Role of airborne soil particulate in transfer of Salmonella spp. to tomato blossoms and consequential fruit contamination

Contaminated tomatoes have become a commonly implicated vehicle of Salmonella outbreaks. Exposure of tomatoes to pathogen could occur in the field. Blossom inoculation with Salmonella can result in contaminated fruit but natural routes of blossom contamination are not well known. Salmonellae have been known to survive in agricultural soil. Since dispersal of soil particulate by wind is a common phenomenon, the potential of airborne soil particulate as a vehicle of Salmonella contamination in tomato blossoms was examined.

It was determined that Salmonella enterica serotype Anatum, Baildon, Braenderup, Montevideo, Newport, Javiana had similar survival patterns in both soil and water. At the end of 40 days, populations of Salmonella in soil dropped by 2.59 log CFU/g and 5.11 log CFU/g when enumerated on Tryptic Soy Agar Yeast Extract (TSAYE) and xylose lysine Tergitol 4 (XLT4) agar respectively. Salmonella populations in water reduced by 2.55 log CFU/ml (TSAYE, enumeration) and 2.94 log CFU/ml (XLT4, enumeration). Blossom to fruit formation takes 20-30 days in tomatoes hence the introduction or presence of the pathogen in agricultural soil and water could increase risk of blossom contamination. Also, it was determined that all Salmonella serotypes tested were capable of biofilm production on glass coverslips and quartz particles. Biofilm based attachment of Salmonella to sand might aid in its dispersal.

To visualize transfer of pathogen from soil particulate to blossom in real-time, bioluminescent S. Baildon, S. Braenderup, S. Newport, S. Javiana and S. Anatum were created.Heat shock procedure was developed to improve electrotransformation efficiency in Salmonella. Transformed strains were compared for bioluminescence production and plasmid stability. S. Newport had the best bioluminescence properties but no difference was observed between strains for plasmid stability.

Imaging of soil particulate - S. Newport mixture inoculated blossoms, indicated that the event led to pathogen transfer to blossom. It was also determined that S. Newport â soil particulate contaminated blossoms developed into fruits that were positive for S. Newport. S. Newport presence in blossom, fruit surface and internal tissue indicates that contaminated soil particulate could serve as a vehicle of tomato contamination.