Susceptibility of bacterial vaginosis (BV)-associated bacteria to secnidazole compared to metronidazole, tinidazole and clindamycin

Secnidazole, a 5-nitroimidazole with a longer half-life, is structurally related to metronidazole and tinidazole. For treatment of Bacterial vaginosis (BV), secnidazole is a suitable single-dose oral drug having a longer serum half-life than metronidazole. The objective of this study was to evaluate the antimicrobial susceptibility of vaginal isolates of facultative and anaerobic bacteria to secnidazole, metronidazole, tinidazole and clindamycin. A total of 605 unique BV-related bacteria and 108 isolates of lactobacilli recovered from the human vagina of US women during the years 2009-2015 were tested for antimicrobial susceptibility by the agar dilution CLSI reference method to determine the minimal inhibitory concentration (MIC). The MIC₉₀ (μg/mL) for secnidazole was similar to metronidazole and tinidazole for Anaerococcus tetradius (secnidazole: MIC₉₀ 2; metronidazole: MIC₉₀ 2; tinidazole: MIC₉₀ 4), Atopobium vaginae (32; >128; 128), Bacteroides species (2; 2; 2), Finegoldia magna (2; 2; 4), Gardnerella vaginalis (128; 64; 32), Mageeibacillus indolicus (2; 2; 2), Megasphaera-like bacteria (0.5; 0.25; 0.5), Mobiluncus curtisii (128; >128; >128) and Mobiluncus mulieris (>128; >128; >128), Peptoniphilus lacrimalis (4; 4; 4) and Peptoniphilus harei (2; 2; 4), Porphyromonas species (0.25; 0.5; 0.25), Prevotella bivia (8; 8; 8), Prevotella amnii (2; 1; 2) and Prevotella timonensis (2; 2; 2). In this evaluation, 14 (40%) of 35 P. bivia, 5 (14%) of 35 P. amnii and 21 (58%) of 36 P. timonensis isolates were resistant to clindamycin with MIC values of >128 μg/mL. Secnidazole, like metronidazole, was superior to clindamycin for Prevotella spp., Bacteroides spp., Peptoniphilus spp., Anaerococcus tetradius and Finegoldia magna. Clindamycin had greater activity against Atopobium vaginae, Gardnerella vaginalis and Mobiluncus spp. compared to the nitroimidazoles. All 27 Lactobacillus crispatus, 26 (96%) of 27 L. jensenii, 5 (19%) of 27 L. gasseri and 18 (67%) of 27 L. iners isolates were susceptible to clindamycin (MIC ≤2) while the MIC₉₀ for all lactobacilli tested was >128 μg/mL for secnidazole, metronidazole and tinidazole. Secnidazole has similar in vitro activity against the range of Microorganisms associated with BV compared to metronidazole or tinidazole. Further, secnidazole spares lactobacilli, a characteristic which is desirable in drugs used to treat Bacterial vaginosis.