oprM

OprM is a bacterial outer membrane protein that functions as the outer membrane component of several tripartite efflux pumps in Gram-negative bacteria, most notably MexAB-OprM in Pseudomonas aeruginosa. It is homologous to other outer membrane factors such as TolC and can interact with multiple inner membrane transporters and periplasmic adaptor proteins to form a continuous conduit that expels toxic compounds from the cell.

In the MexAB-OprM system, OprM forms a trimeric channel that spans the outer membrane and pairs with the inner membrane transporter MexB and the periplasmic adaptor MexA to create a pump that exports a wide range of substrates. This tripartite assembly effectively bridges the inner membrane efflux machinery to the outside, contributing to the organism’s intrinsic and acquired resistance to diverse antibiotics, dyes, and detergents.

Regulation of OprM-mediated efflux is part of broader control of the MexAB-OprM system. Expression of the pump components can be modulated by local regulators and global stress responses; overexpression increases resistance, while loss or mutation of components can restore susceptibility. Because OprM enables extrusion of many drug classes, its activity is a key factor in the multi-drug resistance phenotype of P. aeruginosa and related pathogens.

Structurally, OprM is a beta-barrel outer membrane protein that oligomerizes into a trimer to form the outer membrane channel. Its role as a gateway for efflux pumps makes it a significant factor in understanding and addressing antibiotic resistance in Gram-negative bacteria.