Outer membrane vesicles of carbapenem-resistant clinical Acinetobacter baumannii isolates protect both the vesicle-producing bacteria and non-resistant bacteria against carbapenems

Infections caused by carbapenem-resistant Acinetobacter baumannii (A. baumannii; CRAb) are associated with high patient morbidity and mortality. The serious threat for human health imposed by CRAb was recently underscored by identification of close-to-untouchable carbapenem- and tetracycline-resistant isolates. Since outer membrane vesicles (OMVs) of Gram-negative bacteria may contribute to antimicrobial resistance, our present study was aimed at investigating OMVs produced by the first two carbapenem- and tetracycline-resistant A. baumannii isolates in Europe. These isolates, denoted CRAb1 and CRAb2, contain large, nearly identical plasmids that specify multiple resistances. Both isolates produce OMVs that were analyzed by differential light scattering, transmission electron microscopy and proteomics. By comparison with OMVs from the plasmid-free non-carbapenem-resistant A. baumannii isolate Ab1, which is an isogenic ancestor of the CRAb1 isolate, we show that plasmid carriage by the CRAb1 and CRAb2 isolates leads to an increased OMV size that is accompanied by increased diversity of the OMV proteome. Our analyses show that OMVs from CRAb1 and CRAb2 are major reservoirs of proteins involved in antimicrobial resistance, including the plasmid-encoded carbapenemases New Delhi metallo-β-lactamase-1 (NDM-1), and carbapenem-hydrolyzing oxacillinase OXA-97 (OXA-97). Here we report that these OMV-borne carbapenemases hydrolyze imipenem and protect otherwise carbapenem-sensitive A. baumannii and Escherichia coli (E. coli) isolates against this antibiotic. In conclusion, our findings demonstrate that OMVs from highly drug-resistant CRAb confer protection against last-resort antibiotics to non-resistant bacterial pathogens.