The Effect of Amoxicillin Pre-Exposure on Treatment Outcomes and Antimicrobial Susceptibility in Patients with Urogenital Chlamydia trachomatis Infection

PURPOSE: We investigated the influence of amoxicillin pre-exposure on treatment outcomes, Chlamydia trachomatis (CT) culture, the presence of drug-resistant genes, minimum inhibitory concentrations (MICs), and fractional inhibitory concentrations (FICs) in CT clinical strains. Additionally, we explored the effect of different antimicrobial combinations on CT. PATIENTS AND METHODS: Clinical data of 62 patients with CT infection were recorded. Of these, 33 had pre-exposure to amoxicillin and 29 did not. Among patients with pre-exposure, 17 received azithromycin and 16 received minocycline. Among the patients without pre-exposure, 15 received azithromycin and 14 received minocycline. All patients underwent microbiological cure follow-ups one month after completing the treatment. 23S rRNA gene mutations, acquisition of tet(M) and tet(C) were detected using reverse transcription PCR (RT-PCR) and PCR, respectively. The MICs and FICs of azithromycin, minocycline, and moxifloxacin, alone or in combination, were determined using the microdilution and checkerboard methods, respectively. RESULTS: More cases of treatment failure occurred in pre-exposed patients, in both treatment groups (P <0.05). No 23S rRNA gene mutations or tet(M) and tet(C) acquisitions were found. More inclusion bodies were cultured from patients without amoxicillin pre-exposure than from those with pre-exposure (P <0.0001). The MICs of all antibiotics were higher in pre-exposed patients than in those without pre-exposure (P <0.01). The FICs of azithromycin plus moxifloxacin were lower than those of the other antibiotic combinations (P <0.0001). The synergy rate of azithromycin plus moxifloxacin was significantly higher than those of azithromycin plus minocycline and minocycline plus moxifloxacin (P <0.001). The FICs of all antibiotic combinations were comparable between isolates from the two patient groups (all P >0.05). CONCLUSION: Pre-exposure to amoxicillin in CT patients may inhibit CT growth and decrease sensitivity of CT strains to antibiotics. Azithromycin plus moxifloxacin may be a promising treatment regimen for genital CT infections with treatment failure.