Rifampin-resistant RNA polymerase in spirochetes

Various free-living and host-associated spirochetes isolated by methods not involving rifampin were resistant to relatively high concentrations of this antibiotic. The lowest concentrations of rifampin that were inhibitory for the spirochetes ranged from 50 to more than 200 μ g/ml, depending on the species. The spirochete strains examined were at least 10-fold more resistant to rifampin than Escherichia coli and 10 000-fold more resistant than Staphylococcus aureus. The results support the conclusion that rifampin resistance is a general characteristic of spirochetes. Resistance of Spirochaeta aurantia to rifampin was not the result of detoxification of the antibiotic in the culture medium. The activity of spirochete DNA-dependent RNA polymerase in vitro was completely resistant to 10 μg of rifampin per ml, a concentration that totally inhibited E. coli RNA polymerase. Higher concentrations decreased the spirochetal activity. Thus, rifampin resistance may be due to a low affinity of spirochete RNA polymerase for the antibiotic.