Molecular characterization and correlation with β-lactam resistance of penicillin-binding protein2x, 2b, and 1a of <i>Streptococcus pneumoniae</i> in clinical pneumococcal isolates

Transpeptidases (TPDs), which belong to the penicillin-binding protein (PBP) families, are crucial for bacterial survival and are targeted by β-lactam antibiotics. We sought to determine the prevalence and positions of substitutions across the TPDs of the PBPs and their potential association with β-lactam resistance in pneumococcal strains. A total of 337 pneumococcal isolates were collected from pediatric patients in Suzhou, China, from 2013 to 2022. The serotype, antimicrobial susceptibility, and sequence in PBP1a, 2b, and 2x of these isolates were determined by Quellung reaction, E-test, and polymerase chain reaction sequencing, respectively. Homology modeling was utilized to compare the three-dimensional structure of the aberrant PBP strains with that of a reference R6 strain. Alterations within the PBPs conserved motifs and adjacent sequences SXXK and SXN contributed to penicillin resistance. Substitutions in TPDs were common and have a high degree of diversity. Two insertion amino acid (AA) motif variations, Tyr-Thr-Try(YTW) and Try-Thr-Tyr(WYT), located between AA 424 and 425 in PBP2b, were found to change the conformation of PBP2b in seven strains, and the YTW alteration has not been documented previously. The mutation rate of the TPDs in the PCV13 serotype strains was found to be higher than that observed in strains of non-vaccine serotypes. A high degree of diversity of TPD sequence in different PBPs was observed among the antibiotic non-susceptible pneumococcal isolates. The modeling analysis revealed conformational changes in the PBP2b following the insertion of the AA. IMPORTANCE: Since sequence variations both within and outside the penicillin-binding proteins have been reported to influence β-lactam minimum inhibitory concentrations (MICs), we examined the variability of isolates with transpeptidase (TPD) substitutions. Further quantification of the potential contributions of individual TPD positions and their interactions to β-lactam MICs could provide insights into resistance mechanisms and should be a priority of future investigation.