FACS profile of in vitro chamber-grown B. burgdorferi and in vivo chamber-grown B. burgdorferi. Dialysis chambers were prepared containing low-passage B. burgdorferi N40. Chambers were either placed in vitro at 33°C in BSK medium or implanted into the mouse peritoneal cavity. After 48 h, chambers were removed, and spirochetes were syringe extracted from chambers, counted, and stained for flow cytometry. (A) OspA and flagellin staining of in vitro-, chamber-grown spirochetes and in vivo-, chamber-grown spirochetes. (B) OspA staining of flagellin-positive populations of in vitro-, chamber-grown spirochetes and in vivo-, chamber-grown spirochetes. (C) The average of the MFIs for each of the groups. We determined the MFI of OspA expression for each chamber's flagellin-positive population. MFIs were determined for B. burgdorferi populations extracted from in vitro chambers (n = 3) and from host-adapted, in vivo chambers (n = 8). Error bars indicate the standard error of the mean (SEM). Data are significantly different (P = 0.0003, Student t test). These results are representative of three independent experiments.