Hard tick-borne relapsing fever (HTBRF) is an emerging infectious disease throughout the temperate zone caused by the relapsing fever spirochete Borrelia miyamotoi. Antibiotic treatment of HTBRF is empirically based on the treatment of Lyme borreliosis, however antibiotic susceptibility of B. miyamotoi has not been studied to date. Thus, we set out to determine the in vitro antimicrobial susceptibility of B. miyamotoi.
A microdilution method with 96-well microtiter plates was used to determine antibiotic susceptibility of two B. miyamotoi strains isolated on two different continents (Asia and North America), two Borrelia burgdorferi sensu lato (s.l.) strains and one Borrelia hermsii isolate for comparison. The MIC and MBC were determined by both microscopy and colorimetric assays.
We were able to show that both B. miyamotoi strains and B. hermsii demonstrated greater susceptibility to doxycycline and azithromycin, equal susceptibility to ceftriaxone and proved to be resistant to amoxicillin in vitro as compared to the B. burgdorferi s.l. isolates. The MIC and MBC of amoxicillin for B. miyamotoi evaluated by microscopy were 16-32 mg/L and 32-128 mg/L, respectively. Since B. miyamotoi is susceptible to doxycycline, azithromycin and ceftriaxone in vitro, our data suggests that these antibiotics can be used for treatment of HTBRF. Oral amoxicillin is currently used as an alternative for the treatment of HTBRF, however since we found antimicrobial resistance of the tested B. miyamotoi strains to amoxicillin in vitro, this warrants further study.
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