Purpose: This study evaluated the persistence of nonO157 STEC on fresh produce surfaces.
Methods: Twelve nonO157 serotypes (O26, O45, O103, O111, O121, O145) were evaluated for curli expression, hydrophobicity, and attachment to fresh produce (cabbage, romaine lettuce, spinach) leaves. Fresh produce leaves were cut to disc shaped pieces (22 mm diameter) and inoculated with 50 µl (10 micro-droplets of 5 µl each) of actively growing cultures. Inoculated fresh produce discs were incubated at 4, 10, and 22°C for 48 h and bacterial populations were analyzed at 0, 1, 4, 24, and 48 h by spiral plating on selective media. Curli expression and cell hydrophobicity was analyzed by Congo red (CR) dye and bacterial attachment to hydrocarbon (BATH) assay, respectively.
Results: Persistence of nonO157 serotypes varied with produce surface and incubation temperature. Recovery of attached STEC was two to three log CFU/g lower from their initial populations (~6.5 log CFU/g) when fresh produce was incubated for 24 to 48 h. Significant reductions in STEC populations were observed when spinach was incubated at 22°C for 48 h. Escherichia coli O26:H11 strain 5711 was recovered at significantly higher levels than E. coli O121:H19 strain 5705 on cabbage and romaine lettuce, following 48 h incubation at 4°C. In general, the curli-expressing ability of STEC strains did not influence bacterial attachment.
Significance: Persistence of nonO157 serotypes varied among STEC serotypes, produce surface, and incubation temperature. Bacterial attachment on produce surface was a complex phenomenon and knowledge regarding attachment will be helpful in the evaluation of contamination risk during processing and storage of fresh produce.