Purpose: The aim of this study was to compare survival of Salmonella mutant strains to wild types in soil, water, and on lettuce leaves.
Methods: Eight Salmonella Typhimurium strains (derived from ATCC 14028), including: a series of five Salmonella Pathogenicity Island (SPI) mutants, from SPI-1 to SPI 5; one strain which contains the entire series of SPI 1-5 mutations (potential avirulent surrogate); and two wild type strains, were evaluated for survivability. The eight strains were individually inoculated at ca. 7 log CFU into i) 250 g freshly collected soil samples; ii) 250 ml sterile EPA “worst-case” water samples; and iii) onto the mid-rib of fresh-cut romaine lettuce leaves (n = 6). Soil and water samples were incubated at 25°C; lettuce samples were incubated at 4 and 10°C. Salmonella populations were enumerated at 0, 1, 2, 5, 7, 14, and 21 days or 0, 1, 2, 5, and 7 days for lettuce at 10°C. Significant differences were determined using the Tukey-Kramer test.
Results: All strains declined significantly (ca. 1 to 4 log CFU/g reduction) in soil over 21 days (P < 0.05). Salmonella populations in soil at day 5 and 21 were significantly lower for the potential surrogate strain containing five SPI mutations than the two wild type strains. Salmonella populations in water and on lettuce leaves remained steady for most strains (ca. 6 log CFU), including the wild type strains and the potential surrogate strain containing five SPI mutations.
Significance: The strain of Salmonella with attenuated virulence (five SPI mutations) may be useful as a surrogate in future pilot plant production environment studies.