Identification of Acinetobacter baumannii Serum-Associated Antibiotic Efflux Pump Inhibitors

Adaptive antibiotic resistance is a newly described phenomenon by whichAcinetobacter baumannii induces efflux pump activity in response to host-associated environmental cues that may, in part, account for antibiotic treatment failures against clinically defined susceptible strains. To that end, during adaptation to growth in human serum, the organism induces approximately 22 putative efflux-associated genes and displays efflux-mediated minocycline tolerance at antibiotic concentrations corresponding to patient serum levels. Here, we show that in addition to minocycline, growth in human serum elicitsA. baumannii efflux-mediated tolerance to the antibiotics ciprofloxacin, meropenem, tetracycline, and tigecycline. Moreover, using a whole-cell high-throughput screen and secondary assays, we identified novel serum-associated antibiotic efflux inhibitors that potentiated the activities of antibiotics toward serum-grownA. baumannii . Two compounds,A cinetobacter b aumannii e ffluxp umpi nhibitor 1 (ABEPI1) [(E )-4-((4-chlorobenzylidene)amino)benezenesulfonamide] and ABEPI2 [N -tert-butyl-2-(1-tert-butyltetrazol-5-yl)sulfanylacetamide], were shown to lead to minocycline accumulation withinA. baumannii during serum growth and inhibit the efflux potential of the organism. While both compounds also inhibited the antibiotic efflux properties of the bacterial pathogenPseudomonas aeruginosa , they did not display significant cytotoxicity toward human cells or mammalian Ca2+ channel inhibitory effects, suggesting that ABEPI1 and ABEPI2 represent promising structural scaffolds for the development of new classes of bacterial antibiotic efflux pump inhibitors that can be used to potentiate the activities of current and future antibiotics for the therapeutic intervention of Gram-negative bacterial infections.