 LHR or ClpK2 LHR variants sharing 61 to 68% nucleotide sequence identity, while one LHR appears to be a hybrid. Our observations suggest positive correlation between the number of LHR regions present in isolates and the extent of heat resistance. The isolate exhibiting the highest degree of heat resistance possessed four LHRs belonging to three different variant groups. Maintenance of as many as four LHRs in a single genome emphasizes the benefits of the LHR in bacterial physiology and stress response.