Analysis of amino acid sequences of penicillin-binding protein 2 in clinical isolates of Neisseria gonorrhoeae with reduced susceptibility to cefixime and ceftriaxone

Neisseria gonorrhoeae strains with reduced susceptibility to cefixime and ceftriaxone, with minimum inhibitory concentrations (MICs) of cefixime of 0.125–0.25 μg/ml and ceftriaxone of 0.031–0.125 μg/ml, were isolated from male urethritis patients in Tokyo, Japan, in 2006. The amino acid sequences of PenA, penicillin-binding protein 2, in these strains were of two types: PenA mosaic and nonmosaic strains. In the PenA mosaic strain, some regions in the transpeptidase-encoding domain in PenA were similar to those of Neisseria perflava/sicca, Neisseria cinerea, Neisseria flavescens, Neisseria polysaccharea, and Neisseria meningitidis. In the PenA nonmosaic strain, there was a mutation of Ala-501 to Val in PenA. In addition, we performed homology modeling of PenA wild-type and mosaic strains and compared them. The results of the modeling studies suggested that reduced susceptibility to cephems such as cefixime and ceftriaxone is due to a conformational alteration of the β-lactam-binding pocket. These results also indicated that the mosaic structures and the above point mutation in PenA make a major contribution to the reduced susceptibility to cephem antibiotics.