For the past decade fresh produce has consistently been at the top of the list of foods linked to outbreaks. Tomatoes, melons, leafy greens, fresh herbs and berries leading to illnesses all seem to make an appearance just about annually. Even though they aren’t really fresh produce, low moisture seeds and have also been in the game.

When it comes to production or minimally-processed linked outbreaks (like this, this and this) water is often fingered as a contamination factor. Either irrigation, wash or rain. barfblog friends and contributors Michelle Danyluk and Linda Harris co-authored some work pointing to wet orchards (from rain, a fairly uncommon event during almond harvest season) as a potential enabler for Salmonella migration through almond hulls and shells and into the kernel (the edible part).

In the most recent issue of Journal of Food Protection, rain enthusiast Michelle is at it again, coauthoring an investigation of the ability of rain to spread Salmonella Typhimurium from plastic mulch to a tomato plant.

Dispersal of Salmonella Typhimurium by rain splash onto tomato plants

07.mar.12

Journal of Food Protection, Volume 75, Number 3, March 2012 , pp. 472-479(8)

Cevallos-Cevallos, Juan M.; Danyluk, Michelle D.; Gu, Ganyu; Vallad, Gary E.; van Bruggen, Ariena H.C.

Abstract: Outbreaks of Salmonella enterica have increasingly been associated with tomatoes and traced back to production areas, but the spread of Salmonella from a point source onto plants has not been described. Splash dispersal by rain could be one means of dissemination. Green fluorescent protein-labeled, kanamycin-resistant Salmonella enterica sv. Typhimurium dispensed on the surface of plastic mulch, organic mulch, or soil at 108 CFU/cm2 was used as the point source in the center of a rain simulator. Tomato plants in soil with and without plastic or organic mulch were placed around the point source, and rain intensities of 60 and 110 mm/h were applied for 5, 10, 20, and 30 min. Dispersal of Salmonella followed a negative exponential model with a half distance of 3 cm at 110 mm/h. Dispersed Salmonella survived for 3 days on tomato leaflets, with a total decline of 5 log and an initial decimal reduction time of 10 h. Recovery of dispersed Salmonella from plants at the maximum observed distance ranged from 3 CFU/g of leaflet after a rain episode of 110 mm/h for 10 min on soil to 117 CFU/g of leaflet on plastic mulch. Dispersal of Salmonella on plants with and without mulch was significantly enhanced by increasing rain duration from 0 to 10 min, but dispersal was reduced when rainfall duration increased from 10 to 30 min. Salmonella may be dispersed by rain to contaminate tomato plants in the field, especially during rain events of 10 min and when plastic mulch is used.

I don’t read this as "don’t eat tomatoes grown on plastic mulch that were rained on for 10 min" but info like this could be important to outbreak investigators trying to link an outbreak to a cadre of causative events.