Evidence of a conjugal erythromycin resistance element in the Lyme disease spirochete Borrelia burgdorferi

We report the identification of isolates of Borrelia burgdorferi strain B31 that exhibit an unusual macrolide–lincosamide (ML) or macrolide–lincosamide–streptogramin A (MLSA) antibiotic resistance pattern. Low-passage isolates were resistant to high levels (>100 μg/mL) of erythromycin, spiramycin and the lincosamides but were sensitive to dalfopristin, an analogue of streptogramin B. Interestingly, the high-passage erythromycin-resistant strain B31 was resistant to quinupristin, an analogue of streptogramin A (25 μg/mL). Biochemical analysis revealed that resistance was not due to antibiotic inactivation or energy-dependent efflux but was instead due to modification of ribosomes in these isolates. Interestingly, we were able to demonstrate high-frequency transfer of the resistance phenotype via conjugation from B. burgdorferi to Bacillus subtilis (10−2–10−4) or Enterococcus faecalis (10−5). An intergeneric conjugal system in B. burgdorferi suggests that horizontal gene transfer may play a role in its evolution and is a potential tool for developing new genetic systems to study the pathogenesis of Lyme disease.