The April 2008 issue of Journal of Food Protection contains a cool paper on a survey of Salmonella and E. coli at a greenhouse tomato farm in Mexico. During 2003 and 2004 the authors sampled over 1600 product and environmental samples, before, during and after a couple of environmental disturbances: a flood and the entry of wild animals (opossums, mice and sparrows).

The authors isolated Salmonella Montevideo, Salmonella Newport, and strains of the F serogroup  from tomatoes and go on to state that almost all of the Salmonella Newport strains were isolated from samples collected during or immediately after the flood.

Analysis by pulsed-field gel electrophoresis revealed that some Salmonella Montevideo isolates from tomatoes, opossums, and mice displayed identical genetic patterns, suggesting that these wild animals represented a potential source of contamination.

The fun part of paper is that the authors suggest that dirty work shoes were also thought to be an important vehicle for dissemination of Salmonella into (and possibly throughout) the greenhouses (especially after being worn during the flood incident):

Contaminated worker shoes may be vehicles for contamination with enteric pathogens, from either outside the greenhouses or from one facility to another. The levels of E. coli on personal shoes were higher than those of working  shoes were before the flood. However, there was a higher  level of contamination with Salmonella and E. coli on  working shoes compared with personal shoes after the flood.

The authors go on to say that sanitary mats intended to reduce pathogen movement may not be all that effective the real-world application:

Working shoes were provided by management to the workers to wear inside the greenhouse at the suggestion of our research group after finding that personal shoes were positive for E. coli, even after shoes received a disinfection treatment with quaternary salts solution (800 ppm) on a sanitary mat. However, working shoes were not used exclusively inside the greenhouse, but were also worn to go from one facility to another. Shoes have seldom been mentioned as vehicles of contamination in food production areas. This dissemination mechanism of enteric pathogens should be considered as an important control point  during working procedures in greenhouses.

It’s unclear whether this is just a notable finding, or if it represents a real risk in moving pathogens around food production systems, and needs some further investigation.  Probably don’t want to use boots to stomp garlic though.