Diversity of substitutions within or adjacent to conserved amino acid motifs of penicillin-binding protein 2X in cephalosporin-resistant Streptococcus pneumoniae isolates

Abstract

The sequence of an approximately 1.1-kb DNA fragment of the pbp2x gene, which encodes the transpeptidase domain, was determined for 35 clinical isolates of Streptococcus pneumoniae for which the cefotaxime (CTX) MICs varied. Strains with substitutions within a conserved amino acid motif changing STMK to SAFK and a Leu-to-Val change just before the KSG motif were highly resistant to CTX (MIC, >==2 microgram/ml). Strains with substitutions adjacent to SSN or KSG motifs had low-level resistance. The amino acid substitutions were plotted on the three-dimensional crystallographic structure of the transpeptidase domain of PBP2X. Transformants containing pbp2x from strains with high-level CTX resistance increased the CTX MIC from 0. 016 microgram/ml to 0.5 to 1.0 microgram/ml.

FIG. 1

Deduced amino acid sequences of part of PBP 2X from representative strains from each group. The sequence of PSSP R6 is shown on the top line. Numbering is based on published data on the R6 strain (15). Only amino acids differing from the R6 sequence are shown. Boxes represent conserved amino acid motifs.

FIG. 2

Stereoview of the transpeptidase domain of PBP 2X of the high-level CTX-resistant PRSP strain 31/KU5. The structure was constructed by using the crystallographic coordinates of PBP 2X of Pares et al. (PDB entry code 1 PMD) (21). Cefditoren of oral cephalosporin was shown in the long groove as a β-lactam model. Amino acids differing from the R6 sequence are indicated by colored circles (blue, substitutions within or adjacent to conserved motifs of STMK and KTG; green, substitutions adjacent to SSN and KSG in strains classified into groups I and II; yellow, general substitutions).