Scanning electron microscopic study of virulent and avirulent colonies of Neisseria gonorrhoeae.

A preparation procedure for scanning electron microscopy was developed by which gononcoccal colonies can be studied directly on the agar surface. After glutaraldehyde fixation directly in petri dishes, small agar pieces were cut out and dehydrated stepwise in increasing concentrations of ethanol. The blocks were thereafter transferred to a critical-point drying apparatus, via steps of increased gradients up to 100% amyl acetate. By this method five different gonococcal colony types could be distinguished analogous to light microscopic observations made by others. At higher magnifications an abundance of intercellular strands was found between the cells in virulent type 1 and 2 colonies, but not in the avirulent types 3 through 5. These strands seemed to anchor the cells to each other and to the agar surface. The presence of such structures probably explains the highly convex surface of virulent colonies and explains why colonies of avirulent strains exhibit a radial extension and a flat upper surface. The nature of these filamentous intercellular strands is discussed.