First passage B. burgdorferi strain B31 clone 5A19 spirochetes, isolated from an ear biopsy of a previously infected mouse, were grown in Barbour-Stoenner-Kelly-H medium supplemented with 6% rabbit serum and antibiotics (rifampicin at 45.4 μg/mL, phosphomycin at 193 μg/ml, and amphotericin at 0.25 μg/ml; Sigma-Aldrich, St. Louis, MO) to late logarithmic phase under microaerophilic conditions. An inoculum containing 1 × 108 spirochetes/ml in RPMI 1640 medium (Invitrogen, USA) was prepared as previously described [11 (link)].
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