New Aspects of the Interplay between Penicillin Binding Proteins, <i>murM</i> , and the Two-Component System CiaRH of Penicillin-Resistant Streptococcus pneumoniae Serotype 19A Isolates from Hungary

Abstract

The <i>Streptococcus pneumoniae</i> clone Hungary^19A-6 expresses unusually high levels of β-lactam resistance, which is in part due to mutations in the MurM gene, encoding a transferase involved in the synthesis of branched peptidoglycan. Moreover, it contains the allele <i>ciaH232</i>, encoding the histidine kinase CiaH (M. Müller, P. Marx, R. Hakenbeck, and R. Brückner, Microbiology 157:3104-3112, 2011, https://doi.org/10.1099/mic.0.053157-0). High-level penicillin resistance primarily requires the presence of low-affinity (mosaic) penicillin binding protein (PBP) genes, as, for example, in strain Hu17, a closely related member of the Hungary^19A-6 lineage. Interestingly, strain Hu15 is β-lactam sensitive due to the absence of mosaic PBPs. This unique situation prompted us to investigate the development of cefotaxime resistance in transformation experiments with genes known to play a role in this phenotype, <i>pbp2x</i>, <i>pbp1a</i>, <i>murM</i>, and <i>ciaH</i>, and penicillin-sensitive recipient strains R6 and Hu15. Characterization of phenotypes, peptidoglycan composition, and CiaR-mediated gene expression revealed several novel aspects of penicillin resistance. The <i>murM</i> gene of strain Hu17 (<i>murM</i>_Hu17), which is highly similar to <i>murM</i> of <i>Streptococcus mitis</i>, induced morphological changes which were partly reversed by <i>ciaH232. murM</i>_Hu17 conferred cefotaxime resistance only in the presence of the <i>pbp2x o</i>f strain Hu17 (<i>pbp2x</i>_Hu17). The <i>ciaH232</i> allele contributed to a remarkable increase in cefotaxime resistance in combination with <i>pbp2x</i>_Hu17 and <i>pbp1a</i> of strain Hu17 (<i>pbp1a</i>_Hu17), accompanied by higher levels of expression of CiaR-regulated genes, documenting that <i>ciaH232</i> responds to PBP1a_Hu17-mediated changes in cell wall synthesis. Most importantly, the proportion of branched peptides relative to the proportion of linear muropeptides increased in cells containing mosaic PBPs, suggesting an altered enzymatic activity of these proteins.