Resistance of Escherichia coli O157:H7 ATCC 35150 to ohmic heating as influenced by growth temperature and sodium chloride concentration in salsa

Abstract

Salsa is a liquid-solid food containing jalapeño and serrano peppers, which result in multistate outbreak in 2008. Storage temperature and sodium chloride (NaCl) concentration of salsa vary depending on the climate, season, and type of product. In this regard, effect of growth conditions, namely, low temperature (15 °C) or NaCl concentration (4.5%) on the resistance of Escherichia coli O157:H7 ATCC 35150 to ohmic heating was identified in this study. Cells of E. coli O157:H7 ATCC 35150 grown under different growth conditions was inoculated into prepared salsa sample, and then subjected to ohmic heating. Mechanisms of resistance acquisition were identified by transcriptional responses, membrane fatty acid changes and confirmed with propidium iodide (PI) uptake values. Resistance of the pathogen to ohmic heating decreased when growth temperature decreased from 37 °C to 15 °C while increased resistance was observed for this pathogen when grown with 4.5% NaCl. Several heat stress related genes such as dnaK, rpoH, grpE, groES, htpG, and htpX were up-regulated (≥5 fold change) as growth temperature decreasedwhile groEL, dnaK, rpoH were up-regulated when grown with high NaCl concentration in the present study. The ratio of unsaturated fatty acids (USFA) to saturated fatty acids (SFA) of pathogen increased slightly (+0.16) or significantly (+0.79) with increasing NaCl concentration or decreasing temperature, respectively. These results indicate that the cell membrane of the pathogen grown at low temperature was more susceptible to heat than when grown under optimal conditions or high NaCl concentration. Cell membrane damage measured by PI uptake values of the pathogen grown with high NaCl concentration were not significantly different from those of the control (p > 0.05), while the values were significantly higher for the pathogen grown at low temperature and subjected to ohmic heating (p < 0.05). Based on these results we suggest that resistance of the pathogen grown at low temperature to ohmic heating decreased because of dominant cell membrane damage compared to induced heat stress related genes. The cell membrane damage was dominant by means of an increased ratio of USFA to SFA. On the other hand, pathogen resistance increased when grown in medium of high NaCl concentration because of induced heat stress related genes.