Susceptibility of Rickettsia rickettsii to Tigecycline in a Cell Culture Assay and Animal Model for Rocky Mountain Spotted Fever

Rocky Mountain spotted fever (RMSF), caused by Rickettsia rickettsii, is a severe tick-borne infection endemic to the Americas. Oral doxycycline is effective, but during severe life-threatening disease, intravenous therapy is recommended. Unfortunately, intravenous formulations of doxycycline are not always available. Therefore, we aimed to determine the susceptibility of R. rickettsii to an alternative parenteral agent, tigecycline, in vitro and in vivo. To determine the minimum inhibitory concentration of tigecycline, R. rickettsii-inoculated Vero cells were incubated with medium containing tigecycline. At various time points, monolayers were collected and R. rickettsii was quantified via real-time polymerase chain reaction (PCR). The growth of R. rickettsii was inhibited in the presence of ³ 0.5 μg/mL of tigecycline. To determine the effectiveness of tigecycline in vivo, Guinea pigs were inoculated with R. rickettsii. Five days after inoculation, they were treated twice daily with subcutaneous tigecycline 3.75 mg/kg or subcutaneous doxycycline 5 mg/kg. Treated animals improved, whereas untreated controls remained ill. Tissues were collected for quantitative PCR-determined bacterial loads on day 8. Median bacterial loads in the tigecycline group were less than those in untreated animals: liver (0 versus 2.9 × 10⁴ copies/mg), lung (0 versus 8.3 × 10³ copies/mg), skin (2.6 × 10² versus 2.2 × 10⁵ copies/mg), spleen (0 versus 1.3 × 10⁴ copies/mg), and testes (0 versus 1.0 × 10⁵ copies/mg, respectively). There were no significant differences in the bacterial loads between doxycycline-treated versus tigecycline-treated Guinea pigs. These data indicate that tigecycline is effective against R. rickettsii in cell culture and in an animal model of RMSF.