Phenotypic changes in the resistance of Neisseria gonorrhoeae to killing by normal human serum

Gonococci adapted to growth in chambers implanted subcutaneously into guinea pigs are resistant to killing by human serum. This resistance is lost after a few generations in vitro both in culture medium and in fluid taken from guinea-pig chambers. The rate of loss is too rapid to occur by mutation and selection. Furthermore, the resistance is regained after a few generations when bacteria from the first in vitro culture are inoculated back into guinea-pig chambers in vivo. Hence the loss of serum resistance in vitro and the gain in vivo are probably due to phenotypically controlled events. Such events could be important in the pathogenicity of Neisseria gonorrhoeae.