In Vitro Activity of Tigecycline Against Francisella tularensis Subsp holarctica in Comparison with Doxycycline, Ciprofloxacin and Aminoglycosides

Francisella tularensis is the etiological agent of tularemia which is a zoonosis of the northern hemisphere. For decades, streptomycin was considered the drug of choice, despite possible side effects, and vestibular toxicity in particular. Alternatives are tetracylines and chloramphenicol which are bacteriostatic agents that are associated with a considerable risk of relapse. The aim of the present study was to assess the in vitro susceptibility of F.tularensis subsp. holarctica biovar II strains to tigecycline, a member of a new class of glycylcyclines. Fourteen F.tularensis strains isolated from patients in Central Anatolia region of Turkey were examined. Minimum inhibitory concentration (MIC) values of tigecycline, doxycycline, streptomycin, gentamicin, and ciprofloxacin were determined using the E-test method on glucosecysteine blood agar plates. Interpretation of results was made according to CLSI clinical breakpoints. All strains were susceptible to the antibiotics traditionally used to treat tularemia. Tigecycline showed good in vitro activity to all the isolates (MIC range: 0.094-0.38 mg/L). In this study, tigecycline was more active than doxycycline against F.tularensis subsp. holarctica strains, according to MIC50 (0.19 mg/L) and MIC90 (0.25 mg/L) values. Doxycycline (MIC90: 0.38 mg/L) showed good in vitro activity against all the isolates and MIC values interpreted according to the CLSI criteria for potential bioterrorism agents, have shown ranges below the breakpoint for sensitivity determination (S <= 4 mg/L). Ciprofloxacin had the lowest MIC50 and MIC90 values. In case the other antibiotics can not be used or intravenous therapy is required, tigecycline may be an important therapeutic alternative agent. However, confinement of tigecycline in the treatment of multi-drug resistant bacterial infections, its parenteral way of administration and overall cost were considered as the major limitations of tigecycline in tularemia treatment