Survival and growth of Salmonella baildon in shredded lettuce and diced tomatoes, and effectiveness of chlorinated water as a sanitizer

An outbreak of salmonellosis associated with diced tomatoes occurred in the United States in 1999. Experiments were done to determine the efficacy of chlorine in killing Salmonella baildon, the causative serotype, inoculated onto shredded lettuce and diced tomatoes, and to determine survival characteristics of the organism on these produce items stored at 4 degrees C for up to 12 days and on tomatoes stored at 21 or 30 degrees C for up to 72 h. Populations of S. baildon in lettuce and tomatoes (pH 4.51 +/- 0.02) inoculated with 3.60 log10 and 3.86 log10 cfu/g, respectively, were reduced by less than 1 log when the produce was immersed for 40 s in a 120 or 200 microgram/ml free chlorine solution. Produce inoculated with 0.60-0.86 log10 cfu/g was positive for the pathogen after treatment with 200 microgram/ml chlorine. Initial populations of 3.28 and 3.40 log10 cfu/g of lettuce and tomatoes, respectively, decreased by about 2 log10 cfu/g during storage for 12 days at 4 degrees C. One of six samples of lettuce initially containing 0.28 log10 cfu of S. baildon per gram was positive after storage for 12 days, but the pathogen was not detected in tomatoes analyzed within 15 min of inoculation with 0.40 log10 cfu/g. While the number of viable cells decreased during storage at 4 degrees C, initial populations of 0.28 log10 cfu/g of shredded lettuce and 3.40 log10 cfu/g of diced tomatoes are not reduced to undetectable levels during storage at 4 degrees C for 12 days. Tolerance of S. baildon to an acidic pH (4.5) was not influenced by the pH (4.5, 5.8, or 7.2) of the medium in which it was grown, suggesting that this strain possesses unusual resistance to acid pH. The pathogen grew in diced tomatoes (pH 4.40 +/- 0.01) from an initial population of 0.79 log10 cfu/g to 5.32 and 7.00 log10 cfu/g within 24 h at 21 and 30 degrees C, respectively.