Thermal Inactivation of Acid-Adapted Escherichia coli O157:H7 in Dairy Compost

Abstract

It is well documented that stress-adapted microorganisms can develop cross-resistance to other unrelated stress. This study was designed to evaluate the thermal resistance of acid-adapted Escherichia coli O157:H7 in both fresh and finished dairy composts. A three-strain mixture of E. coli O157:H7, either acid-adapted or non-adapted (control), was inoculated into dairy compost to a final concentration of approximately 10⁷ CFU/g. The inoculated compost was kept in an environmental chamber which was programmed to raise temperature from room to target temperatures (50°C, 55°C, and 60°C) in 2 days, simulating the early phase of composting. In fresh dairy compost with 2 days of come-up time, acid-adapted and control E. coli O157:H7 survived for 19 and 17 days at 50°C, respectively, and 6 and 4 days for both types of culture at 55°C and 60°C, respectively. Overall, pathogen survival was non-significant (p>0.05) between control and acid-adapted cultures at all tested temperatures. In finished compost, the same trend in pathogen survival between control and acid-adapted cultures was observed at 55°C. However, the duration of survival for both cultures was short in comparison to that in fresh compost. In fresh compost with short come-up time (15 min), acid-adaptation provided E. coli O157:H7 some cross-protection to heat at 55°C up to 30 min of exposure. The effect of heating medium on thermal resistance of acid-adapted E. coli O157:H7 revealed that in saline, acid-adapted E. coli O157:H7 was inactivated slower (p<0.05) with 0.5 and 1 h of heat exposure at 55°C as compared to control culture. Our results revealed that cross-protection against heat in E. coli O157:H7 due to acid-adaptation was demonstrated in saline but lost in fresh dairy compost with 2 days of come-up time during composting. Additionally, the type of compost and heating medium can influence the rate of pathogen inactivation at composting temperatures.