Antibiotic susceptibility in vitro of Francisella tularensis subsp. holarctica isolates from Germany

Background: Tularaemia is a zoonotic disease caused by the bacterium Francisella tularensis. In Germany, the disease is still rare (e.g. 34 human cases reported in 2015). There is a lack of data about the susceptibility of F. tularensis strains to antibiotics, because many cases are diagnosed using serological assays only. Objectives: The antibiotic susceptibility in vitro of F. tularensis subsp. holarctica strains isolated in Germany was assessed to determine whether the currently recommended empirical therapy is still adequate. Methods: A total of 128 F. tularensis strains were investigated that were collected between 2005 and 2014 in Germany from wild animals, ticks and humans. All isolates were genotyped using real-time PCR assays targeting canonical SNPs, and antibiotic susceptibility was tested using MIC test strips on agar plates. MIC values were interpreted using CLSI breakpoints. Results: The strains were susceptible to antibiotics commonly recommended for tularaemia therapy, i.e. aminoglycosides (MIC90 values: gentamicin 1 mg/L; streptomycin 4.0 mg/L), tetracyclines (MIC90 values: tetracycline 0.5 mg/L; doxycycline 1.5 mg/L) and quinolones (MIC90 value: ciprofloxacin 0.064 mg/L). Chloramphenicol (MIC90 value: 3.0 mg/L) may be of value in treatment of tularaemia meningitis. Ninety-four isolates were susceptible to erythromycin, which defines biovar I (genotypes B.4 and B.6); 34 were resistant (biovar II; genotype B.12). Conclusions: The F. tularensis isolates investigated in this study showed the typical antibiotic susceptibility pattern that was previously observed in other countries. Therefore, recommendations for empirical antibiotic therapy of tularaemia can remain unchanged. However, antibiotic susceptibility testing of clinical isolates should be performed whenever possible.