Conjugative plasmid transfer between Salmonella enterica Newport and Escherichia coli within the gastrointestinal tract of the lesser mealworm beetle, Alphitobius diaperinus (Coleoptera: Tenebrionidae)

Abstract

The objective of this study was to determine if conjugative transfer of antimicrobial resistance plasmids could occur between donor and recipient bacteria within the gastrointestinal tract of lesser meal-worm beetles, a common pest in poultry production facilities. In 3 replicate studies (n = 40 overall), beetles were allowed to feed for 2 h on brain heart infusion agar inoculated with a multidrug-resistant Salmonella enterica serotype Newport strain (SN11 that carried plasmid replicons A/C and N) at 1.0 × 108 cfu/mL. Beetles were surface-disinfected and allowed to feed for 16 h on brain heart infusion agar inoculated with nalidixic acid- and rifampicin-resistant Escherichia coli JM109 at 9.0 × 106 cfu/mL. After bacterial exposure, beetles were surface-disinfected, homogenized, and selectively plated for transconjugants. Serial dilutions were done for conjugation frequencies. In vitro filter conjugations were performed simultaneously with beetle conjugations. Transconjugants were produced in all beetles exposed to both donor and recipient bacteria. Ninety-five percent of the beetle and 100% of the in vitro filter transconjugants were positive for the N plasmid replicon. The A/C replicon, which was also detected in the SN11 donor strain, did not transfer in any of the conjugation studies. None of the transconjugants displayed resistance to extended-spectrum cephalosporins. The geometric mean conjugation frequency in the beetle gut was 1.07 × 10−1. The average conjugation frequencies for the beetle gut were 2 logs higher than those for the filter conjugations 4.1 × 10−3. This study demonstrates that horizontal transfer of antimicrobial resistance plasmids can occur between Salmonella and E. coli within the gut of beetles and that beetles may be used as an in vivo model to study resistance gene transfer.