A nutrient-limited screen unmasks rifabutin hyperactivity for extensively drug-resistant Acinetobacter baumannii

Abstract

Industry screens of large chemical libraries traditionally have relied on rich media to ensure rapid bacterial growth in high-throughput testing. We used eukaryotic, nutrient-limited growth media in a compound screen that unmasked a previously unknown hyper-activity of the old antibiotic, rifabutin (RBT), against highly resistant Acinetobacter baumannii . In nutrient-limited, but not rich media, RBT was 200-fold more potent than rifampin (RIF). RBT was also substantially more effective in vivo . The mechanism of enhanced efficacy was a Trojan horse-like import of RBT but not RIF through FhuE , only in nutrient-limited conditions. These results are of fundamental importance to efforts to discover antibacterial agents.

Pharmacology, Dosing, and Side Effects of Rifabutin as a Possible Therapy for Antibiotic-Resistant Acinetobacter Infections

“…Doses >900 mg/d were not associated with further increases in Cmax, ranging from 0.4 µg/mL to 1 µg/mL [ 23 , 24 ]. Although its Cmax is low, the rifabutin MICs of most A. baumannii strains tested in nutrient-depleted media were <0.1 µg/mL [ 4 ], readily exceedable by standard rifabutin dosing.…”

“…We recently found that rifabutin possesses hitherto unknown hyperactivity against most strains of A. baumannii [ 4 ]. Rifabutin, which has been approved for use in humans for >30 years, has minimal inhibitory concentrations (MICs) against A. baumannii strains that are >50-fold below those of rifampin.…”

Recent Advances in Genetic Tools for Acinetobacter baumannii

“…Transposon mutant libraries demonstrate their versatility in the foundation of studies and as demonstrated by Luna et al (2020) can be repurposed years in the future to investigate and validate novel findings.…”

“…To ensure that unnecessary selection pressures are avoided, freezing of the pooled mutants must occur very shortly after selection ( Kazi et al, 2020 ). However, as research progresses, storing and exposing strains of interest to new conditions/stress allows investigation of novel effects as demonstrated for an XDR clinical lung isolate HUMC1 ( Luna et al, 2020 ). The authors repurposed a previously generated mutant library in AB5075 in order to validate results observed in HUMC1 ( Gallagher et al, 2015 ).…”

“…A notable hit, resulting in an increase in susceptibility to rifabutin, led to the investigation of the molecular mechanism in HUMC1 and also in AB5075. Transposon mutant libraries demonstrate their versatility in the foundation of studies and as demonstrated by Luna et al (2020) can be repurposed years in the future to investigate and validate novel findings.…”