Purpose: The aim of this study is to compare ten different serotypes of non-O157 STEC with O157:H7 in growth kinetics, chlorine sensitivity and the extent of transfer during lettuce washing, to provide information for preventing pathogen transfer in post-harvest processes.
Methods: The characterization of growth kinetics was conducted by using the Bioscreen C automated growth curve analysis system. Chlorine sensitivity was evaluated by analyzing the lag phase extension of growth in the presence of a sub-lethal concentration of chlorine (0.15 ppm), compared to no-chlorine controls. Pathogen transfer during washing was assessed in lettuce washing experiments. After inoculation of 106 CFU cells onto 3 pieces of lettuce leaf (0.5 g), the lettuce was washed in 50 ml cold water with uninoculated lettuce strips (0.5 g) for 1 min. Populations of the E. coli remaining on the inoculated leaves, in the wash water and transferred to the uninoculated leaves were enumerated by plate counts on selective agar.
Results: Thirty non-O157 STEC strains (including the USDA defined “Big Six”, O104:H4 and three other O serotypes) showed growth kinetics similar to that of O157:H7. Doubling time was 25 to 30 min, lag phase duration was 3 hrs and maximum population was 109 CFU/ml, approximately. With only one strain in O111 group and another strain in O121 group showing less chlorine resistance, no significant difference in chlorine sensitivity was observed for the other “Big Six” non-O157 STEC strains and O104:H4 group when compared with that of O157:H7. On the other hand, O29, O73 and O78 strains exhibited higher chlorine sensitivity.
Preliminary cross-contamination experiments using the O104:H4 isolate from the 2011 German sprout outbreak and O157:H7 isolate from the 2006 US spinach outbreak showed similar transfer efficiencies by the two strains during lettuce washing. Log reduction from washed lettuce was approximately 2.3; log transferred from the lettuce to water was approximately 3.8 after 1 min, and to the uninoculated leaves was 2.8.
Significance: The information generated by this study will be useful to assess pathogen risk and to determine effective sanitization procedures for preventing STEC cross-contamination during washing of leafy greens.