Persistence of non-O157 Shiga-Toxigenic <i>Escherichia coli</i> on Equipment Surfaces and Fresh Produce

Fresh produce has been associated with non-O157 Shiga toxin-producing <i>Escherichia coli</i> (STEC) outbreaks. Biofilm formed by non-O157 STEC on the equipment surfaces are resistant to disinfectants and could contaminate food. The purpose of this study was to evaluate the biofilm formation ability of non-O157 STEC on equipment surfaces. Further, the persistence of non-O157 STEC on fresh produce surfaces was investigated. Twelve non-O157 serotypes (O26, O45, O103, O111, O121, O145) were evaluated for biofilm formation on equipment surfaces (polytetrafluoroethylene [PTFE], polycarbonate [PC], and Stainless Steel [SS]) using center for disease control (CDC) reactor for a 48 h incubation. The attachment and persistence to fresh produce (cabbage, Romaine lettuce, spinach) leaves were studied by inoculating disk-shaped fresh produce pieces (22-mm dia) with 50 µL (10 micro-droplets of 5 µL each) of actively growing cultures with initial populations of ∼6.5 log CFU/cm², followed by storage 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. Biofilm formation of non-O157 STEC on equipment surfaces varied with surface and strains. In general, lower non-O157 STEC populations were recovered from SS surfaces than from PTFE or PC surfaces with SS showing a 0.1-0.7 log CFU lower recovery compared with PC and a 0.3-0.7 log CFU lower compared with PTFE. The persistence of non-O157 serotypes varied depending on the produce surface, incubation temperature, and the specific STEC serotype. Recovery of attached STEC was -0.5 to 1.2 log CFU/cm² lower from their initial populations (∼6 log CFU/cm²) when fresh produce was incubated for 24-48 h. Significant reductions (∼2.0 log CFU) in STEC populations were observed when spinach was incubated at 22°C for 48 h. Bacterial attachment on produce surface is quite a complex phenomenon, and its knowledge will be helpful in the evaluation of contamination risk during processing and storage of fresh produce.