Phenotypic antibiotic resistance of Mycoplasma genitalium and its variation between different macrolide resistance-associated mutations.

Objectives: Mycoplasma genitalium, a sexually transmitted bacterium, faces increasing antibiotic resistance, particularly to azithromycin. However, presence of macrolide resistance-associated mutations (MRAMs) does not evidently implicate azithromycin treatment failure. This study aimed to establish an in vitro co-culture system of M. genitalium isolates and perform phenotypic susceptibility testing for different antibiotics, focusing on azithromycin to evaluate genotypic and phenotypic resistance across MRAMs.

Methods: Urine specimens testing positive for M. genitalium via nucleic acid amplification were co-cultured with Vero cells. Phenotypic susceptibility testing was performed for eight antibiotics. Growth inhibition and MIC of M. genitalium by azithromycin were compared across different MRAMs.

Results: M. genitalium was cultured from 20/40 (50.0%) positive urine samples, with phenotypic susceptibility tested in a subset. MICs ranged as follows: azithromycin (0.008->32 mg/L), levofloxacin (1-4 mg/L), moxifloxacin (<0.25-1 mg/L), sitafloxacin (<0.032-0.25 mg/L), minocycline (<0.25-1 mg/L), doxycycline (<0.125-2 mg/L), spectinomycin (<2.5->25 mg/L) and lefamulin (<0.004-0.063 mg/L). Isolates with A2058T demonstrated 24-, 7-, 15- and 12-fold increases in growth inhibition compared with A2058G at azithromycin concentrations of 4, 8, 16 and 32 mg/L, respectively (P < 0.01). MRAMs ranked from low to high impact on MIC range were as follows: wildtype (0.008-0.016), A2058T (8-32), A2059G (≥32) and A2058G (>32).

Conclusions: This study revealed that M. genitalium isolates vary in azithromycin-induced growth inhibition across MRAMs, potentially explaining differences in clinical treatment efficacy. Phenotypic susceptibility testing for other antibiotics demonstrated relatively low MICs. Future studies should incorporate clinical treatment efficacy and symptom severity to optimize treatment for M. genitalium.