Purpose: This study quantifies cross contamination by Enterobacter aerogenes, avirulent E. coli O157:H7, pathogenic E. coli O157:H7 during lettuce washing.
Methods: The experimental variables were post-inoculation drying time (10 min vs. 2 h), water volume (100 ml vs. 1L), and ratio of inoculated to non-inoculated lettuce pieces (1:5 vs. 1:20). Overnight cultures were centrifuged and resuspended in peptone water prior to inoculation. A single piece of baby romaine lettuce was spot inoculated with ~ 6 log CFU of bacteria, dried, and washed with the non-inoculated pieces in a stainless-steel bowl for 30 seconds. Bacterial reduction and cross-contamination were quantified. Data were analyzed using ANOVA in Microsoft Excel.
Results: In every case, most (~99%) of the bacterial contamination was transferred to the water, and ~1% of the inoculated bacteria transferred to the non-inoculated pieces. Water volume (P=0.02) and drying time (P<0.0001) had a significant effect of log-reduction. The ratio of inoculated to non-inoculated leaves and organism type did not have a significant effect of log reduction (P>0.05). Water volume (P<0.001), drying time (P=0.04) had a significant effect on log % transfer. Ratio of inoculated to non-inoculated lettuce did not have a significant influence on log % transfer (P=0.12). E. aerogenes showed significantly different transfer than pathogenic E. coli O157:H7 (P<0.001) but avirulent E. coli O157:H7 transfer results were not significantly different than pathogenic E. coli O157:H7 (P=0.1).
Significance: These results show that avirulent E. coli O157:H7 can be used as surrogate for pathogenic E. coli O157and that water volume and drying time are important variables to consider for leafy green cross-contamination research.