X-ray Structure of Catenated Lytic Transglycosylase SltB1

Fluorescence Assessment of the AmpR-Signaling Network of Pseudomonas aeruginosa to Exposure to β-Lactam Antibiotics

Gram-negative bacteria have evolved an elaborate pathway to sense and respond to exposure to β-lactam antibiotics. The β-lactam antibiotics inhibit penicillin-binding proteins, whereby the loss of their activities alters/damages the cell-wall peptidoglycan. Bacteria sense this damage and remove the affected peptidoglycan into complex recycling pathways. As an offshoot of these pathways, muropeptide chemical signals generated from the cell-wall recycling manifest the production of a class C β-lactamase, which hydrolytically degrades the β-lactam antibiotic as a resistance mechanism. We disclose the use of a fluorescence probe that detects the activation of the recycling system by the formation of the key muropeptides involved in signaling. This same probe additionally detects natural-product cell-wall-active antibiotics that are produced in situ by cohabitating bacteria.

Cell-Wall Recycling of the Gram-Negative Bacteria and the Nexus to Antibiotic Resistance

The importance of the cell wall to the viability of the bacterium is underscored by the breadth of antibiotic structures that act by blocking key enzymes that are tasked with cell-wall creation, preservation, and regulation. The interplay between cell-wall integrity, and the summoning forth of resistance mechanisms to deactivate cell-wall-targeting antibiotics, involves exquisite orchestration among cell-wall synthesis and remodeling and the detection of and response to the antibiotics through modulation of gene regulation by specific effectors. Given the profound importance of antibiotics to the practice of medicine, the assertion that understanding this interplay is among the most fundamentally important questions in bacterial physiology is credible. The enigmatic regulation of the expression of the AmpC β-lactamase, a clinically significant and highly regulated resistance response of certain Gram-negative bacteria to the β-lactam antibiotics, is the exemplar of this challenge. This review gives a current perspective to this compelling, and still not fully solved, 35-year enigma.

Total Syntheses of Bulgecins A, B, and C and Their Bactericidal Potentiation of the β-Lactam Antibiotics

The bulgecins are iminosaccharide secondary metabolites of the Gram-negative bacterium Paraburkholderia acidophila and inhibitors of lytic transglycosylases of bacterial cell-wall biosynthesis and remodeling. The activities of the bulgecins are intimately intertwined with the mechanism of a cobiosynthesized β-lactam antibiotic. β-Lactams inhibit the penicillin-binding proteins, enzymes also critical to cell-wall biosynthesis. The simultaneous loss of the lytic transglycosylase (by bulgecin) and penicillin-binding protein (by β-lactams) activities results in deformation of the septal cell wall, observed microscopically as a bulge preceding bacterial cell lysis. We describe a practical synthesis of the three naturally occurring bulgecin iminosaccharides and their mechanistic evaluation in a series of microbiological studies. These studies identify potentiation by the bulgecin at subminimum inhibitory concentrations of the β-lactam against three pathogenic Gram-negative bacteria and establish for the first time that this potentiation results in a significant increase in the bactericidal efficacy of a clinical β-lactam.