Comparison of growth and morphology of Borrelia burgdorferi sensu lato in BSK-H and BSK-II media stored for prolonged periods

Barbour-Stoenner-Kelly II (BSK-II) and BSK-H media were used for cultivation and isolation of fastidious Borrelia species - the causative agents of Lyme borreliosis. Culture media have a limited shelf life and require adequate storage. Our goal was to assess how the growth of Borrelia would be affected by prolonged storage of media and inadequate storage conditions (BSK-H stored at +4 °C for 2.5 years and BSK-II stored at −20 °C for 11 years). Growth of different Borrelia afzelii, Borrelia garinii, Borrelia lusitaniae and Borrelia valaisiana strains was assessed during 2 weeks of incubation at 33 °C. Monitored parameters included cell count per mL, morphology and motility. The results of this study have shown weaker growth of borrelia strains in BSK-H at +4 °C (median final cell number of 1.5 × 10⁶/mL) than in BSK-II at −20 °C (median final cell number of 7.75 × 10⁶/mL) and in fresh BSK-H media (median final cell number of 8.95 × 10⁶/mL). Duration of storage of media had no impact on Borrelia morphology and motility. Our results indicate that temperature of −20 °C is optimal for long-term storage of medium, BSK-II stored for 11 years provided effective support to growth of Borrelia and may be employed for cultivation.