Bacterial strains and growth conditions
B. burgdorferi low-passage virulent B31-5A4 was cultivated in Barbour-Stoenner-Kelly II (BSK-II) complete medium at 34°C (59). Cultures were passaged to 1 × 105 cells/ml and grown to 1-3 × 107 cells/ml. Cell density was determined using a Cellometer counting chamber (Nexcelom). E. coli strains, HI-control 10G or BL21(DE3) cells, were grown overnight in lysogeny broth (LB) at 37°C to a density of ~1 OD600. Cells were then passaged 1:100 unless stated otherwise. Descriptions and genotypes of all strains used (E. coli and B. burgdorferi ) can be found in Table S1.