Influence of colony type on susceptibility of gonococci to killing by human serum

The virulence of gonococci is related to colony morphology and has been attributed to pili found only on the virulent types. Because complement-mediated serum killing is a major defense against other Gram-negative bacteria, we tested the resistance to normal human serum of virulent and avirulent types from two strains. Virulent types were more resistant than avirulent types. This stable, relative serum resistance of virulent colony types persisted during prolonged passage on agar. In addition, virulent types acquired complete resistance after exposure to serum which was rapidly lost during passage on agar. This acquired, complete resistance helps explain the observation that freshly isolated gonococci are resistant to their host's serum, but lose this resistance when passed on agar. The capacity to become resistant to the serum of newly infected patients may contribute to virulence of T1 and T2 gonococci.