Selection of Opa(+) Neisseria gonorrhoeae by limited availability of normal human serum

Experimental infections of human male volunteers with Neisseria gonorrhoeae have provided valuable insights into the early stages of gonorrheal disease. Bacterial variants expressing outer membrane opacity (Opa) proteins appear to be selected from the inoculum during a period in which total recoverable numbers of bacteria decrease rapidly. This apparent survival advantage occurs simultaneously with the onset of an inflammatory response, characterized by local production of interleukin 6 (IL-6) and IL-8 and the appearance of leukocytes in urine. Since the inflammatory response may also result in the presence of serum factors on the mucosal surface, we investigated the possibility that killing in normal human serum (NHS) leads to the selection of Opa(+) variants. We therefore studied killing of separate populations and mixtures of Opa(-) and Opa(+) N. gonorrhoeae MS11mk in NHS. Expression of an Opa protein conferred a survival advantage upon the organism; i.e., the Opa(+) variants were more serum resistant than their isogenic Opa(-) counterparts, resulting in a selection for Opa(+) phenotypes when a mixture of Opa(+) and Opa(-) gonococci (GC) was exposed to submaximal doses of NHS. This selection was observed in three different lipooligosaccharide (LOS) backgrounds, indicating that it was not due to a difference in LOS expression between Opa(-) and Opa(+) phenotypes. Incubation in NHS of sialylated GC resulted in a similar selection for Opa(+) variants. The presence of normal human urine during the serum killing assay had no effect on the selection phenomenon but drastically depleted NHS of bactericidal activity, which was found to be at least partly due to complement inhibition. The results suggest that serum killing may contribute to the transition from Opa(-) to Opa(+) phenotypes during the early stages of infection of the male urethra.