Antimicrobial activity and spectrum of rifaximin, a new topical rifamycin derivative

Rifaximin, a rifamycin derivative, was evaluated in vitro to assess its spectrum and potency against a wide variety of bacteria, yeasts, viruses, and parasites. High concentrations of rifaximin were often used to reflect topically achieved levels since this compound is poorly absorbed by oral route. Like rifampin, rifaximin possessed best activity against Staphylococcus spp. (MIC50 < or = 0.015 microgram/ml), Streptococcus spp. (MIC50s, < or = 0.03-0.12 microgram/ml), Enterococcus spp. (MIC50s, 0.25-2 micrograms/ml), Bacillus cereus (MIC50, 0.06 microgram/ml), Moraxella catarrhalis (MIC50, < or = 0.03 microgram/ml), and Haemophilus influenzae (MIC50, 0.25 microgram/ml). Rifaximin demonstrated potential use as a topical agent for bacterial vaginosis by inhibiting Bacteroides bivius-disiens, Gardnerella vaginalis, Lactobacillus spp., and Mobiluncus spp. strains (all MICs < or = 1 microgram/ml). Strains of Haemophilus ducreyi and Neisseria gonorrhoeae (MIC50s, 0.25 microgram/ml) were also inhibited. However, some organisms associated with genital tract infections were rifaximin resistant, for example, Candida spp., herpes virus, mycoplasmas, Trichomonas vaginalis, and Ureaplasma urealyticum. Clinical trials appear warranted using rifaximin topical concentrations that will minimize mutations to rifamycin resistance.