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.