The NorM efflux pump of Neisseria gonorrhoeae and Neisseria meningitidis recognizes antimicrobial cationic compounds

Abstract

In Neisseria gonorrhoeae and Neisseria meningitidis, we identified a gene that would encode a protein highly similar to NorM of Vibrio parahaemolyticus (Y. Morita et al., Antimicrob. Agents Chemother. 42:1778-1782, 1998). A nonpolar insertional mutation in either the gonococcal or meningococcal norM gene resulted in increased bacterial sensitivity to compounds harboring a quaternary ammonium on an aromatic ring (e.g., ethidium bromide, acriflavine hydrochloride, 2-N-methylellipticinium, and berberine). The presence of point mutations within the -35 region of a putative norM promoter or a likely ribosome binding site resulted in an increased resistance of gonococci and meningococci to the same compounds, as well as to norfloxacin and ciprofloxacin. Structure-activity relationship studies with putative NorM substrates have found that a cationic moiety is essential for NorM recognition.

FIG. 1.

Multiple sequence alignment of the most conserved regions of NorM with representative homologues. Ng, N. gonorrhoeae FA19; Nm, N. meningitidis Z2491; Vc, V. cholerae N16961; Vp, V. parahaemolyticus; Ec, E. coli. Amino acid numbering is shown at the beginning of the sequence presented. Identical residues (present in more than 50% of the five sequences) are presented in boldface. TMs are overlined. The GKFGXP sequences, characteristic of the MATE proteins from the NorM cluster, are boxed.

FIG. 2.

Structural attributes of NME compared to those of ELL, BC, and TPP.

FIG. 3.

Agar dilution of CIP and PD 0132927 against isogenic N. gonorrhoeae strains harboring −35 and −35/RBS norM promoter mutants in wild-type (GC805 and GC806) and quinolone-resistant (GC807 and GC808) backgrounds. Five-microliter spots containing 1 × 10⁴ to 4 × 10⁴ CFU were incubated for 24 h. No growth was observed on plates containing 8 μg of CIP per ml.