B. burgdorferi is microaerophilic andcanbe grown with some difficulty in a specialized artificial culture medium. The doubling time under these conditions is long (8 to 24 hours), so growth for isolation takes many days to weeks. B.burgdorferi consists of at least 10 different subspecies (eg, B.burgdorferi sensu stricto, B. afzelii,B.garini), which differ in geographic distribution and some clinical manifestations. All will be referred to as B.burgdorferi here. As with other species of Borrelia, there are multiple classes of OMPs, many of which undergo antigenic variation. Recent studies have focused on a class called outer surface proteins (Osps), which have been linked to aspects of pathogen- esis and immunity. Two of these, OspA and OspC, are differentially expressed depending on the stage of tick or mammalian infection.