We examined the isolation of fimbriae from Bacteroides nodosus. It was found that the best preparations were obtained from the supernatant of washed cells cultured on solid medium, from which fimbriae could be recovered in high yield and purity by a simple one-step procedure. Analysis of such preparations by sodium dodecyl sulfate gel electrophoresis showed that greater than 98% of the protein consisted of fimbrial structural subunits whose molecular weight was ca. 17,000. These preparations also usually exhibited minor contamination with a polypeptide of ca. 80,000 molecular weight, as well as trace amounts of lipopolysaccharide. Attempts to release additional fimbriae by the traditional means of subjecting the bacterial cells to physical stress, such as shearing or heating, resulted primarily in an increase in the level of contamination, without significant gain in the yield of fimbriae. Removal of the 80,000-dalton component could not be achieved by any of a variety of techniques normally used in fimbriae purification, including isoelectric precipitation, MgCl2 precipitation, and CsCl gradient ultracentrifugation, implying a direct physical association with the fimbrial strand. Electron micrographs of fractions containing this protein show cap-shaped structures attached to the ends of what appeared to be fimbrial stubs. These observations suggest that the 80,000-dalton polypeptide may actually constitute the basal attachment site which anchors the fimbria to the outer membrane, analogous to a similar protein recently described in enterotoxigenic strains of Escherichia coli. In B. nodosus, this 80,000-* Corresponding author. which is then followed by a lengthy series of purification steps (e.g., see reference 10). However, using these procedures, we noted substantial contaminants and, in particular, the presence and persistence of a protein of about 80,000 molecular weight. Our results indicate, first, that the current methods used for the production offimbriae from B. nodosus may be inappropriate, even counterproductive, and we present a simplified protocol for the isolation of fimbriae in high purity. Second, our evidence suggests that the 80,000dalton component may actually be physically associated with fimbriae in vivo and in vitro, and that it probably represents the basal protein which anchors the fimbrial strand to the outer surface of the cell.MATERIALS AND METHODS Bacterial culture. Bacteroides nodosus 198, originally obtained from the Commonwealth Scientific and Industrial Research Organization McMaster Laboratory (Sydney, Australia) culture collection, was used in these studies. This strain is the prototype assigned to serogroup A, subtype 1 and has been reclassified as VCS 1001 (3). Strain VCS 1006 (serogroup B, subtype 1) was also used in some experiments. Seeding cultures were purified from freeze-dried samples by two subcultures on 4% hoof-agar (HA) medium (35). For large-scale cultures, cells were grown on 2% HA medium and harvested by being scraped into 10 mM sodium phosphate buffer (pH 7.2) co...