Survival, growth, and potential of horizontal gene transfer of antibiotic resistance genes of bovine Salmonella on the cantaloupe surface.

Abstract

Salmonella outbreaks are often linked to contaminated cantaloupes, and bacterial multiple antibiotic resistance is frequently observed. Research is needed to understand the growth and survival of Salmonella on cantaloupe surfaces, and the mechanisms of antibiotic resistance in that environment. This study investigated the growth, survival, and capacity for horizontal gene transfer of antibiotic resistance genes in Salmonella on the surface of cantaloupe. Bovine-derived Salmonella isolates were inoculated onto the surface of cantaloupe and incubated at 35°C under 65-75% humidity for 5 days. Each isolate was co-inoculated with donor E. coli CSH56 harboring helper plasmid RP4-8 and mobilizable plasmid pSC27, which carries kanamycin, neomycin, and ampicillin resistance genes. One hundred microliters of the Salmonella suspension (1 × 10⁸ CFU/mL) and 100 µL of the E. coli suspension (1 × 10⁷ CFU/mL) was applied to each cantaloupe (1:10 ratio). Salmonella survival and growth were monitored on Hektoen agar, transconjugants were enumerated on Hektoen agar with kanamycin, and donor E. coli were quantified on MacConkey agar. All Salmonella isolates survived and exhibited growth within the 5-day period, with increases ranging from 0.94 log₁₀ CFU/mL to 3.43 log₁₀ CFU/mL. Notably, all Salmonella isolates possess the capability to accept and integrate sequences derived from the pSC27 plasmid from E. coli into their chromosome, both in vitro and on cantaloupe. After conjugation, the count of Salmonella mutants carrying the plasmid was 2.79 log₁₀ CFU/mL in vitro. The survival, growth, and horizontal gene transfer of Salmonella on the surface of cantaloupe pose significant risks to public health.IMPORTANCESalmonella, a leading cause of foodborne diseases, poses a global health threat, with multidrug resistance emerging as a severe problem. Cantaloupes have been implicated in salmonellosis outbreaks in multiple countries over the last three decades. We hypothesize that acquisition of antibiotic resistance genes from other bacteria may occur on the surface of the fruit. This study established that Salmonella can survive, proliferate, and acquire antibiotic resistance genes on cantaloupe surfaces. This observation underscores the potential risks on the growth and transfer of genes between pathogens such as Salmonella and E. coli in foods. This impact goes beyond disease production and contributes to the rapid escalation of antimicrobial resistance and transmission of virulence genes in foods and the environment, which is a significant threat to public health.