Mechanisms of ceftriaxone resistance in neisseria gonorrhoeae isolated from Thai patients

Abstract

N. gonorrhoeae is the causative agent of gonorrhea, which is one of the most important sexually transmitted disease. The multidrug resistance in N. gonorrhoeae has been increasingly reported worldwide. The objectives of this study were to investigate the prevalence and the mechanisms of ceftriaxone resistance in N. gonorrhoeae Thai isolates, and to investigate the synergistic activities of antibiotic combinations against N. gonorrhoeae isolates with reduced susceptibility or resistance to ceftriaxone. A total of 134 N. gonorrhoeae isolates were included in this study. The resistance rates of ciprofloxacin, tetracycline, penicillin G, gentamicin, azithromycin, ertapenem, fosfomycin, cefixime, and ceftriaxone were 90.30%, 82.09%, 73.13%, 59.70%, 13.43%, 1.49%, 0%, 0%, and 0%, respectively. The high prevalence of multidrug-resistant N. gonorrhoeae was found in 82.84%. Although all isolates were susceptible to ceftriaxone (MIC range of 0.001 to 0.125 mg/L with MIC50 and MIC90 of 0.002 and 0.015 mg/L, respectively.), two (1.49%) of them displayed reduced susceptibility to ceftriaxone. This is the first report of 2 N. gonorrhoeae isolates with reduced susceptibility to ceftriaxone, NG-083 (ST723) and NG-091 (new ST) in Thailand. Both isolates had mosaic PBP2 pattern XXXIV, L421P substitution in PBP1, an adenine deletion in the 13-bp inverted repeat sequence of the mtrR promoter region, a H105Y substitution in the MtrR repressor, and G120K and A121N substitutions in PorB porin. The result from NG-MAST showed the heterogeneous populations (29 STs) among 31 representative strains of N. gonorrhoeae . The activities of ceftriaxone plus azithromycin, fosfomycin, gentamicin, or ertapenem were evaluated by checkerboard method against the 2 isolates with reduced susceptibility to ceftriaxone. There was no synergistic activity of these antibiotic combinations. The best combination was ceftriaxone plus azithromycin, which showed the lowest FICIs of additive and was confirmed by time-kill method. The results showed that this combination had no synergistic activity but had bactericidal activity. This study showed high resistance rates of antibiotics previously used for treatment of gonorrhea, e.g., penicillin G, tetracycline, and ciprofloxacin. However, no isolates were resistant to ceftriaxone, suggesting that it is still active against N. gonorrhoeae Thai isolates. Mutations in, penA, mtrR promoter, mtrR, and porB genes were associated with reduced susceptibility to ceftriaxone. Therefore, antimicrobial resistance surveillance and rapid diagnosis are important to limit the emergence and spread of antibiotic-resistant N. gonorrhoeae isolates.