Orientia tsutsugamushi Is Highly Susceptible to the RNA Polymerase Switch Region Inhibitor Corallopyronin A In Vitro and In Vivo

Scrub typhus is a potentially lethal infection caused by the obligate intracellular bacteriumOrientia tsutsugamushi . Reports on the emergence of doxycycline-resistant strains highlight the urgent need to develop novel antiinfectives against scrub typhus. Corallopyronin A (CorA) is a novel α-pyrone compound synthesized by the myxobacteriumCorallococcus coralloides that was characterized as a noncompetitive inhibitor of the switch region of the bacterial RNA polymerase (RNAP). We investigated the antimicrobial action of CorA against the human-pathogenic Karp strain ofO. tsutsugamushi in vitro andin vivo . The MIC of CorA againstO. tsutsugamushi was remarkably low (0.0078 μg/ml), 16-fold lower than that againstRickettsia typhi . In the lethal intraperitonealO. tsutsugamushi mouse infection model, a minimum daily dose of 100 μg CorA protected 100% of infected mice. Two days of treatment were sufficient to confer protection. In contrast to BALB/c mice, SCID mice succumbed to the infection despite treatment with CorA or tetracycline, suggesting that antimicrobial treatment required synergistic action of the adaptive immune response. Similar to tetracycline, CorA did not prevent latent infection ofO. tsutsugamushi in vivo . However, latency was not caused by acquisition of antimicrobial resistance, sinceO. tsutsugamushi reisolated from latently infected BALB/c mice remained fully susceptible to CorA. No mutations were found in the CorA-binding regions of the β and β′ RNAP subunit genesrpoB andrpoC . Inhibition of the RNAP switch region ofO. tsutsugamushi by CorA is therefore a novel and highly potent target for antimicrobial therapy for scrub typhus.