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IN RECENT YEARS THERE HAS BEEN INCREASED INTEREST IN examining bacteria obtained directly from patients and infected animals for two reasons: to confirm that putative determinants of virulence indicated by experiments in vitro are, in fact, produced in vivo, and to discover virulence determinants that are induced in vivo but have not yet been seen in vitro. In almost all experiments, however, the in vivo conditions are accepted as a whole. There have been hardly any attempts to dissect out specific host factors that are responsible for the production in vivo of defined virulence determinants. Specification of the influence of factors in the host environment on virulence determinants should be done in future studies on pathogenicity. This paper describes an example of what can be done. Gonococci obtained directly from patients resist killing by human serum. Most people believe it is important in pathogenicity, and it is of two types. There is a type stable on subculture notably shown by strains from disseminated infection. For most strains, however, (certainly those from u rethral pus) there is a rapid loss of resistance. It was not long before we showed that the change was so rapid that it must be a phenotypic change (1). We then went on to show that in human serum and in human red blood cell extracts there was a factor which would induce the gonococcus to resistance. This occurred in vitro if one added the host factor (2-4). We called it resistance-inducing factor (RIF) and we have been trying to answer the following five questions. First of all, does it appear important in gonorrhea? If it does, what is it? How does it induce resistance? What is the gonococcal determinant concerned? And the object of the exercise does it all work in vivo? In order to answer the first two questions, we needed an assay for RIF. We took serum-susceptible gonococci and induced them to resistance over 3 h with dilutions of the inducer. We took a small numoer of these gonococci and incubated them for a short time with either fresh human serum or heated human serum (as controls). Viable counts gave us the percentage resistance of the experimental samples. Plotting percentage resistance against dilution yielded the dilution giving 50% resistance, which we arbitrarily called 1 RIF unit (4,5). RIF was important in gonorrhea (6); it was present in male and female genital secretions, in about 20% human sera, with the proportion greater (31 %) in females infected for the first time (7), in polymorphonuclear and mononuclear phagocyte extracts which are very active (200x red blood cell lysates, 6000x sera) (8) and it affects different gonococcal strains (30 tested). Thirty isolates from Paris venereal disease clinics, in fact, were induced to resistance by RIF. So we had something that looked important. What was it?

Gonococci in vivo: Host CMP-NANA, Sialylated Lipopolysaccharide and Serum Resistance