Acinetobacter baumannii is a nosocomial pathogen of serious healthcare concern that is becoming increasingly difficult to treat due to antibiotic treatment failure. Recent studies have revealed that clinically defined antibiotic-susceptible strains upregulate the expression of a repertoire of putative drug efflux pumps during their growth under biologically relevant conditions, e.g., in human serum, resulting in efflux-associated resistance to physiologically achievable antibiotic levels within a patient. This phenomenon, termed Adaptive Efflux Mediated Resistance (AEMR), has been hypothesized to account for one mechanism by which antibiotic-susceptible A. baumannii fails to respond to antibiotic treatment. In the current study, we sought to identify genetic determinants that contribute to A. baumannii serum-associated AEMR by screening a transposon mutant library for members that display a loss of the AEMR phenotype. Results revealed that mutation of a putative pirin-like protein, YhaK, results in a loss of AEMR, a phenotype that could be complemented by a wild-type copy of the yhaK gene and was verified in a second strain background. Ethidium bromide efflux assays confirmed that the loss of AEMR phenotype due to pirin-like protein mutation correlated with reduced overarching efflux capacity. Further, flow cytometry and confocal microscopy measures of a fluorophore 7-(dimethylamino)-coumarin-4-acetic acid (DMACA)-tagged levofloxacin isomer, ofloxacin, further verified that YhaK mutation reduces AEMR-mediated antibiotic efflux. RNA-sequencing studies revealed that YhaK may be required for the expression of multiple efflux-associated systems, including MATE and ABC families of efflux pumps. Collectively, the data indicate that the A. baumannii YhaK pirin-like protein plays a role in modulating the organism’s adaptive efflux-mediated resistance phenotype.

New antitubercular agents acting through previously unexploited mechanisms are urgently needed. Using a drug-repurposing platform, we identified TI-374, a hydroxamic acid containing compound that inhibits Mycobacterium tuberculosis ( Mtb ) with sub-micromolar potency. Systems analysis, resistance mapping, supplementation assays, and biochemical studies showed that TI-374 inhibits two PLP-dependent aminotransferases, AlaA and HisC1. However, its activity is driven primarily by irreversible inhibition of AlaA, whereas HisC1 inhibition is only partially reversible, revealing differential reversibility between the two targets. Optimization yielded TI-801, a low-nanomolar AlaA inhibitor. Both compounds remained active against intracellular Mtb in a macrophage infection model, where alanine supplementation did not rescue growth, indicating that host-derived alanine is unlikely to bypass AlaA inhibition. Genetic deletion of alaA attenuated Mtb survival in a murine infection model. Together, these findings support AlaA as a host-relevant metabolic vulnerability in Mtb and TI-801 as a mechanistic chemical probe for its validation as an antitubercular target.