Mechanism of β-Lactam Action in Streptococcus pneumoniae: the Piperacillin Paradox

Abstract

fThe human pathogen Streptococcus pneumoniae has been treated for decades with ␤-lactam antibiotics. Its resistance is now widespread, mediated by the expression of mosaic variants of the target enzymes, the penicillin-binding proteins (PBPs). Understanding the mode of action of ␤-lactams, not only in molecular detail but also in their physiological consequences, will be crucial to improving these drugs and any counterresistances. In this work, we investigate the piperacillin paradox, by which this ␤-lactam selects primarily variants of PBP2b, whereas its most reactive target is PBP2x. These PBPs are both essential monofunctional transpeptidases involved in peptidoglycan assembly. PBP2x participates in septal synthesis, while PBP2b functions in peripheral elongation. The formation of the "lemon"-shaped cells induced by piperacillin treatment is consistent with the inhibition of PBP2x. Following the examination of treated and untreated cells by electron microscopy, the localization of the PBPs by epifluorescence microscopy, and the determination of the inhibition time course of the different PBPs, we propose a model of peptidoglycan assembly that accounts for the piperacillin paradox.

Substitutions in PBP2b from β-Lactam-resistant Streptococcus pneumoniae Have Different Effects on Enzymatic Activity and Drug Reactivity

“…With cefotaxime, R6-PBP2b is 4 ϫ 10 7 -fold less reactive than PBP2x (9). Qualitatively, the reactivities measured in vitro here and in other studies for R6 PBPs (9,10,14) are in agreement with the acylations observed in vivo (18,26).…”

“…ments (12,18,22). Note that if the c 50 is considered, 5204-PBP2b is slightly less susceptible to ␤-lactam than HybridPBP2b.…”

“…An allele conferring a resistance can therefore easily be introduced and selected, provided that flanking regions are provided to allow recombination. With PBP2b, however, the whole gene could not be introduced because recombination repeatedly occurred within the coding region, resulting in a gene with the 5Ј-region from the resistant strain and the 3Ј-region retained from the susceptible strain (12,18,22). Characterization of the resulting Hybrid-PBP2B might shed light on this incomplete incorporation of the 5204-pbp2b allele in the R6 strain.…”

Profiling of β-Lactam Selectivity for Penicillin-Binding Proteins in Streptococcus pneumoniae D39

“…We report here that direct inhibition studies confirm previous findings that cefotaxime and piperacillin are coselective for PBP3 and PBP2x and that piperacillin does possess a partial relative affinity for PBP2b in our assay. Importantly, a recent study confirmed this piperacillin inhibition profile and also provided compelling evidence that PBP2b is a critical target of this drug and that combined inhibition of PBP2x and PBP2b may be key to the unusual resistance pattern seen with this molecule (43).…”

Molecular mechanisms and epidemiology of resistance in Streptococcus pneumoniae in the Middle East region

“…Recently, PBP2x seems to have been the primary target for piperacillin and not PBP2b [19]. Another recent genomewide association study identified a set of 301 single nucleotide polymorphisms, from which 73 caused amino acid alterations in cell-wall synthesis genes [20].…”