ANALYSIS OF BORRELIA-BURGDORFERI MEMBRANE ARCHITECTURE BY FREEZE-FRACTURE ELECTRON-MICROSCOPY

Freeze fracture electron microscopy was used to investigate the membra ne architectures of high-passage Borrelia burgdorferi B31 and low- and high-passage isolates of B. burgdorferi N40. In all three organisms, fractures occurred almost exclusively through the outer membrane (OM), and the large majority of intramembranous particles were distributed randomly throughout the concave OM leaflet. The density of intramembra nous particles in the concave OM leaflet of the high-passage N40 isola te was significantly greater than that in the corresponding leaflet of the low-passage N40 isolate. Also noted in the OMs of all three organ isms were unusual structures, designated linear bodies, which typicall y were more or less perpendicular to the axis of the bacterium. A comp arison of freeze-fractured B. burgdorferi and Treponema pallidum, the syphilis spirochete, revealed that the OM architectures of these two p athogens differed markedly. All large membrane blebs appeared to be bo unded by a membrane identical to the OM of B. burgdorferi whole cells; in some blebs, the fracture plane also revealed a second bilayer clos ely resembling the B. burgdorferi cytoplasmic membrane. Aggregation of the lipoprotein immunogens outer surface protein A (OspA) and OspB on the bacterial surface by incubation of B. burgdorferi B31 with specif ic polyclonal antisera did not affect the distribution of OM particles , supporting the contention that lipoproteins do not form particles in freeze-fractured OMs. The expression of poorly immunogenic, surface-e xposed proteins as virulence determinants may be part of the parasitic strategy used by B. burgdorferi to establish and maintain chronic inf ection in Lyme disease.