Several Salmonella enterica outbreaks have been traced back to contaminated tomatoes. In this study, the internalization of S. enterica Typhimurium via tomato leaves was investigated as affected by surfactants and bacterial rdar morphotype, which was reported to be important for the environmental persistence and attachment of Salmonella to plants. Surfactants, especially Silwet L-77, promoted ingress and survival of S. enterica Typhimurium in tomato leaves. In each of two experiments, 84 tomato plants were inoculated two to four times before fruiting with GFP-labeled S. enterica Typhimurium strain MAE110 (with rdar morphotype) or MAE119 (without rdar). For each inoculation, single leaflets were dipped in 109 CFU/ml Salmonella suspension with Silwet L-77. Inoculated and adjacent leaflets were tested for Salmonella survival for 3 weeks after each inoculation. The surface and pulp of ripe fruits produced on these plants were also examined for Salmonella. Populations of both Salmonella strains in inoculated leaflets decreased during 2 weeks after inoculation but remained unchanged (at about 104 CFU/g) in week 3. Populations of MAE110 were significantly higher (P<0.05) than those of MAE119 from day 3 after inoculation. In the first year, nine fruits collected from one of the 42 MAE119 inoculated plants were positive for S. enterica Typhimurium. In the second year, Salmonella was detected in adjacent non-inoculated leaves of eight tomato plants (five inoculated with strain MAE110). The pulp of 12 fruits from two plants inoculated with MAE110 was Salmonella positive (about 106 CFU/g). Internalization was confirmed by fluorescence and confocal laser microscopy. For the first time, convincing evidence is presented that S. enterica can move inside tomato plants grown in natural field soil and colonize fruits at high levels without inducing any symptoms, except for a slight reduction in plant growth.

The survival on tomato fruits (Lycopersicum esculentum) of a rifampicin-resistant strain of Salmonella montevideo (Centers for Disease Control and Prevention [CDC] isolate G4639), the alleged source of the 1993 multistate outbreak of salmonellosis, was affected by inoculum dose and inoculation site (unbroken surface or wounds and stem scars), as well as by the medium (distilled water, Butterfield's buffer, or trypticase soy broth [TSB]) used to deliver the bacterium, This bacterium inoculated at 4 log10 CFU (colony forming units) per site in distilled water survived for 20 h on tomato skin. However, comparable survival occurred at the stem scars and growth cracks with smaller inoculum doses (3 log10 CFU). The bacterial populations increased rapidly on puncture wounds and tomato slices but decreased on the unbroken surface and stem scar. With unbroken skin and approximately 4 log10 CFU per site, the population survived for at least 48 h but could not be consistently detected after 5 days. By contrast, the stem scar population survived for at least 7 days and decreased only 1 to 2 log10 units. The inherently low pH of the tomatoes did not inhibit bacterial growth. Treatment with 100 ppm of aqueous chlorine for up to 2 min failed to kill all bacteria at these inoculation sites. This was especially true when the bacterial suspensions were prepared in TSB. TSB supported better bacterial survival and/or growth and also protected against the bactericidal effect of aqueous chlorine.

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The efficacy levels of different physical and chemical washing treatments in the reduction of viral and bacterial pathogens from inoculated strawberries were evaluated. Escherichia coli O157:H7, Salmonella Montevideo, poliovirus 1, and the bacteriophages PRD1, phiX174, and MS2 were used as model and surrogate organisms. Chemicals readily available to producers and/or consumers were evaluated as antimicrobial additives for the production of washes. The gentle agitation of contaminated strawberries in water for 2 min led to reductions in microbial populations ranging from 41 to 79% and from 62 to 90% at water temperatures of 22 and 43 degrees C, respectively. Significant reductions (> 98%) in numbers of bacteria and viruses were obtained with sodium hypochlorite (50 to 300 ppm of free chlorine), Oxine or Carnebon (200 ppm of product generating "stabilized chlorine dioxide"), Tsunami (100 ppm of peroxyacetic acid), and Alcide (100 or 200 ppm of acidified sodium chlorite) washes. Overall, 200 ppm of acidified sodium chlorite produced the greatest reductions of microorganisms. Hydrogen peroxide (0.5%) was slightly less effective than free chlorine in a strawberry wash and caused slight fruit discoloration. Cetylpyridinium chloride (0.1%) was effective in the reduction of bacterial species, while trisodium phosphate (1%) was effective against viruses. The consumer-oriented produce wash Fit was very effective (> 99%) in reducing the numbers of bacteria but not in reducing the numbers of viruses. Another wash, Healthy Harvest, was significantly less effective than Fit in reducing bacterial pathogens but more effective for viruses. The performance of automatic dishwashing detergent was similar to that of Healthy Harvest and significantly better than that of liquid dishwashing detergent. Solutions containing table salt (2% NaCl) or vinegar (10%) reduced the numbers of bacteria by about 90%, whereas only the vinegar wash reduced the numbers of viruses significantly (ca. 95%).

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