Understanding Salmonella biofilm formation and sanitizer efficacy in the produce industry

Abstract

In recent years, Salmonella has caused multiple foodborne outbreaks linked to fresh produce. The ability of Salmonella to form biofilms is predicted to be a strategy to increase persistence and survival. When harvesting equipment is not effectively cleaned and sanitized, microorganisms can persist and potentially cross-contaminate produce. Preventing contamination during harvest is therefore important to ensure food safety. The objectives of this study were to: 1) study attachment properties of Salmonella biofilms on nylon, HDPE, and wood; and 2) evaluate the efficacy of five commercially available sanitizers to control Salmonella biofilms on experimentally inoculated coupons representative of harvesting bins and picking bags. For objective 1, multi-strain Salmonella biofilms were grown in a Centers for Disease Control and Prevention (CDC) biofilm reactor at 22 ± 2°C on nylon, HDPE, and wood coupons for up to 96-hour. Cell attachment and biofilm hydrophobicity were evaluated by a rinse assay and contact angle measurements, respectively. Laser Scanning Confocal Microscopy (LSCM) was utilized to visualize biofilm formation and architecture. Nylon and wood surfaces showed higher hydrophilic properties as compared to HDPE and resulted in greater Salmonella attachment (P<0.05). For objective 2, multi-strain Salmonella biofilms were grown on coupons following the protocol developed in objective 1. Biofilms were exposed to 500 ppm free chlorine, 500 ppm peroxyacetic acid, 75psi steam, 5% silver dihydrogen citrate (SDC) for 1 or 2 minutes and 100 ppm chlorine dioxide gas for 24-hour. After treatment, coupons were neutralized, and surviving cells were enumerated. Nylon, wood, and HDPE control coupons resulted in initial Salmonella biofilm populations of 9.59 ± 0.29, 9.34 ± 0.40, and 8.77 ± 0.34 log CFU/coupon, respectively. Overall sanitizer, surface type, and application time significantly (P<0.05) impacted the remaining Salmonella population. All treatments resulted in a significant reduction of Salmonella cells when compared to the control (P<0.05). Chlorine dioxide gas was the only sanitizer to completely inactivate Salmonella biofilm on all surfaces tested. Overall, the results obtained in this research highlight the importance of linking the physiochemical and attachment characteristics of biofilms with the antimicrobial properties of sanitizers in order to control Salmonella biofilm formation on harvesting equipment commonly used in the produce industry.