Simpson's index of diversity was used to produce a single numerical value to compare the abilities of single or combined typing schemes to discriminate between unrelated isolates. This calculation was used to compare the discriminating power of auxotype and serovar determination and plasmid content analysis, either singly or in combination, for Neisseria gonorrhoeae isolates having different antimicrobial susceptibilities (i.e., antibiotic-susceptible isolates and those that produce penicillinase, carry plasmid-mediated resistance to tetracycline, have chromosomally mediated penicillin resistance, or both produce penicillinase and carry plasmid-mediated resistance to tetracycline). Plasmid content analysis and auxotype determination produced the lowest level of discrimination, while a combination of auxotype and serovar typing schemes generally provided higher levels of discrimination. Addition of plasmid content analysis to auxotype and serovar typing provided additional discrimination only with penicillinase-producing isolates. For isolates that carried plasmid-mediated resistance to tetracycline, isolates that were tetracycline resistant, isolates that both produced penicillinase and carried plasmid-mediated resistance to tetracycline, or isolates that had chromosomally mediated penicillin resistance, none of the typing methods produced high discriminatory indices, indicating that these isolates are probably derived from relatively few clones.
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