Membrane vesicles derived from Enterococcus faecalis promote the co-transfer of important antibiotic resistance genes located on both plasmids and chromosomes.

Background: Bacterial membrane vesicles (BMVs) are novel vehicles of antibiotic resistance gene (ARG) transfer in Gram-negative bacteria, but their role in the spread of ARGs in Gram-positive bacteria has not been defined. The purpose of this study was to evaluate the role of MVs in the transmission of antimicrobial resistance in Gram-positive bacteria.

Methods: A linezolid-resistant Enterococcus faecalis CQ20 of swine origin was selected as the donor strain. Linezolid-susceptible E. faecalis SC032 of human origin, Enterococcus faecium BM4105 and Escherichia coli were selected as recipient strains. The presence of plasmids (pCQ20-1 and pCQ20-2) and an optrA-carrying transposon Tn6674 in CQ20, MVs and vesiculants was verified by WGS or PCR. MVs were isolated with density gradient centrifugation, and MV-mediated transformation was performed to assess the horizontal transferability of MVs. The MICs for CQ20 and its vesiculants were determined by the broth microdilution method.

Results: CQ20-derived MVs (CQ20-MV) were isolated, and PCR identified the presence of two plasmids and the optrA gene in the CQ20-MVs. MV-mediated transformation to E. faecalis SC032 and E. faecium BM4105 was successfully performed, and the WGS data also showed that both plasmids pCQ20-1 and pCQ20-2 and optrA-carrying transposon Tn6674 were transferred to E. faecalis SC032 and E. faecium BM4105, but failed for E. coli. Additionally, vesiculants that had acquired ARGs still had the ability to spread these genes via MVs.

Conclusions: To our knowledge, this is the first report of MV-mediated co-transfer of ARG-carrying plasmids and transposons in the Gram-positive bacterium E. faecium.