In vitro activities of antimicrobial combinations against clinical isolates of Neisseria gonorrhoeae

The Centers for Disease Control and Prevention (CDC) now recommend combination therapy with ceftriaxone 250 mg plus azithromycin (AZM) 1 g as a first-line regimen for gonorrhea because the increase of Neisseria gonorrhoeae resistant to multiple antimicrobial agents. However, reports on the in vitro activity of antimicrobial combinations against clinical isolates of N. gonorrhoeae are very rare. In the present study, a checkerboard method was utilized to examine the in vitro activity of ceftriaxone (CTRX), cefodizime (CDZM), spectinomycin (SPCM), or gentamicin (GM) in combination with AZM against 25 clinical isolates of N. gonorrhoeae. The SPCM + AZM combination demonstrated the lowest mean fractional inhibitory concentration index (FICI) of 0.69, followed by the CDZM + AZM combination (mean FICI, 0.75), the CTRX + AZM combination (mean FICI, 0.81), and the GM + AZM combination (mean FICI, 0.83). Additivity/indifference effect was detected for the SPCM + AZM combination, the CDZM + AZM combination, the CTRX + AZM combination, and the GM + AZM combination, against 96 %, 72 %, 92 %, and 100 % of the isolates, respectively. There was no antagonism for any of the antimicrobial combinations against the 25 N. gonorrhoeae isolates. These results suggest that the antimicrobial combinations may be worthy of clinical evaluation as an alternative regimen for gonococcal infections caused by antimicrobial-resistant strains.