Characterization of outer membrane vesicles from Acinetobacter haemolyticus and their role in the dissemination of carbapenem resistance

Species outside the Acinetobacter calcoaceticus–baumannii (Acb) complex have recently emerged as new clinically relevant carbapenem-resistant pathogens, such as A. haemolyticus. A new mechanism that could contribute to the dissemination of this resistance is the secretion of outer membrane vesicles (OMVs). We investigate OMVs secreted by A. haemolyticus AN54, a Mexican clinical strain carrying a plasmid-borne blaNDM−1 gene and by its plasmid-cured derivative AN54Δe. Active secretion of OMVs formed by a lipid membrane, measuring 10–50 nm in both strains, was observed by TEM. The OMVs from AN54 possess hydrolytic capacity against carbapenem. The OMV protein profile varied depending on the antibiotic concentration during induction, with additional protein bands detected at 8 and 32 µg/ml of antibiotic exposure. Mass spectrometry identified proteins involved in different metabolic pathways and in resistance-related processes. The porins, efflux pumps, and ADC enzyme were more abundant in the OMVs from AN54Δe than in those from AN54 harboring the active NDM-1 enzyme. The OMVs protected the sensitive strains from the same and different genera against the antibiotic action for a limited period. However, the OMVs transferred the blaNDM−1-carrying plasmid (pAhaeAN54e) only to the AN54Δe strain, conferring permanent resistance to carbapenems. Together, these findings suggest that OMVs can help protect surrounding bacteria from antibiotic action and serve as vehicles for disseminating resistance genes. These results show the importance of studying OMVs as a novel mechanism for the dissemination of antimicrobial resistance.