test of the novel antibiotic lefamulin alone and in combination with doxycycline against .

, a sexually transmitted bacterium, is a significant cause of urethritis in men and various reproductive tract infections in women, including cervicitis, pelvic inflammatory disease, endometritis, and potentially infertility. Treatment has become increasingly challenging due to the emergence of resistance to both first-line (azithromycin) and second-line (moxifloxacin) antibiotics. The need for new treatment options is critical. This study evaluates the efficacy of the novel antibiotic lefamulin against 54 . isolates, including highly resistant variants. Additionally, the potential synergistic effects of combining lefamulin with doxycycline were assessed in eight selected isolates. Lefamulin exhibited strong antibacterial activity across all tested isolates, with minimal inhibitory concentrations (MICs) ranging from 0.0005 to 0.064 µg/mL. Minimal bactericidal concentrations ranged from 0.001 to 0.128 µg/mL and were equal to the MIC in 40 of 54 isolates and within two- and fourfold MIC in the rest of the isolates. Notably, lefamulin's MIC values were significantly lower than those of azithromycin, doxycycline, and moxifloxacin, underscoring its potent efficacy. Checkerboard assays revealed no antagonistic interaction between lefamulin and doxycycline, with some additive effects observed in certain isolates. These findings highlight lefamulin's potential as a highly effective treatment for infections, particularly those involving multi-drug-resistant strains. Given the increasing rates of resistance and the limitations of current therapies, lefamulin may represent a promising new option for managing this challenging pathogen. Further clinical studies are warranted to confirm these results and explore the therapeutic potential of lefamulin in combination with doxycycline.