The Bacterial Cell Wall

The bacterial cell wall represents a very complex structure disconnecting the interior of single-cell organisms from the environment, thus protecting, but also enabling, them to interact with the surrounding milieu and to exchange both substances and information. Knowledge of the biochemistry of the cell wall (components) and the genetic background helps to understand their significance with regard to microbiology and immunology of bacteria. This book represents the second edition of a publication which was presented nearly 20 years ago in the German language (Die bakterielle Zellwand). Since that time our knowledge in this field has been significantly enlarged. Therefore, the manuscript had to be completely revised and updated. To maintain both the size and the introductory character of the book at least to a great extent, the authors had to restrict the presented material to that which appears basic and most important. This requirement must inevitably bring about many subjective factors. As pointed out in the first edition, the term cell wall was not taken too strictly. Since the constituents located outside the cytoplasmic membrane are frequently difficult to divide in structure, localisation, and/or function into true cell wall components and supplementary substances, they are all at least briefly mentioned.

This book provides an up-to-date overview of the architecture and biosynthesis of bacterial and archaeal cell walls, highlighting the evolution-based similarities in, but also the intriguing differences between the cell walls of Gram-negative bacteria, the Firmicutes and Actinobacteria, and the Archaea. The recent major advances in this field, which have brought to light many new structural and functional details, are presented and discussed. Over the past five years, a number of novel systems, e.g. for lipid, porin and lipopolysaccharide biosynthesis have been described. In addition, new structural achievements with periplasmic chaperones have been made, all of which have revealed amazing details on how bacterial cell walls are synthesized. These findings provide an essential basis for future research, e.g. the development of new antibiotics. The book’s content is the logical continuation of Volume 84 of SCBI (on Prokaryotic Cytoskeletons), and sets the stage for upcoming volumes on Protein Complexes.

Studies of the bacterial cell wall emerged as a new field of research in the early 1950s, and has flourished in a multitude of directions. This excellent book provides an integrated collection of contributions forming a fundamental reference for researchers and of general use to teachers, advanced students in the life sciences, and all scientists in bacterial cell wall research. Chapters include topics such as: Peptidoglycan, an essential constituent of bacterial endospores; Teichoic and teichuronic acids, lipoteichoic acids, lipoglycans, neural complex polysaccharides and several specialized proteins are frequently unique wall-associated components of Gram-positive bacteria; Bacterial cells evolving signal transduction pathways; Underlying mechanisms of bacterial resistance to antibiotics.

This volume brings together the most widely used and important protocols currently being employed in researching and understanding bacterial cell wall homeostasis. Chapters in Bacterial Cell Wall Homeostasis cover a variety of subjects, such as: modern microscopy techniques and other biophysical methods used to characterize the subcellular structure of the bacterial cell wall; high-throughput approaches that can be used to identify all the genes and proteins that participate in the correct functioning of an organism’s cell wall; protocols for assaying individual gene products for specific cell wall functions or identify chemicals with inhibitory activity against the cell wall; and methods for analyzing the non-protein components of the cell wall and the increasing use of computational approaches for predicting and modeling cell wall related functions and processes. Written in the highly successful Methods in Molecular Biology series format, chapters include introduction to their respective topics, lists of the necessary material and reagents, step-by-step, readily reproducible laboratory protocols, and tips on troubleshooting and avoiding known pitfalls. Thorough and cutting-edge, Bacterial Cell Wall Homeostasis: Methods and Protocols emphasizes the diversity of the research taking place in bacterial cell wall homeostasis, and explains how the integration of information from across multiple disciplines is going to be essential if a holistic understanding of this important process is to be obtained.

Bacterial cells are encased in a cell wall, which is required to maintain cell shape and to confer physical strength to the cell. The cell wall allows bacteria to cope with osmotic and environmental challenges and to secure cell integrity during all stages of bacterial growth and propagation, and thus has to be sufficiently rigid. Moreover, to accommodate growth processes, the cell wall at the same time has to be a highly dynamic structure: During cell enlargement, division, and differentiation, bacteria continuously remodel, degrade, and resynthesize their cell wall, but pivotally need to assure cell integrity during these processes. Finally, the cell wall is also adjusted according to both environmental constraints and metabolic requirements. However, how exactly this is achieved is not fully understood. The major structural component of the bacterial cell wall is peptidoglycan (PG), a mesh-like polymer of glycan chains interlinked by short-chain peptides, constituting a net-like macromolecular structure that has historically also termed murein or murein sacculus. Although the basic structure of PG is conserved among bacteria, considerable variations occur regarding cross-bridging, modifications, and attachments. Moreover, different structural arrangements of the cell envelope exist within bacteria: a thin PG layer sandwiched between an inner and outer membrane is present in Gram-negative bacteria, and a thick PG layer decorated with secondary glycopolymers including teichoic acids, is present in Gram-positive bacteria. Furthermore, even more complex envelope structures exist, such as those found in mycobacteria. Crucially, all bacteria possess a multitude of often redundant lytic enzymes, termed “autolysins”, and other cell wall modifying and synthesizing enzymes, allowing to degrade and rebuild the various structures covering the cells. However, how cell wall turnover and cell wall biosynthesis are coordinated during different stages of bacterial growth is currently unclear. The mechanisms that prevent cell lysis during these processes are also unclear. This Research Topic focuses on the dynamics of the bacterial cell wall, its modifications, and structural rearrangements during cell growth and differentiation. It pays particular attention to the turnover of PG, its breakdown and recycling, as well as the regulation of these processes. Other structures, for example, secondary polymers such as teichoic acids, which are dynamically changed during bacterial growth and differentiation, are also covered. In recent years, our view on the bacterial cell envelope has undergone a dramatic change that challenged old models of cell wall structure, biosynthesis, and turnover. This collection of articles aims to contribute to new understandings of bacterial cell wall structure and dynamics.

It is a common statement that because of its simplicity the bacterial cell makes an ideal model for the study of a wide variety of biological systems and phenomena. While no-one would dispute that much of our under standing of biological function derives from the study of the humble bacterium, the concept of a simple life-form would be hotly disputed by any scientist engaged in the determination of the relationship between structure and function within the bacterial cell. Bacteria are particularly amenable to intensive study; their physiology can be probed with powerful biochemical, genetical and immunological techniques. Each piece of information obtained inevitably raises as many questions as answers, and can lead to a highly confused picture being presented to the lay reader. Nowhere is this more evident than in the study of the surface layers of the bacterial cell. Examination of the early electron micrographs suggested that the bacterial cytoplasm was surrounded by some sort of semi-rigid layer, possessing sufficient intrinsic strength to protect the organism from osmotic lysis. The belief that the surface layers were rather passive led to their neglect, while researchers concentrated on the superficially more exciting cytoplasmic components. Over the last twenty years our view of the bacterial envelope has undergone extensive revision, revealing a structure of enormous complexity.

Crystalline surface layers (S-layers) represent an almost universal feature of archaebacterial cell envelopes and can be found in gram-positive and gram-negative eubacterial species from nearly all phylogenetic branches. S-layers consist of a single protein- or glycoprotein species and thus can be considered as one of the most primitive membrane structures developed during evolution. Prokaryotes carrying S-layers are ubiquitously found in every part of the biosphere. This supports the concept of a general supramolecular "porous crystalline surface layer" fulfilling a broad spectrum of functions which are strongly dependent on the particular environmental and ecological conditions. Their structural simplicity makes S-layers a suitable model for analyzing structure-function relationships as well as dynamic aspects of membrane morphogenesis.