Phenotypically determined resistance of gonococci to killing by normal human serum can be generated by growth of susceptible organisms in media containing guinea pig, calf or human serum. However, even in the best medium tested, i.e. defined medium containing 50% (v/v) guinea pig serum, resistance was greatly reduced after 24 h incubation and the maximum number of colony-forming units generated was 10 to 10 ml. Resistance was not acquired after incubation in guinea pig serum at low temperature (8°C), supporting previous indications that metabolic activity was necessary for the generation of resistance. Alteration of the concentration of glutamine, proline, lactate or iron had little or no effect on the generation of serum resistance under the conditions used. Optimum conversion to resistance occurred at pH 6.0 to 6.5 and both non-diffusible and diffusate fractions of dialysed guinea pig serum promoted resistance. Furthermore, resistant organisms could be produced by incubation in defined medium containing the diffusate from guinea pig serum and 0.1% bovine serum albumin, a step which should facilitate identification of the resistance-promoting factor.


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