Secondary Transfer and Expression of vanA in Enterococcus faecium Derived from a Commensal Vancomycin-Susceptible Enterococcus faecium Multi-Component Food Isolate

Abstract

We investigated the potential for vancomycin-susceptible Enterococcus (VSE) from multi-component salads to disseminate vanA from four clinical vancomycin-resistant enterococci to 14 streptogramin-resistant enterococci (SRE) of food and animal origin. Strains were selected from a previous study based on the presence of streptogramin susceptibility and/or vanA, vanB, vatD, vatE, agg, cpd, and gelE genes. Transconjugants were isolated on brain heart infusion agar containing vancomycin and selective antibiotics. Thirty-nine matings using a 1:10 donor-recipient ratio for filter and broth methods resulted in transfer of vanA between an agg⁺cpd⁺gelE⁺ Enterococcus faecalis donor and an agg⁻cpd⁻gelE⁻ streptogramin-susceptible Enterococcus faecium salad recipient at a frequency of 10⁻⁸ per recipient by filter method. Secondary mating of the transconjugant with SR/VSE strains resulted in a two- to four-log-fold greater frequency of transfer. Reverse transcription–polymerase chain reaction revealed vanA RNA products in the transconjugant cultivated in nutrient broth and salad at 37°C in the presence and absence of recipient filtrate. This study demonstrated that native salad VSE disseminated vanA to SRE carrying agg, cpd, and/or gelE. An increase in transfer efficiency resulted from secondary conjugation using the native vancomycin- and streptogramin-susceptible salad strain as the donor.