Evidence for the presence of a capsule in Vibrio vulnificus.

Vibrio vulnificus strain FCC, isolated from a patient with a wound infection, and reference strain ATCC 27562, were examined by electron microscopy for the presence of capsules. Both strains had a layer heavily stained with ruthenium red. The number of stained cells was high in strain FCC and low in strain ATCC 27562. The proportion of stained cells correlated with virulence against mice and with susceptibility to the bactericidal activity of normal human serum. Rapid freezing and substitution fixation, a mild method, revealed on the cell surface a fibrous layer of relatively low electron density, which we considered to represent a capsule.