Mycobacteria hit a limit on efflux-based drug resistance

Antibiotic resistance is a moving target, and one of the tools bacteria use to survive drugs are efflux pumps—molecular machines that push toxic compounds out of the cell. But these pumps come with a cost: when highly active they demand a lot of energy, and under normal conditions many remain transcriptionally quiet. Sarika Mehra and colleagues set out to understand just how far bacteria can push efflux-based defenses before physiology gets in the way. To do this they focused on mycobacteria, using Mycobacterium smegmatis mc²155 (Msm) as a model and also testing the findings in Mycobacterium tuberculosis H37Ra (Mtb). The team compared wild-type strains with strains lacking key pumps (Δ lfrA and Δ efpA) and then pushed these populations through long-term selection in the lab using the broad-spectrum efflux substrate ethidium bromide (EtBr). Their goal was to reveal the maximum level of resistance that can be achieved by ramping up efflux alone, and to see what cellular trade-offs accompany that process.

The researchers used adaptive laboratory evolution to expose Msm wild-type and efflux pump-deleted strains (Δ lfrA and Δ efpA) to ethidium bromide (EtBr) over prolonged periods. Despite continued selection for survival, resistance consistently stalled at a ceiling of roughly 8–16× MIC. Detailed analysis of evolved populations showed several coordinated changes: faster efflux kinetics and lower intracellular EtBr accumulation, together with mutations in global regulators, ribosomal genes, and membrane components. When lfrA was deleted (Δ lfrA), cells often compensated by upregulating alternative efflux systems, revealing redundancy among pumps. Beyond changes in efflux, evolved strains rewired their metabolism—downregulating the tricarboxylic acid cycle while boosting lipid biosynthesis and β-oxidation—which shifted NAD⁺/NADH homeostasis and altered membrane potential. Repeating the approach in Mycobacterium tuberculosis H37Ra (Mtb) produced similar constraints and metabolic changes, indicating these adaptive limits are conserved across mycobacterial species.

Taken together, these findings map a hard upper bound on how much resistance bacteria can gain by cranking up efflux alone. The plateau at 8–16× MIC shows that cellular physiology and global costs—energy demand, altered redox balance, and membrane energetics—restrict runaway efflux overexpression. The study also reveals that achieving this ceiling requires more than turning on pumps: bacteria must reprogram core metabolism and accept trade-offs in growth or energy use, and they can reroute regulation when particular pumps are lost. For clinicians and drug developers, the work suggests a new angle: instead of only trying to block efflux, it may be effective to target the broader regulatory and metabolic networks that make high-level efflux possible. By exploiting the vulnerabilities exposed when bacteria push their physiology to the limit, we may better limit the evolution and spread of multidrug resistance.