Attachment and survival of Salmonella enterica and Listeria monocytogenes on tomatoes (Solanum lycopersicum) as affected by relative humidity, temperature and time of storage

Tomatoes (Solanum lycopersicum) are one of the most consumed fruits worldwide. The fruit can become contaminated with Salmonella and Listeria monocytogenes at different stages of the production and supply chain, and these pathogens may survive under different storage conditions. The effect of relative humidity, temperature and time of storage on the attachment and survival characteristics of both pathogens on the surface of tomatoes was investigated. Fresh whole Roma tomatoes were inoculated with a cocktail of Salmonella or L. monocytogenes and stored at 5, 12, 25, 30, or 35 ºC for 10 days. Every day during storage, relative humidity and temperature were measured and tomatoes were removed to enumerate cells of these pathogens that were loosely attached (LA, cells detached from the tomato surface by simple rinse) and strongly attached (SA, cells that required sonication to detach. Enumeration was achieved by spread-plate method. Surviving populations of LA and SA cells were obtained and the attachment strength (SR) was calculated to express the proportion of SA cells on the tomato surface. The LA initial counts of Salmonella and L. monocytogenes on the surface of the fruit after inoculation were 6.6 and 6.5 log CFU/tomato, and for SA were 5.1 and 5.6 log CFU/tomato, respectively. For both pathogens, the LA counts were higher (p < 0.05) than the SA counts. Also, the LA and SA counts varied significantly as a function of temperature, relative humidity and time of storage. The S R for Salmonella was affected by the time but not the temperature of storage, while the S R for L. monocytogenes was affected by the temperature, relative humidity and time of storage (p < 0.05). Understanding the attachment and survival of Salmonella and L. monocytogenes on tomatoes stored at different temperature conditions may be useful in determining ways to prevent/reduce the establishment of pathogens and designing improved decontamination methods for tomatoes.