Differences in morphology of Bacteroides nodosus attributable to culture media

A single strain of Bacteroides nodosus was cultured under controlled conditions on hoof agar or in either biphasic medium or hoof broth. The gross and ultrastructural appearances of organisms were compared one with the other and with B. nodosus as seen in necrotic detritus obtained from a case of non-progressive foot rot. The possible implications of those morphological differences with regard to the antigenic structure of cells and their suitability for vaccine production, are briefly discussed. The results reported in this paper are confined to the gross and ultrastructural differences seen when a single strain of B. nodosus was cultured under controlled conditions either on solid or in liquid media. The “rough” colony form was predominant on hoof agar and considered to be normal but both “smooth” and intermediate colony forms were also observed either when plates were dried insufficiently or after repeated subculture of organisms in hoof broth. B. nodosus organisms seen in smears of necrotic detritus, were surrounded by a clear halo, bore filamentous appendages thought to be pili and possessed a layered cell envelope typical of Gram-negative bacteria. Electron-dense polychromatic granules were either small and scattered through the nucleoplasm or were much larger and occurred singly, often near the poles. B. nodosus cells grown on hoof agar were sometimes longer but in other morphological respects were similar to those seen in necrotic detritus. No capsular material was demonstrable to account for the clear zone surrounding the cell envelope. After 24-hr incubation in either biphasic medium or hoof broth, B. nodosus showed evidence of suboptimal cultural conditions as indicated by absence of pili, wrinkling of the cell envelope, appearance of amorphous extracellular structures, cytoplasmic vacuolation and cell lysis.