Mechanistic Basis for Decreased Antimicrobial Susceptibility in a Clinical Isolate of Neisseria gonorrhoeae Possessing a Mosaic-Like <i>mtr</i> Efflux Pump Locus

Abstract

Recent reports suggest that mosaic-like sequences within the <i>mtr</i> (<i>m</i>ultiple <i>t</i>ransferable <i>r</i>esistance) efflux pump locus of <i>Neisseria gonorrhoeae</i>, likely originating from commensal <i>Neisseria</i> sp. by transformation, can increase the ability of gonococci to resist structurally diverse antimicrobials. Thus, acquisition of numerous nucleotide changes within the <i>mtrR</i> gene encoding the transcriptional repressor (MtrR) of the <i>mtrCDE</i> efflux pump-encoding operon or overlapping promoter region for both along with those that cause amino acid changes in the MtrD transporter protein were recently reported to decrease gonococcal susceptibility to numerous antimicrobials, including azithromycin (Azi) (C. B. Wadsworth, B. J. Arnold, M. R. A. Satar, and Y. H. Grad, mBio 9:e01419-18, 2018, https://doi.org/10.1128/mBio.01419-18). We performed detailed genetic and molecular studies to define the mechanistic basis for why such strains can exhibit decreased susceptibility to MtrCDE antimicrobial substrates, including Azi. We report that a strong <i>cis</i>-acting transcriptional impact of a single nucleotide change within the -35 hexamer of the <i>mtrCDE</i> promoter as well gain-of-function amino acid changes at the C-terminal region of MtrD can mechanistically account for the decreased antimicrobial susceptibility of gonococci with a mosaic-like <i>mtr</i> locus.<b>IMPORTANCE</b> Historically, after introduction of an antibiotic for treatment of gonorrhea, strains of <i>N. gonorrhoeae</i> emerge that display clinical resistance due to spontaneous mutation or acquisition of resistance genes. Genetic exchange between members of the <i>Neisseria</i> genus occurring by transformation can cause significant changes in gonococci that impact the structure of an antibiotic target or expression of genes involved in resistance. The results presented here provide a framework for understanding how mosaic-like DNA sequences from commensal <i>Neisseria</i> that recombine within the gonococcal <i>mtr</i> efflux pump locus function to decrease bacterial susceptibility to antimicrobials, including antibiotics used in therapy of gonorrhea.