In vitro activity of tigecycline alone and antimicrobial combinations against clinical Neisseria gonorrhoeae isolates

In this study, we determined the in vitro activity of various combinations of antimicrobial agents against 54 Neisseria gonorrhoeae isolates. The combined activity of ceftriaxone (CRO) and azithromycin (AZM), CRO and doxycycline (DOX), CRO and spectinomycin (SPT), cefixime (CFX) and AZM, CFX and DOX, and CFX and SPT was determined using a checkerboard method. The fractional inhibitory concentration index (FICI) values for all combinations were either additive or indifferent, and no synergistic or antagonistic effects were found. The FICI comparison in each combination did not show any difference according to the N.gonorrhoeae-resistant phenotypes and genotypic characteristics, including penicillinase-producing N. gonorrhoeae, tetracycline-resistant N. gonorrhoeae, stratified MIC of all antibiotics, and N. gonorrhoeae multiantigen sequence typing. MIC50 and MIC90 of tigecycline by agar dilution were 0.5 mg/L and 0.5 mg/L, respectively, which were lower than that of tetracycline and DOX. Additive/indifference results could suggest that combinations that include CRO may be used safely without a significant likelihood of generating resistance.