In a survey of negatively stained preparations of the prototype strains of Neisseria meningitidis, pili were detected on three strains. However, these pili were detected on fewer than 5% of cells in populations of these three strains. Those individual cells with pili were seldom observed to contain more than two or three pili per cell. In contrast, nearly all cells of the nonprototype group B strain ATCC 13090 had numerous pili on their surfaces. When viewed in frozen-etched replicas, a few pili were observed lying on the cell surface of this latter strain. An annular structure was also found in frozen-etched replicas. This structure usually consisted of a series of concentric rings that were always found on the flat side of these bean-shaped cells. It is concluded that such structures represent a differentiated portion of the cell wall, which is involved in cross-wall formation during synthesis of the cell septum in dividing cells
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