Growth Characteristics of Non-O157 Shiga Toxin-producing Escherichia coli under Stressed Conditions Induced by Sodium Hypochlorite, pH and Sodium Chloride

Introduction: Among Shiga toxin-producing Escherichia coli (STEC), E. coli O157:H7 has been responsible for majority of foodborne illnesses. Recently, non-O157 STEC have been increasingly implicated in foodborne illnesses caused by STEC. It has been shown that environmental stresses could select resistant O157 cells and allow them to persist in the food processing environment. However, the survival behavior of non-O157 STEC in stressed environments is not well understood.

Purpose: The purpose of this study was to examine the effect of environmental stresses on survival of non-O157 STEC.

Methods: One strain of O157:H7 and six strains of non-O157 STEC (O26:H11, O103:H1, O104:H4, O111:NM, O121:NM, and O145:NM) were subjected to three stresses: 8 log CFU/ml of each strain were exposed to chlorine (0, 2, and 5 ppm) at 22°C for 24h; 3 log of each exposed to water activity (a_w 0.95, 0.96, 0.97, and 0.98) or pH (4, 5, 6, and 7) at 12°C for 3 weeks. Survivor curves were plotted and compared using Student’s t-test.

Results: There were significant differences (P < 0.05) in the surviving populations of STEC during stress exposures.  Higher populations were observed for O26:H11, O103:H1, and O145:NM strains than others including O157:H7. For instance, after 24h in 2 ppm chlorine, survived cells of O26:H11 and O103:H1 were 6.1 and 5.6 log CFU/ml, respectively, while other strains were less than 3 log CFU/ml. Likewise, 2.2 log CFU/ml of O103:H1 survived for 7 d at pH 4 and 5.6 log CFU/ml of O145:NM survived for 21 d at a_w 0.97, while other strains were not detected or less than 3 log CFU/ml.

Significance: The data suggest that O26:H11, O103:H1, and O145:NM are more resistant to environmental stresses than O157:H7. This information will provide insight into the growth characteristics of non-O157 STEC from O157.