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Conjugative plasmid transfer between Salmonella enterica Newport and Escherichia coli within the gastrointestinal tract of the lesser mealworm beetle, Alphitobius diaperinus (Coleoptera: Tenebrionidae)¹

USDA, Agricultural Research Service, Food and Feed Safety Research Unit, 2881 F&B Road, College Station, TX 77845

² Corresponding author: toni.poole{at}ars.usda.gov

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 x 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 x 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 x 10^–1. The average conjugation frequencies for the beetle gut were 2 logs higher than those for the filter conjugations 4.1 x 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.

Key Words: darkling beetle • Alphitobius diaperinus • antimicrobial resistance • poultry litter

¹ Proprietary or brand names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by USDA implies no approval of the product, or exclusion of others that may be suitable.