Evaluation of alternative antibiotics for susceptibility of gonococcal isolates from China

The efficacy of the currently recommended first-line treatments for gonococcal infections - ceftriaxone monotherapy or ceftriaxone/azithromycin dual therapy - is waning rapidly, and efficient alternative antimicrobials are needed urgently to ensure that future treatment of gonorrhoea remains available. As such, the aim of this study was to screen alternative clinically approved antimicrobials for in-vitro activity against Neisseria gonorrhoeae. The susceptibility levels of 504 clinical isolates from Zhejiang Province, China to ertapenem, tigecycline, gentamicin, fosfomycin, gemifloxacin, doxycycline and rifampicin were investigated using the agar dilution method. The presence of resistance determinants was identified by polymerase chain reaction and sequencing. The minimum inhibitory concentration inhibiting 90% of growth (MIC₉₀) was 0.06 mg/L for ertapenem, 0.25 mg/L for tigecycline, 16 mg/L for doxycycline, 4 mg/L for gemifloxacin, 16 mg/L for gentamicin, 32 mg/L for fosfomycin and 128 mg/L for rifampicin. All strains appeared to be susceptible to tigecycline (MIC ≤0.5 mg/L), while a poor correlation between tigecycline and tetracycline susceptibility was observed, indicating that tetracycline resistance determinants have little impact on tigecycline susceptibility. For ertapenem, 30 isolates showed an MIC >0.125 mg/L, but the correlation between ertapenem and ceftriaxone susceptibility was low and only two strains showed an MIC >0.125 mg/L for both antibiotics. Therefore, it appeared that most ceftriaxone-resistant isolates were still susceptible to ertapenem. In conclusion, tigecycline and ertapenem showed good activity against N. gonorrhoeae and limited cross-resistance with previously used antibiotics. Therefore, they might be interesting candidates for further evaluation of their suitability as alternative antigonococcal therapies.