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.