Resistance of Borrelia burgdorferi Sensu Lato Isolates from Serbia to Human Complement

Serbia is characterised by the high prevalence and diversity of Borrelia burgdorferi sensu lato strains in ticks, but the reported incidence of Lyme borreliosis is much lower than that in regions with a similar prevalence of Borrelia in ticks. To evaluate the susceptibility and viability profiles of Borrelia strains circulating in local enzootic cycles, we analysed their response to human complement in vitro using short-term incubation periods. A total of 31 strains, comprising 27 Serbian tick isolates, two tick isolates from Spain, one tick isolate from Portugal, and one human skin isolate from Portugal (12 Borrelia afzelii, 12 Borrelia lusitaniae, three Borrelia bavariensis, two Borrelia garinii, and two Borrelia valaisiana), were analysed using serum susceptibility testing. The motility of spirochetes was assessed one and three hours after incubation with normal human serum (NHS) and heat-inactivated serum (HIS) used as a control. The absence of a statistically significant decrease in strain motility after incubation with NHS compared with HIS indicated the resistance of the strains. All tested B. afzelii and B. bavariensis strains and one B. valaisiana strain were serum-resistant, whereas both B. garinii strains were serum-susceptible. Based on statistical analysis, all B. lusitaniae strains were classified as serum-susceptible; however, two local tick isolates and the human reference strain demonstrated distinct biological heterogeneity, maintaining significantly higher residual motility after short-term exposure to human serum, which underscores strain-specific and geographic variations in complement tolerance.