Bacitracin enhances ceftriaxone susceptibility of the high-level ceftriaxone-resistant gonococcal FC428 clone

Neisseria gonorrhoeae is a multidrug-resistant bacterial pathogen for which ceftriaxone mono-antimicrobial therapy and ceftriaxone plus azithromycin dual-antimicrobial therapy are the only remaining effective therapies. However, ceftriaxone efficacy is threatened by the global dissemination of the high-level ceftriaxone-resistant FC428 clone, while azithromycin has in recent years been removed from the recommended dual therapy in some countries as a result of increasing (high-level) azithromycin resistance. Inclusion of an alternative second antimicrobial could be an effective strategy to protect ceftriaxone from (further) resistance development. In this study, we investigated gonococcal susceptibility to bacitracin and determined its synergistic anti-gonococcal activity with ceftriaxone. Gonococcal susceptibility to bacitracin was investigated for 449 contemporary clinical isolates using the agar dilution method. Bacitracin displayed consistent activity against the N. gonorrhoeae strain collection, with a minimum inhibitory concentration (MIC) range of 2-32 mg/L and a MIC90 of 32 mg/L. Furthermore, bacitracin was bactericidal in time-kill assays, particularly against the tested FC428-associated isolate, which showed 10(5)-fold inactivation after 8 hours of exposure to a 4x MIC dose. Importantly, bacitracin and ceftriaxone displayed synergistic activity, with a fractional inhibitory concentration index <= 0.5 for the majority of tested strains. Finally, spot assays showed strong synergistic activity against the FC428-associated isolates, with up to 200-fold reduced plating efficacy for bacitracin combined with ceftriaxone compared with ceftriaxone only. In conclusion, bacitracin shows synergistic anti-gonococcal activity with ceftriaxone and might therefore be an interesting candidate for inclusion in a dual-antimicrobial therapy with ceftriaxone.