Susceptibility of Neisseria gonorrhoeae to rifampicin and thiamphenicol: correlation with protein I antigenic determinants.

Tests of susceptibility to thiamphenicol and rifampicin were done for 85 strains of Neisseria gonorrhoeae, including 28 beta-lactamase-producing and three spectinomycin-resistant isolates. Strains were serogrouped by co-agglutination with 14 monoclonal antibodies that are specific for different antigenic determinants on protein I of the gonococcal outer membrane. Thus the isolates could be classified into one of the serogroups WI, WII, or WIII and could be subgrouped further into several serovars. The minimal inhibitory concentration (MIC) of thiamphenicol ranged between 0.125 and 4.0 micrograms/ml and that of rifampicin between 0.016 and 4.0 micrograms/ml. All isolates with a MIC of rifampicin of greater than or equal to 0.5 microgram/ml had a MIC of thiamphenicol of greater than or equal to 2.0 micrograms/ml. Decreased susceptibility (thiamphenicol MIC, greater than or equal to 1.0 microgram/ml; rifampicin MIC, greater than or equal to 0.5 microgram/ml) to both drugs was correlated with serogroup WII specificity and also correlated with strains that belonged to serovars characterized by a positive reaction with the WII monoclonal antibody 2G2.