Background Antimicrobial resistance (AMR) of Neisseria gonorrhoeae (Ng) is increasing. With recent resistance to last resort extended-spectrum cephalosporins, combination therapy of azithromycin (AZ) and ceftriaxone (TX) is now widely recommended. We used 2 methods to study in vitro synergy of recommended and new dual antibiotic combinations.

Methods A panel of 15 Ng strains with a minimal inhibitory concentration (MIC) of 0.064–8 for AZ and 0.012–2 for TX was tested for in vitro synergy, using both Etest and agar dilution checkerboard methods. Combinations of cefixime with AZ, colistin, ertapenem, gentamicin and moxifloxacin were also tested using the Etest method on 10 stains of the panel. Etests were placed crosswise at the MIC of each antibiotic in a 90° angle. All tests were performed in duplicate. MIC’s were red after 16–18 hours (Etest) or 24–48 hours (checkerboard) incubation. Synergy was defined as a fractional inhibitory concentration index (FICI) ≤ 0.5.

Results Using the Etest method no synergy was found in any strain for any of the used combinations. Mean FICI for each combination was between 0.77–1.27. Individual FICI’s varied between 0.49–2.00. Values ≤ 0.5 could not be confirmed in repeat testing. No antagonism was found. Mean FICI for AZ+TX was 1.27 (0.58–2.00). The results of the checkerboard for AZ+TX indicated synergy for only 2 of the 15 strains (FICI: 0.16 and 0.5). The mean FICI of all strains was 0.64 (0.16–1.01). Adding AZ to TX could not lower the TX MIC below 0.25 for one TX resistant strain (MIC for TX alone: 2).

Conclusion The recommended combination therapy against Ng (AZ+TX) showed no in vitro synergy using either the Etest or the agar dilution method. Other combinations of antibiotics from various groups showed no indication of in vitro synergy using the Etest method.