Emergence of Resistance to Rifampin and Rifalazil in Chlamydophila pneumoniae and Chlamydia trachomatis

Although rifamycins have excellent activity against Chlamydophila pneumoniae and Chlamydia trachomatis in vitro, concerns about the possible development of resistance during therapy have discouraged their use for treatment of chlamydial infections. Rifalazil, a new semisynthetic rifamycin with a long half-life, is the most active antimicrobial against C. pneumoniae and C. trachomatis in vitro, indicating its potential for treatment of acute and chronic C. pneumoniae and C. trachomatis infections. We investigated the effect of serial passage of two C. pneumoniae isolates and two serotypes of C. trachomatis in subinhibitory concentrations of rifalazil and rifampin on the development of phenotypic and genotypic resistance. C. trachomatis developed resistance to both antimicrobials within six passages, with higher level resistance to rifampin (128 to 256 g/ml) and lower level resistance to rifalazil (0.5 to 1 g/ml). C. pneumoniae TW-183 developed only low-level resistance to rifampin (0.25 g/ml) and rifalazil (0.016 g/ml) after 12 passages. C. pneumoniae CWL-029 failed to develop resistance to either drug. Two unique mutations emerged in the rpoB gene of rifampin (L456I) and rifalazil (D461E)-resistant C. pneumoniae TW-183. A single mutation (H471Y) was detected in both rifampin- and rifalazil-resistant C. trachomatis UW-3/Cx/D, and a unique mutation (V136F) was found in rifalazil-resistant BU-434/L2. No mutations were detected in the entire rpoB gene of rifampin-resistant BU-434/L2. This is the first description of antibiotic resistance-associated mutations in C. pneumoniae and of rifampin resistance in C. trachomatis not associated with mutations in the rpoB gene. Chlamydophila pneumoniae is a common cause of respiratory infections, and Chlamydia trachomatis is the most common sexually transmitted bacterial disease. In addition, persistent C. trachomatis and C. pneumoniae infections have been associated with the development of infertility, asthma, chronic obstructive pulmonary diseases, atherosclerosis, and Alzheimer’s disease (3, 12, 16, 19, 20), and therapeutic trials with antimicrobial agents are in progress for some of these conditions. Yet the optimal treatment regimens for C. pneumoniae are unknown, and resistance to available antibiotics is beginning to emerge in C. trachomatis (21) and Chlamydia suis, a related veterinary pathogen (17). Alternative or improved therapies for acute and chronic chlamydial infections would be desirable. Rifamycins have excellent activity against C. pneumoniae and C. trachomatis in vitro, but concerns about the emergence of resistance during therapy have discouraged their use for treatment of chlamydial infections in humans (25). Resistance to this class of antibiotics frequently occurs in other species, correlating with point mutations in their target gene, encoding RNA polymerase beta (rpoB) (2, 4, 14). The emergence of resistance to rifampin in vitro has been demonstrated in C. trachomatis (9, 15, 22, 25, 26). In contrast, no resistance has developed in C. trachomatis propagated in the presence of the rifamycin derivatives rifabutin and 3-azinomethyl-rifamycin under the same in vitro conditions (26, 29). Rifalazil is a new semisynthetic rifamycin with a long half