The virulence of transparent (Tr) and opaque (Op) colony types of in the genital tract of female mice was evaluated at two stages of oestrous. Isogenic pairs of Tr and Op variants were isolated from strain 57–120. Both variants exhibited a T2 morphology, but only the Op variant possessed protein II (P. II) in outer-membrane fractions. When administered by intravaginal inoculation Op gonococci were highly infective only for mice in late pro-oestrous, whereas Tr gonococci were virulent for mice at both late pro-oestrous and dioestrous. Gonococci recovered from the uterus were of both Tr and Op phenotypes in equal proportions when mice were infected at dioestrous with Tr cells. In contrast, ≫90% of recovered colonies were of Op phenotype when mice were infected at late pro-oestrous with either Op or Tr cells. These results indicate that the virulence of gonococci for the genital tract of female mice differs from that for the chicken embryo. Furthermore, gonococcal survival in the female genital tract might be attributable to phase variation from Tr to Op phenotypes.


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