The outer membranes of gram-negative bacteria are unique structures. This comparison of three gram-negative diplococci--Neisseria gonorrheae, N. cinnerea, and N. meningitidis--illustrates that outer membranes vary quite a bit from one organism to the next. Characteristically, outer membranes appear heavily textured in electron microscopic views. Here, the extent of the texture on the cell surface is shown to be slight on N. gonorrheae (Fig. 1), moderate on N. cinnerea (Fig. 2), and very extensive on N. meningitidis (Fig. 3). Other studies have indicated that much of the toxicity of N. meningitidis(causative agent of septicemia and meningitis) arises from its release of endotoxin in the form of membranous protrusions called blebs. Here, the propensity of the N. meningitidis outer membrane to form blebs is obvious when directly compared with other membranes of the same genus.
Students may find these three electron micrographs interesting because of the diversity revealed about outer membrane structure, the connection indicated between virulence and ultrastructure, and the close-up views of the characteristic diplococcus morphology.