Evaluating the Cross-Contamination Risks of Salmonella and Generic Escherichia coli on Agricultural Ground Covers in Produce Pre-Harvest Production

The US Food and Drug Administration Food Safety Modernization Act (FSMA) Produce Safety Rule (PSR) prohibits the harvest of dropped fruits and vegetables due to potential microbial contamination. Under the FSMA PSR dropped produce includes (i) produce that has detached from the parent plant and unintentionally contacts the ground and (ii) produce that is attached to the parent plant and unintentionally contacts the ground. Due to the benefits of plant growth and pest management, agricultural ground covers are a common horticultural practice implemented in the fresh produce production environment and produce may come into contact with these ground covers. Thus, this thesis aims to (i) quantify the survival of bacteria on different agricultural ground cover types and in different production environments and (ii) evaluate the cross-contamination risk of mulch to fresh produce from different drop heights and contact times. A seven-strain Salmonella cocktail was spot inoculated on coupons of biodegradable mulch, landscape fabric, and plastic mulch, and held in a growth chamber (23°C, 55% RH). At 0, 0.06, 0.17, 1, 2, 3, 5, 7, 30, 60, 90, and 140 days post-inoculation (dpi), coupons were enumerated for Salmonella. Coupons of plastic mulch were also spot inoculated with a green-fluorescent protein-tagged generic Escherichia coli and held in a growth chamber, greenhouse, and field environment for enumeration at 0, 0.06, 0.17, 0.41, 1, 2, 3, 5, and 7 dpi. Fresh cucumber, jalapeño, and tomato were dropped from 0, 1, 2, 4, and 6 ft using height-modified PVC (polyvinyl chloride) pipes onto generic E. coli inoculated plastic mulch, as well as tomato onto inoculated biodegradable mulch. Produce samples were enumerated after 3 s of mulch contact. Fresh cucumber, jalapeño, and tomato were also grown in contact with generic E. coli inoculated plastic mulch for 0, 1, 3, 5, and 7 days post-placement in the field. Salmonella survived on all ground covers for up to 140 dpi in the growth chamber. From 0 to 30 dpi, biodegradable mulch had the lowest Salmonella reduction, followed by landscape fabric and then plastic mulch (P < 0.05). No significant differences in ground cover type and Salmonella reduction were observed at 90 dpi (P > 0.05). Plastic mulch had the highest reduction of generic E. coli in the field followed by the greenhouse and growth chamber over 7 dpi (P < 0.05) with field and greenhouse coupons achieving approximately a 6-log reduction by 0.17 and 7 dpi, respectively. Ground cover type and environment impacted bacterial survival and highlighted the importance of growing location in risk management. Cucumber and tomato samples dropped from 4 (33%; 17%) and 6 (100%; 43%) ft were damaged, respectively. In general, generic E. coli transferred to the tested commodities regardless of drop height or contact time. These findings support that dropped produce should not be harvested due to potential damage and when surfaces were contaminated, transfer was likely to occur. Similarly, if surfaces were contaminated, regardless of contact time (0, 1, 3, 5, and 7 d), transfer was likely to occur indicating cross-contamination poses a food safety risk despite unintentional or intentional ground contact. Food safety efforts should focus on minimizing visible contamination, as outlined in the FSMA PSR, that may contaminate fresh produce in the production environment. Growing produce in contact with the ground alone may not be the sole factor in the contamination of fresh produce, as a contamination event is needed.