Axial fibrils of anaerobic spirochetes: Ultrastructure and chemical characteristics
Axial fibrils isolated from three species of Spirochaeta were examined by electron microscopy. Each axial fibril consisted of three main morphological components similar to those present in bacterial flagella: an insertion apparatus, a proximal hook near the insertion apparatus, and a filamentous portion. The filamentous portion appeared to be composed of a core surrounded by a non-striated sheath. The core consisted of globules arranged in a helical pattern. Striated tubular structures were observed in preparations of disrupted cells.
Purified axial fibril cores were dissociated by acid, alkali, urea, or guanidine-HCl, but were stable to the action of various enzymes. The proximal hooks were more resistant than the fibril cores to treatment with acid, alkali, or other chemicals. Aspartic and glutamic acids, alanine, leucine, glycine, and serine were the most abundant amino acids in axial fibril core hydrolysates. At pH 2.4 in 4 M urea, acid-dissociated axial fibril cores yielded two protein bands in polyacrylamide gel electrophoresis. Structures morphologically closely resembling the native axial fibril cores were assembled in vitro by raising the pH of acid-dissociated fibril core preparations. The data show that striking similarities exist between spirochetal axial fibrils and bacterial flagella with regard to their fine structure and physical and chemical characteristics. The results support the view that axial fibrils play a role in spirochetal motility.
Spirochetes from which the axial fibrils had been removed by treatment with acid maintained their coiled shape. The peptidoglycan layer purified from two species of Spirochaeta frequently retained its coiled configuration. It is concluded that, in the spirochetes studied, the axial fibrils are not responsible for maintaining the coiled shape of the cells, but that this function is served by the peptidoglycan.
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