Ultrastructural examinations of the flagellum of Compylobacter (Vibrio) fetus were performed throughout the growth cycle. Filament diameters, exceeding 17.6 nm during the exponential phase, were substantially greater than those reported for unsheathed flagella of other genera with the exception of Pseudomonas fluorescens. Filament diameters increased during growth, reaching a mean width of 21.2 nm in middle to late stationary phase. Internal flagellar structure, principally of the parallel lined variety, was observed during the later periods of growth but not during exponential or early stationary phase. Despite the unusually large filament sizes, no evidence of a flagellar sheath was observed after selected treatments (0.01 N HCl, 6 M urea, tris(hydroxymethyl) amino-methane-hydrochloride buffer, warm water) or examination of thin sections. To determine whether alterations in filament size and variable ability to demonstrate filament fine structure were correlated with progressive changes in serological activity, agglutination and immobilization tests were conducted with antisera directed against intact flagella, the principal flagellar antigen, the O antigen, and a superficial glycoprotein which has been found in association with the flagellum and the cell envelope. Significant differences in the serological activity of cells at different growth intervals were not noted with any of the sera employed.

The outer membrane proteins of Campylobacter fetus have been isolated by extraction of cell envelopes both in Triton X-100 and ethylenediaminetetraacetate (EDTA) and in sodium dodecyl sulfate (SDS). Each method yielded a major protein component, which migrated identically in SDS-polyacrylamide gel electrophoresis (SDS-PAGE) and proved comparable in electrophoretic and molecular weight characteristics to the analagous protein from Escherichia coli. In addition, the surface of SDS-extracted C. fetus cells displayed a subunit structure similar to that observed in E. coli. The major envelope protein isolated with SDS appeared antigenically identical with one of the proteins isolated with Triton-EDTA on the basis of immunodiffusion reactions with specific antisera. Antibodies directed to the major envelope protein were not reactive in agglutination, immobilization, bactericidal, or opsonization reactions. Strains of C. fetus belonging to each of the three 0 serotypes possessed major envelope proteins comparable in SDS-PAGE but distinguished antigenically in a fashion paralleling the 0 serotype classification.The cell envelope of gram-negative bacteria is a complex structure consisting of the outer membranous layer, the murein layer, and the cytoplasmic membrane. The outer membrane contains substantial amounts of protein, phospholipid, and lipopolysaccharide (LPS) and appears to be heterogeneous with respect to structural organization (14,17,28). In Enterobacteriaceae, a low-molecular-weight lipoprotein is covalently bound to the murein layer and extends into the outer membrane (2, 3).The results of comparative studies of cell envelopes of a variety of gram-negative organisms have also revealed the presence of a major protein band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) (21). This protein, which represents approximately 70% of the total outer membrane protein, is an integral structural component and is difficult to solubilize without dissolution of the membrane (23). Based on the results of chromatographic separation and cyanogen bromide cleavage, Schnaitman (24) has reported that the 42,000molecular weight major envelope protein in Escherichia coli 0111 is a complex consisting of at least four distinct polypeptide species (proteins 1, 2, 3a, and 3b). In a recent investigation, Rosenbusch (18) reported that separation of a major envelope protein ("matrix protein") from the murein layer in Escherichia coli BE by SDS ' Present address:

Immobilization tests were conducted on a wild-type strain of Campylobacter fetus subsp. intestinalis and on a mutant lacking an antiphagocytic cell surface component. Highly effective immobilization of the mutant, both as single cells and clumps of cells, was produced with an antiserum containing antibodies specific for the flagellar hook and filament and for the O antigen. Damage to flagellar hooks after reaction with this antiserum was observed only with cells of the mutant. Single-cell immobilization of the mutant was also produced with an antiserum specific for a heat-stable somatic antigen which was distinct from the O antigen and was exposed on the cell surface only of the mutant. Minimal immobilization of the wild strain was brought about by either of these antisera. It was shown also that O antibodies had no effect on the motility of either the wild strain or mutant. These findings indicate that antibody-mediated immobilization may be brought about by effects on the flagellar hook or cell body, as well as on the flagellar filament. Furthermore, the protection from immobilization afforded the bacterium by the antiphagocytic surface structure suggests a dual function for this virulence factor in the infected animal.