Abstract
Outbreaks of foodborne illness attributed to the consumption of Salmonella-tainted cantaloupe have occurred repeatedly, but understanding of the ecology of Salmonella on cantaloupe fruit surfaces is limited. We investigated the interactions between Salmonella enterica Poona, the plant pathogenic bacterium Erwinia tracheiphila, and cantaloupe fruit. Fruit surfaces were inoculated at the natural cracking stage by spreading S. enterica and E. tracheiphila, 20 µl at 107 cfu/ml, independently or together, over a 2×2 cm rind area containing a crack. Microbial and microscopic analyses were performed at 0, 9 and 24 days post inoculation (DPI). Even at 24 DPI (fruit maturity) S. enterica was detected on 14% and 40% of the fruit inoculated with S. enterica alone and the two-pathogen mixture, respectively. However, the population of S. enterica declined gradually after initial inoculation. E. tracheiphila, inoculated alone or together with Salmonella, caused watersoaked lesions on cantaloupe fruit; but we could not conclude in this study that S. enterica survival on the fruit surface was enhanced by the presence of those lesions. Of fruit inoculated with E. tracheiphila alone and sampled at 24 DPI, 61% had watersoaked lesions on the surface. In nearly half of those symptomatic fruits the watersoaking extended into the sub-rind mesocarp, and E. tracheiphila was recovered from that tissue in 50% of the symptomatic fruit. In this work, E. tracheiphila internalized through natural cracks on developing fruits. S. enterica was never detected in the fruit interior (ca. 2–3 mm below rind surface) under the limited conditions of our experiments, but the possibility that it, or other human pathogens that contaminate fresh produce, might also do so should be investigated under a wider range of conditions and produce types.
Highlights
Incidences of the contamination of fresh fruits and vegetables with human pathogens and resulting foodborne illness outbreaks have been increasing in the United States and around the world [1,2,3]
Fruit appearance and symptom development Newly formed, healthy cantaloupe fruit were smooth-skinned with fine hairs, but at about 10–12 days of age small cracks appeared in the rind near the blossom end
After E. tracheiphila or E. tracheiphila + S. enterica inoculation of cantaloupe fruit rind, small watersoaked lesions appeared at the inoculated site within 4–7 days on 28 (58%) of the inoculated fruit
Summary
Incidences of the contamination of fresh fruits and vegetables with human pathogens and resulting foodborne illness outbreaks have been increasing in the United States and around the world [1,2,3]. Salmonella enterica, the causal agent of salmonellosis, is one of the most common human pathogenic bacteria contaminating fresh produce world-wide [2,4]. The first documented salmonellosis outbreak, caused by consumption of salad bar cantaloupes contaminated with S. enterica Chester in 1990, involved 245 reported cases in 30 U.S states [11]. Cantaloupe fruits have pronounced rind netting containing sheltered niches likely to harbor microorganisms and hamper effective sanitation [12,13,14,15]. As few as 150 bacteria cm on the rind surface can contaminate the edible mesocarp upon slicing [14]
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