Purpose: The objective of this study was to determine how curli affect cross-contamination of E. coli during washing from inoculated to non-inoculated lettuce pieces considering different inoculation locations.
Methods: Overnight cultures of E. coli O157:H7 strain 0018 (± for curli) were centrifuged and resuspended in peptone water. A single piece of baby romaine lettuce was spot inoculated either on the surface, freshly cut edges, or both the surface and the edge with approximately six log CFU. Leaves were dried for 2 hours, and washed with 10 uninoculated pieces in a stainless-steel bowl for 30 seconds. Bacterial reduction and cross-contamination were quantified. Data were plotted and analyzed using Microsoft Excel and Statplus.
Results: Fewer curli producing E. coli transferred to water than non-curli producing E. coli (p<0.05). The percent transfer to water ranged between 80-99% when E. coli was inoculated on the lettuce surface and 24-80% when the cut-edges were inoculated. Log reduction from washing was higher for the non-curli producing E. coli and significantly different when the lettuce surface was inoculated (p=0.01). There was no significant difference in washing log reduction for curli and non-curli producers when the cut-edges were inoculated (p=0.42). The transfer to uninoculated pieces was significantly less for the non-curli strain than curli producing strain, regardless of the inoculation location (p<0.01).
Significance: The E. coli O157:H7 strain lacking curli more readily transferred to the water, but appeared to lack the ability to readily reattach to lettuce leaves. These findings suggest that targeting curli might be an effective means of combating E. coli transfer during washing of leafy greens.