Analysis of mutations in multiple loci of Neisseria gonorrhoeae isolates reveals effects of PIB, PBP2 and MtrR on reduced susceptibility to ceftriaxone.

OBJECTIVES To elucidate loci in Neisseria gonorrhoeae implicated in reduced susceptibility to ceftriaxone. METHODS N. gonorrhoeae isolates were collected in Shanghai, China, in 2005 and 2008. Twenty-eight isolates with reduced susceptibility to ceftriaxone (CRO(Red); MIC = 0.125-0.25 mg/L) were studied for mutations in PorB (porB), MtrR (mtrR), PBP2 (penA) and PBP1 (ponA). The mutation profiles of the 28 CRO(Red) isolates were compared with those of 32 ceftriaxone-susceptible isolates (CRO(S); MIC = 0.004-0.016 mg/L). porB-based DNA sequence typing and N. gonorrhoeae multi-antigen sequence typing (NG-MAST) analyses were performed. RESULTS Significantly more CRO(Red) isolates (89.3%) exhibited a PIB phenotype as compared with the CRO(S) isolates (59.4%) (P = 0.02). Double mutations (G45D/H105Y or A39T/H105Y) in MtrR were associated with CRO(Red) phenotypes. A 'wild-type' MtrR protein characterized CRO(Red) isolates (50.0%, 14/28), while a single H105Y mutation was observed only in CRO(S) isolates (43.8%, 14/32). Both CRO(Red) and CRO(S) isolates carried an '-A' deletion in the mtrR promoter. Six of 15 mutation patterns observed in PBP2 were new. Mutation patterns XIII (17.9% of CRO(Red) isolates) and XVII or XVIII (25.0% of CRO(Red) isolates) of PBP2 comprised A501V/G542S or A501V/P551S double mutations and were associated with a CRO(Red) phenotype. The mosaic PBP2 (pattern X) was not observed. The L421P mutation in PBP1 was observed in all CRO(Red) and in 97.0% of CRO(S) isolates. CRO(Red) isolates were non-clonal. CONCLUSIONS Reduced susceptibility to ceftriaxone in N. gonorrhoeae is mediated by porB1b alleles and is associated with specific mutations in PBP2 and in the DNA binding and dimerization domains of MtrR.