Gonococcal color and opacity variants: virulence for chicken embryos.

Previous studies have noted that the prevalence of certain gonococcal colony types is influenced by the physiological state of the human host. Consequently eight different gonococcal strains were passed on clear typing medium, and opaque (Op) and transparent (Tr) variants of heavily pilated (P++) bacteria were selected. P++Op and P++Tr were injected into 11-day-old chicken embryos, and the resulting mortality was determined. In every case the Tr variants were more lethal to the embryos than were the Op variants (P = 0.01). P++Tr were stable in vivo, but chicken embryos injected with P++Op strains developed bacteremia with P++Tr organisms. However, chicken blood was not bactericidal for either colonial variant. These results indicate that transparent pilated bacteria may be more virulent than opaque pilated bacteria and that in vivo selection of transparent forms occurs.