In Vitro Antimicrobial Susceptibility of Clinical Isolates of Borrelia miyamotoi

Borrelia miyamotoi is an emerging relapsing fever (RF) Borrelia species that is reported to cause human disease in regions in which Lyme borreliosis is endemic. We recently showed that B. miyamotoi tick isolates are resistant to amoxicillin in vitro; however, clinical isolates have not been studied. Therefore, our aim was to show the antimicrobial susceptibility of recently obtained clinical isolates of B. miyamotoi. A dilution series of various antibiotics was made in modified Kelly-Pettenkofer medium with 10% fetal calf serum. The susceptibilities of different B. miyamotoi clinical, B. miyamotoi tick, RF Borrelia, and Borrelia burgdorferi sensu lato isolates were tested by measuring MICs through colorimetric changes and by counting motile spirochetes by dark-field microscopy after 72 h of incubation. The ceftriaxone and azithromycin MIC ranges of the six B.miyamotoi clinical isolates tested were 0.03 to 0.06 mg/liter and 0.0016 to 0.0032 mg/liter, respectively. These values are similar to MICs for RF Borrelia strains and B. 0miyamotoi tick isolates. All tested RE Borrelia strains were susceptible to doxycycline (microscopic MIC range, 0.0625 to 0.25 mg/liter). In contrast to the MICs of the tested B. burgdorferi sensu lato strains and in line with our previous findings, the amoxicillin MICs (range, 8 to 32 mg/liter) of all RF Borrelia strains, including B. 0miyamotoi clinical isolates, were above the clinical breakpoint for resistance (<= 4 mg/liter). Clinical isolates of B. miyarnotoi are highly susceptible to doxycycline, azithromycin, and ceftriaxone in vitro. Interestingly, as described previously for tick isolates, amoxicillin shows poor in vitro activity against B. miyamotoi clinical isolates.