Dual mechanism of action of the atypical tetracycline chelocardin.

Classical tetracyclines targeting the protein biosynthesis machinery are commonly applied in human and veterinary medicine. The development and spread of resistance seriously compromise the successful treatment of bacterial infections. The atypical tetracycline chelocardin holds promise as it retains activity against tetracycline-resistant strains. It has been suggested that chelocardin targets the bacterial membrane, thus differing in mode of action from that of classical tetracyclines. We investigated the mechanism of action of chelocardin using global proteome analysis. The proteome profiles after sublethal chelocardin stress were compared to a reference compendium containing antibiotic response profiles of Bacillus subtilis. This approach revealed a concentration-dependent dual mechanism of action. At low concentrations, like classical tetracyclines, chelocardin induces the proteomic signature for peptidyl transferase inhibition demonstrating that protein biosynthesis inhibition is the dominant physiological challenge. At higher concentrations B. subtilis mainly responds to membrane stress indicating that at clinically relevant concentrations the membrane is the main antibiotic target of chelocardin. Studying the effects on the membrane in more detail, we found that chelocardin causes membrane depolarization but does not lead to formation of large pores. We conclude that at growth inhibiting doses chelocardin not only targets protein biosynthesis but also corrupts the integrity of the bacterial membrane. This dual mechanism of action might prove beneficial in slowing the development of new resistance mechanisms against this atypical tetracycline.