Decreased azithromycin susceptibility of Neisseria gonorrhoeae due to mtrRmutations

Single-dose azithromycin therapy has recently been used in Uruguay for the treatment of uncomplicated gonococcal infections. As part of an active surv eillance study to monitor the emergence of antibiotic resistance in gonococ cal isolates, we examined the levels of azithromycin susceptibility in 51 c onsecutive isolates obtained from males with uncomplicated gonococcal ureth ritis, Isolates with decreased susceptibility to azithromycin (MICs, 0.25 t o 0.5 mu g/ml) were common, and these isolates often displayed cross-resist ance to hydrophobic antimicrobial agents (erythromycin and Triton X-100), R esistance to erythromycin and Triton X-100 is frequently due to overexpress ion of the mtrCDE-encoded efflux pump mediated by mutations in the mtrR gen e, which encodes a transcriptional repressor that modulates expression of t he mtrCDE operon. Accordingly, me questioned whether clinical isolates that express decreased azithromycin susceptibility harbor mtrR mutations. Promo ter mutations that would decrease the level of expression of mtrR as well a s a missense mutation at codon 45 in the mtrR-coding region that would resu lt in a radical amino acid replacement within the DNA-binding motif of MtrR were found in these strains. When these mutations were transferred into az ithromycin-susceptible strain FA19 by transformation, the susceptibility of gonococci to azithromycin was decreased by nearly 10-fold, The mtrCDE-enco ded efflux pump system was responsible for this property since insertional inactivation of the mtrC gene resulted in enhanced susceptibility of gonoco cci to azithromycin. We conclude that the mtrCDE-encoded efflux: pump can r ecognize azithromycin and that the emergence of gonococcal strains with dec reased susceptibility to azithromycin can, in part, be explained by mtrR mu tations.