The locus of heat resistance (LHR) mediates heat resistance in Salmonella enterica, Escherichia coli and Enterobacter cloacae
Citation
Mercer, R.G., Walker, B.D., Yang, X., McMullen, L.M., Gänzle, M.G. (2017). The locus of heat resistance (LHR) mediates heat resistance in Salmonella enterica, Escherichia coli and Enterobacter cloacae, 64 96-103. http://dx.doi.org/10.1016/j.fm.2016.12.018
Plain language summary
In food production and food preparation, enteric pathogens, bacterial organisms capable of causing disease in human small intestines, are often controlled by thermal treatment. Some bacteria may be more resistant to heat and this heat resistance is bacterial strain dependent, likely conferred by a group of genes in their genome. In this study, we investigated the contribution of these relevant genes to heat stability of three enteric bacterial species, including Escherichia coli, which is often used as a hygienic indicator of food processing environments, as well as Salmonella enterica and Enterobacter cloacae, which are both foodborne pathogens. This study found the isolates of the three species possessing heat-resistance relevant genes were much more stable to heat intervention, 60oC for 5 minutes, compared to the isolates absent of these genes. The introduction of heat-resistance relevant genes into heat sensitive isolates increased their stability to heat. This study indicates that enteric bacteria with heat-resistant genes may pose a risk to food safety and public health.
Abstract
Enterobacteriaceae comprise food spoilage organisms as well as food-borne pathogens including Escherichia coli. Heat resistance in E. coli was attributed to a genomic island called the locus of heat resistance (LHR). This genomic island is also present in several other genera of Enterobacteriaceae, but its function in the enteric pathogens Salmonella enterica and Enterobacter cloacae is unknown. This study aimed to determine the frequency of the LHR in food isolates of E. coli, and its influence on heat resistance in S. enterica and Enterobacter spp. Cell counts of LHR-positive strains of E. coli, S. enterica and E. cloacae were reduced by less than 1, 1, and 4 log (cfu/mL), respectively, after exposure to 60 °C for 5 min, while cell counts of LHR-negative strains of the same species were reduced by more than 7 log (cfu/mL). Introducing an exogenous copy of the LHR into heat-sensitive enteropathogenic E. coli and S. enterica increased heat resistance to a level that was comparable to LHR-positive wild type strains. Cell counts of LHR-positive S. enterica were reduced by less than 1 log(cfu/mL) after heating to 60 °C for 5 min. Survival of LHR-positive strains was improved by increasing the NaCl concentration from 0 to 4%. Cell counts of LHR-positive strains of E. coli and S. enterica were reduced by less than 2 log (cfu/g) in ground beef patties cooked to an internal core temperature of 71 °C. This study indicates that LHR-positive Enterobacteriaceae pose a risk to food safety.