The Response of Verotoxin-producing Escherichia coli (VTEC) to Cold Stress on Plants and Fresh-cut Romaine Lettuce

Introduction: Verotoxin-producing Escherichia coli (VTEC) have emerged as important foodborne pathogens with the majority of non-O157 VTEC infections attributed to six serotypes: O26, O45, O103, O111, O121 and O145.  Compared to O157, little is known about the fitness and stress response in non-O157 serotypes rendering it difficult to draw conclusions about their behavior under adverse conditions. 

Purpose: The objective of this study was to evaluate the impact of cold stress on the behavior of non-O157 VTEC on Romaine lettuce plants (16°C) and fresh-cut lettuce (4°C).  

Methods: In duplicate trials, nine week old Romaine lettuce (Lactuca sativa cv. Parris Island) plants (n = 3, average weight 145 g) were spot inoculated with 10⁹ CFU of VTEC serotypes O26, O44, , O103, O111, O145 and O157 and maintained at 16 + 2.0°C for 7 days in a controlled environment chamber.  Plants were then harvested and leaves were cut into 5 mm strips across the midrib and stored in PD961EZ film bags at 4°C for 14 d. Samples (10 g) were taken  on day 0, 3, 7, 10 and 14 and bacterial populations were determined by serial dilution plating on MacConkey agar.  

Results: On Romaine lettuce plants at 16°C, serotype O103 was the most resistant to cold stress with populations increasing 0.67 log CFU/g over 7 d. Survival was variable (P < 0.05) among serotypes on fresh-cut lettuce stored at 4°C with a significant decline (P < 0.05) in numbers observed in serotypes O26 and O145 with 3.27 and 3.84 log CFU/g recovered on day 14, respectively.  

Significance: Results indicate the variability in response to cold stress among VTEC serotypes. Specific information regarding the effects of cold stress on VTEC survival on leafy greens is essential to develop science-based food safety guidelines for the produce industry.