Peptidoglycan DD-peptidases have distinct activities that impact fitness of Acinetobacter baumannii

Abstract

The Gram-negative cell envelope is a vital interface between the bacterial cell and its environment. It acts as a selective barrier, blocking harmful agents while permitting nutrient uptake. Additionally, it enables environmental sensing and adaptive responses. Structurally, it is composed of the outer membrane, the cytoplasmic (inner) membrane, and the periplasm, which contains the peptidoglycan layer. Peptidoglycan is a conserved polymer that provides structural integrity, allowing the cell to withstand the internal turgor. It consists of glycan strands connected by short peptides, forming a mesh-like structure. In Gram-negative bacteria, the majority of the peptidoglycan subunits contain tetrapeptides. Tetrapeptides are generated through the action of DD-carboxypeptidases (DD-CPases), which cleave the terminal D-alanine from pentapeptides. Gram-negative bacteria encode multiple DD-CPases, but their precise role in maintaining cell shape and structural integrity remain poorly understood. The nosocomial pathogen Acinetobacter baumannii encodes three putative DD-CPases. To investigate the role of these enzymes, we generated single mutants, as well as double mutants in dacC, dacD, and pbpG, which encode the homologs of Escherichia coli DD-CPases PBP5, PBP6a, PBP6b, and the endopeptidase (DD-EPase) PBP7, respectively. We assessed the mutants for changes in cell morphology, growth dynamics, and stress tolerance. Additionally, we analyzed the composition of their peptidoglycan layers to determine the biochemical consequences of their inactivation. Each mutant exhibited distinct alterations in coccobacillary morphology and growth. Peptidoglycan analysis confirmed the enzymes possess DD-CPase activity, and PBP6b also demonstrated endopeptidase activity. Together, our results demonstrate that each peptidoglycan-modifying enzyme contributes uniquely to cell growth and morphology. These findings underscore their non-redundant functions and suggest their specific activities may serve as valuable targets for developing new antimicrobial therapies. ImportanceDD-peptidases, including carboxypeptidases and endopeptidases are crucial for maintaining cell envelope homeostasis, with distinct roles for each enzyme in cell wall biogenesis and structural integrity. The enzymatic characterization presented in this study not only advance our understanding of fundamental A. baumannii biology but also highlight these enzymatic activities as targets for development of innovative therapeutic strategies to combat infections caused by this multidrug-resistant microbe.