<i>In Vitro</i> Antimicrobial Susceptibility of Clinical Isolates of Borrelia miyamotoi

Abstract

<i>Borrelia miyamotoi</i> is an emerging relapsing fever (RF) <i>Borrelia</i> species that is reported to cause human disease in regions in which Lyme borreliosis is endemic. We recently showed that <i>B. miyamotoi</i> tick isolates are resistant to amoxicillin <i>in vitro</i>; however, clinical isolates have not been studied. Therefore, our aim was to show the antimicrobial susceptibility of recently obtained clinical isolates of <i>B. miyamotoi</i> A dilution series of various antibiotics was made in modified Kelly-Pettenkofer medium with 10% fetal calf serum. The susceptibilities of different <i>B. miyamotoi</i> clinical, <i>B. miyamotoi</i> tick, RF <i>Borrelia</i>, and <i>Borrelia burgdorferi</i><i>sensu lato</i> 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 <i>B. miyamotoi</i> 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 <i>Borrelia</i> strains and <i>B. miyamotoi</i> tick isolates. All tested RF <i>Borrelia</i> strains were susceptible to doxycycline (microscopic MIC range, 0.0625 to 0.25 mg/liter). In contrast to the MICs of the tested <i>B. burgdorferi sensu lato</i> strains and in line with our previous findings, the amoxicillin MICs (range, 8 to 32 mg/liter) of all RF <i>Borrelia</i> strains, including <i>B. miyamotoi</i> clinical isolates, were above the clinical breakpoint for resistance (≤4 mg/liter). Clinical isolates of <i>B. miyamotoi</i> are highly susceptible to doxycycline, azithromycin, and ceftriaxone <i>in vitro</i> Interestingly, as described previously for tick isolates, amoxicillin shows poor <i>in vitro</i> activity against <i>B. miyamotoi</i> clinical isolates.