DISCRIMINATORY POWER OF TYPING SCHEMES BASED ON SIMPSON INDEX OF DIVERSITY FOR NEISSERIA-GONORRHOEAE

Simpson's index of diversity was used to produce a single numerical va lue to compare the abilities of single or combined typing schemes to d iscriminate between unrelated isolates. This calculation was used to c ompare the discriminating power of auxotype and serovar determination and plasmid content analysis, either singly or in combination, for Nei sseria gonorrhoeae isolates having different antimicrobial susceptibil ities (i.e., antibiotic-susceptible isolates and those that produce pe nicillinase, carry plasmid-mediated resistance to tetracycline, have c hromosomally mediated penicillin resistance, or both produce penicilli nase and carry plasmid-mediated resistance to tetracycline). Plasmid c ontent analysis and auxotype determination produced the lowest level o f discrimination, while a combination of auxotype and serovar typing s chemes generally provided higher levels of discrimination. Addition of plasmid content analysis to auxotype and serovar typing provided addi tional discrimination only with penicillinase-producing isolates. For isolates that carried plasmid-mediated resistance to tetracycline, iso lates that were tetracycline resistant, isolates that both produced pe nicillinase and carried plasmid-mediated resistance to tetracycline, o r isolates that had chromosomally mediated penicillin resistance, none of the typing methods produced high discriminatory indices, indicatin g that these isolates are probably derived from relatively few clones.