In vitro evolution provides insights into mechanisms of Mycoplasma genitalium resistance to moxifloxacin.

Background: Current knowledge of Mycoplasma genitalium fluoroquinolone resistance is based on sequence analyses of clinical samples, an approach with limited scope. Many potential resistance mutations have been described but their impact on moxifloxacin efficacy is unclear.

Objective: To investigate the impact of individual mutations on fluoroquinolone resistance through selection of moxifloxacin-resistant mutants in vitro.

Methods: M. genitalium G37 was passaged sequentially in sub-inhibitory concentrations of moxifloxacin. MIC values were determined for moxifloxacin, sitafloxacin, levofloxacin and ciprofloxacin. Bacterial populations were profiled using amplicon sequencing.

Results: Across three independent experiments, four moxifloxacin mutants were isolated, with mutations encoding the following protein sequence variations: (i) GyrA D99Y/ParE E468K, (ii) ParC S83I/GyrA D99Y/ParE E468K, (iii) ParC D87V/GyrB P462S and (iv) GyrA M95I/ParE E468dup. Moxifloxacin MICs were elevated 16- to 32-fold for mutants with a single GyrA or ParC variation, and 128-fold for the dual GyrA/ParC mutant. Sitafloxacin MICs were elevated but remained lower than moxifloxacin MICs. Mutations did not have a substantial impact on in vitro growth dynamics. Population analysis showed that multiple mutations attained detectable population-wide frequencies, with evidence of clonal interference dynamics, with a minority becoming fixed in the population.

Conclusion: Mutations in multiple genes conferring fluoroquinolone resistance appeared with regularity in vitro. Findings of an additive effect for ParC/GyrA changes, and greater effectiveness of sitafloxacin against resistant bacteria compared with moxifloxacin, are both consistent with clinical data. Improved understanding of fluoroquinolone resistance will inform the development of diagnostic assays predicting fluoroquinolone susceptibility.