Salmonella enterica colonization and fitness in pre-harvest cantaloupe production

Cantaloupes have emerged as significant vehicles of widespread foodborne illness outbreaks caused by bacterial pathogens, including Salmonella. The purpose of this study was to investigate the efficiency of Salmonella colonization and internalization in cantaloupes by relevant routes of contamination. Cantaloupe plants (Cucumis melo 'reticulatus') from two cultivars 'Athena' (Eastern) and 'Primo' (Western) were grown from commercial seed. Plants were maintained in the NCSU BSL-3P phytotron greenhouse. Salmonella enterica (a cocktail of cantaloupe associated outbreak serovars Javiana, Newport, Panama, Poona and Typhimurium) contamination was introduced via blossoms or soil at ca. 4.4 log(10) CFU/blossom or 8.4 log(10) CFU/root zone, respectively. Cantaloupes were analyzed for Salmonella by enrichment in accordance with modified FDA-BAM methods. Five randomly chosen colonies from each Salmonella-positive sample were typed using the Agilent 2100 bioanalyzer following multiplex PCR. Data were analyzed for prevalence of contamination and serovar predominance in fruit, stems and soil. Of the total cantaloupe fruit harvested from Salmonella-inoculated blossoms (n = 63), 89% (56/63) were externally contaminated and 73% (46/63) had Salmonella internalized into the fruit. Serovar Panama was the most commonly isolated from the surface of fruit while S. Panama and S. Poona were the most prevalent inside the fruit. When soil was inoculated with Salmonella at one day post-transplant, 13% (8/60) of the plants were shown to translocate the organism to the lower stem (ca. 4 cm) by 7 days post-inoculation (dpi). We observed Salmonella persistence in the soil up to 60 dpi with S. Newport being the predominant serovar at 10 and 20 dpi. These data demonstrate that contaminated soil and blossoms can lead to Salmonella internalization into the plant or fruit at a relatively high frequency.