efflux pump
An efflux pump is a protein in a microbe's membrane that grabs drug molecules and shoves them back out before they can reach a harmful concentration inside the cell. Picture a tireless bouncer at the cell's door, throwing antibiotics out as fast as they get in.
These pumps are membrane transporters powered by cellular energy — often the proton gradient or ATP — and they actively move a drug against its concentration gradient. Some are narrow, expelling one drug class; the most troubling, like the RND-family pumps of Gram-negative bacteria, are remarkably promiscuous, exporting many unrelated antibiotics, biocides, and dyes at once, which produces multidrug resistance from a single mechanism.
Efflux is a major reason Gram-negative bacteria are so hard to treat, and overexpressing a pump can raise a drug's MIC enough to cross from susceptible to resistant. Researchers are developing efflux-pump inhibitors to restore drug activity, but achieving selectivity and safety has been difficult, so for now the honest state is that no efflux inhibitor has reached the clinic.
In Pseudomonas aeruginosa, upregulated RND efflux pumps export multiple antibiotic classes at once, a key driver of its notorious multidrug resistance.
One promiscuous pump can confer resistance to many drugs at once.
Efflux pumps are not unique to microbes: human cells use related transporters (like P-glycoprotein) that pump drugs out and shape oral absorption and drug resistance in cancer.