Quantification of Salmonella enterica transfer between tomatoes, soil, and plastic mulch

Tomatoes have been linked to Salmonella outbreaks in the United States (US). Plasticulture systems, that combine raised beds, plastic mulch, drip irrigation and fumigation, are common in commercial staked fresh tomato production in the US. The US FDA Produce Safety Rule prohibits the distribution of any produce covered by the rule (including fresh market tomatoes) that drops to the ground before harvest. This research was undertaken to better characterize the risks posed by tomatoes that touch plastic mulch or soil immediately before or during harvest. Research was conducted in three states (Florida, Maryland, and Ohio). Each state utilized tomatoes from their state at the point of harvest maturity most common in that state. Each state used indigenous soil and plastic mulch for transfer scenarios. New plastic mulch obtained directly from the application roll and used plastic mulch that had been present on beds for a growing season were evaluated. A five-strain cocktail of Salmonella enterica isolates obtained from tomato outbreaks was used. Mulch (new or used), soil, or tomatoes were spot inoculated with 100 μl of inoculum to obtain a final population of ~6 log CFU/surface. Items were either touched to each other immediately (1-2 s) after inoculation (wet contact) or allowed to dry at ambient temperature for 1 h or 24 h (dry contact). All surfaces remained in brief (1-5 s) or extended (24 h) contact at ambient temperature. Transfer of Salmonella between a tomato and plastic mulch or soil is dependent on contact time, dryness of the inoculum, type of soil, and contact surface. Transfer of Salmonella to and from the mulch and tomatoes for wet and 1 h dry inocula were similar with mean log % transfers varying from 0.7 ± 0.2 to 1.9 ± 0.1. The transfer of Salmonella between soil or plastic mulch to and from tomatoes was dependent on moisture with wet and 1 h dry inocula generally yielding significantly (p < 0.05) higher transfer than the 24 h dry inoculum. Results indicate that harvesting dry tomatoes significantly (p < 0.05) reduces the risk of contamination from soil or mulch contact. Transfer to tomatoes was generally significantly greater (p < 0.05) from new and used plastic mulch than from soil. If contamination and moisture levels are equivalent and contact times are equal to or <24 h before harvest, significantly (p < 0.05) more Salmonella transfers to tomatoes from mulch than from soil. Our findings support that harvesting tomatoes from soil has similar or lower risk than harvesting from plastic mulch.