On the Offensive: the Role of Outer Membrane Vesicles in the Successful Dissemination of New Delhi Metallo-β-lactamase (NDM-1)

The emergence and worldwide dissemination of carbapenemase-producing Gram-negative bacteria are a major public health threat. Metallo-beta-lactamases (MBLs) represent the largest family of carbapenemases. Regrettably, these resistance determinants are spreading worldwide. Among them, the New Delhi metallo-beta-lactamase (NDM-1) is experiencing the fastest and largest geographical spread. NDM-1 beta-lactamase is anchored to the bacterial outer membrane, while most MBLs are soluble, periplasmic enzymes. This unique cellular localization favors the selective secretion of active NDM-1 into outer membrane vesicles (OMVs). Here, we advance the idea that NDM-containing vesicles serve as vehicles for the local dissemination of NDM-1. We show that OMVs with NDM-1 can protect a carbapenem-susceptible strain of Escherichia coli upon treatment with meropenem in a Galleria mellonella infection model. Survival curves of G. mellonella revealed that vesicle encapsulation enhances the action of NDM-1, prolonging and favoring bacterial protection against meropenem inside the larva hemolymph. We also demonstrate that E. coil cells expressing NDM-1 protect a susceptible Pseudomonas aeruginosa strain within the larvae in the presence of meropenem. By using E. coil variants engineered to secrete variable amounts of NDM-1, we demonstrate that the protective effect correlates with the amount of NDM-1 secreted into vesicles. We conclude that secretion of NDM-1 into OMVs contributes to the survival of otherwise susceptible nearby bacteria at infection sites. These results disclose that OMVs play a role in the establishment of bacterial communities, in addition to traditional horizontal gene transfer mechanisms. IMPORTANCE Resistance to carbapenems, last-resort antibiotics, is spreading worldwide, raising great concern. NDM-1 is one of the most potent and widely disseminated carbapenem-hydrolyzing enzymes spread among many bacteria and is secreted to the extracellular medium within outer membrane vesicles. We show that vesicles carrying NDM-1 can protect carbapenem-susceptible strains of E. coil and P. aeruginosa upon treatment with meropenem in a live infection model. These vesicles act as nanoparticles that encapsulate and transport NDM-1, prolonging and favoring its action against meropenem inside a living organism. Secretion of NDM-1 into vesicles contributes to the survival of otherwise susceptible nearby bacteria at infection sites. We propose that vesicles play a role in the establishment of bacterial communities and the dissemination of antibiotic resistance, in addition to traditional horizontal gene transfer mechanisms.

Automated summary

The emergence and dissemination of carbapenemase-producing Gram-negative bacteria pose a major public health threat worldwide. NDM-1, a potent carbapenem-hydrolyzing enzyme, is rapidly spreading and evolving with the ability to be secreted into outer membrane vesicles (OMVs). These OMVs act as vehicles for local dissemination, protecting susceptible bacteria against meropenem. The protective effect correlates with the amount of NDM-1 secreted into vesicles, suggesting a role in bacterial community establishment and antibiotic resistance dissemination.