Purpose: To compare STEC serotypes and non-pathogenic E. coli in the extent of transfer and attachment to lettuce during washing.
Methods: Transfer of eight STEC strains (O157:H7, O104:H4, and the “Big Six” non-O157 STEC) and non-pathogenic E. coli Nissle 1917 (EcN) was compared in 30-ml washes of lettuce with or without chlorine. E. colipopulations remaining on inoculated leaves, in wash water, and transferred to un-inoculated leaves after a 1-min wash were enumerated by plate counts on selective agar in triplicate trials. An attachment assay was performed to determine the number of attached cells after repeated washes.
Results: During an un-chlorinated water wash, > 90% of inoculated cells was transferred (all strains). Transfer of EcN, O111, and O145 did not differ from that of O157:H7 (P > 0.05); but the other STEC strains showed less transfer than O157:H7 (P < 0.05). Approximately 2 - 3 log CFU transferred onto un-inoculated leaves. In a 1-ppm chlorine wash, EcN showed significantly more tolerance to the chlorine wash (P < 0.0001); more cross-contamination in wash water was detected for O111 (P = 0.0027), compared to O157:H7.
Preliminary attachment data showed that O157:H7 and O145 attached less than the other STEC strains, and EcN demonstrated stronger attachment than O157:H7 (P = 0.0057).
Significance: Data of STEC transfer and attachment during washing will be valuable for assessing cross-contamination risks in post-harvest washing of lettuce. These methods also will be useful for identifying potential STEC surrogates for process validation.