Expression of Stress and Virulence Genes in Escherichia coli O157:H7 in Fresh Dairy Compost

Introduction: Composting process has been used to inactivate foodborne pathogens in animal wastes on farm. However, the extended survival of these pathogens during composting has been reported. Heat-adaptation of pathogen during the mesophilic phase of the composting process may induce stress response, thereby extending their survival.

Purpose: This study was to understand the mechanisms for pathogen survival during composting by analyzing expression patterns of selected stress and virulence genes.

Methods: A two-step real-time PCR assay was used to evaluate expression of stress and virulent genes in Escherichia coli O157:H7 heat-shocked in compost. E. coli O157:H7 (strain F07-020-1) was inoculated in autoclaved fresh dairy compost which was heat-shocked at 47.5 °C for 10 min in water bath. To serve as medium control, heat-shock of the pathogen was also conducted in tryptic soy broth (TSB).

Results: In compost, heat-shock genes (clpB, dnaK, groEL) and alternative sigma factor (rpoH) of E. coli O157:H7 were all up-regulated significantly (P < 0.05). There was no significant (P > 0.05) difference in the expression of trehalose synthesis genes. Virulent genes such as stx1 and fliC were up-regulated while the rest of genes were down-regulated with no significant difference (P > 0.05). In toxin-antitoxin system, toxin genes, mazF, hipA, and yafQ, were up-regulated with no significant (P > 0.05) difference, whereas antitoxin gene dinJ was up-regulated with level of expression significantly (P < 0.05) different. Most of other antitoxin genes were down-regulated. In broth as the heat-shock medium, all heat-shock genes were up-regulated with relative fold change significantly (P < 0.05) different. There was no significant (P > 0.05) change in trehalose synthesis genes in broth medium either. Except for eaeA, the rest of virulent genes were down-regulated with no significant (P > 0.05) change. Majority of the toxin-antitoxin genes were down-regulated with relative fold change in toxin genes hipA and chpB only significantly (P < 0.05) different.

Significance: Our results suggest that induction of heat-shock response in pathogens plays an important role in providing protection to pathogens against lethal temperature during composting. Activation of toxin-antitoxin (TA) system in addition to heat-shock response may also be supporting pathogen survival in compost as a parallel mechanism.