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

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 10(7) CFU/g. The inoculated compost was kept in an environmental chamber which was programmed to raise temperature from room to target temperatures (50 degrees C, 55 degrees C, and 60 degrees 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 degrees C, respectively, and 6 and 4 days for both types of culture at 55 degrees C and 60 degrees 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 degrees 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 degrees 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 degrees 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.