Cryo-electron microscopy of cell division in Staphylococcus aureus reveals a mid-zone between nascent cross walls

Functional profiling of CHAP domain-containing peptidoglycan hydrolases of Staphylococcus aureus USA300 uncovers potential targets for anti-staphylococcal therapies

The bacterial pathogen Staphylococcus aureus employs a thick cell wall for protection against physical and chemical insults. This wall requires continuous maintenance to ensure strength and barrier integrity, but also to permit bacterial growth and division. The main cell wall component is peptidoglycan. Accordingly, the bacteria produce so-called peptidoglycan hydrolases (PGHs) that cleave glycan strands to facilitate growth, cell wall remodelling, separation of divided cells and release of exported proteins into the extracellular milieu. A special class of PGHs contains so-called 'cysteine, histidine-dependent amidohydrolase/peptidase' (CHAP) domains. In the present study, we profiled the roles of 11 CHAP PGHs encoded by the core genome of S. aureus USA300 LAC. Mutant strains lacking individual CHAP PGHs were analysed for growth, cell morphology, autolysis, and invasion and replication inside human lung epithelial cells. The results show that several investigated CHAP PGHs contribute to different extents to extracellular and intracellular growth and replication of S. aureus, septation of dividing cells, daughter cell separation once the division process is completed, autolysis and biofilm formation. In particular, the CHAP PGHs Sle1 and SAUSA300_2253 control intracellular staphylococcal replication and the resistance to β-lactam antibiotics like oxacillin. This makes the S. aureus PGHs in general, and the Sle1 and SAUSA300_2253 proteins in particular, attractive targets for future prophylactic or therapeutic anti-staphylococcal interventions. Alternatively, these cell surface-exposed enzymes, or particular domains of these enzymes, could be applied in innovative anti-staphylococcal therapies.

Stay on track - revelations of bacterial cell wall synthesis enzymes and things that go by single-molecule imaging

In this review, we explore the regulation of septal peptidoglycan (sPG) synthesis in bacterial cell division, a critical process for cell viability and proper morphology. Recent single-molecule imaging studies have revealed the processive movement of the FtsW:bPBP synthase complex along the septum, shedding light on the spatiotemporal dynamics of sPG synthases and their regulators. In diderm bacteria (E. coli and C. crescentus), the movement occurs at two distinct speeds, reflecting active synthesis or inactivity driven by FtsZ-treadmilling. In monoderm bacteria (B. subtilis, S. pneumoniae, and S. aureus), however, these enzymes exhibit only the active sPG-track-coupled processive movement. By comparing the dynamics of sPG synthases in these organisms and that of class-A penicillin-binding proteins in vivo and in vitro, we propose a unifying model for septal cell wall synthesis regulation across species, highlighting the roles of the sPG- and Z-tracks in orchestrating a robust bacterial cell wall constriction process.

Models versus pathogens: how conserved is the FtsZ in bacteria?

Combating anti-microbial resistance by developing alternative strategies is the need of the hour. Cell division, particularly FtsZ, is being extensively studied for its potential as an alternative target for anti-bacterial therapy. Bacillus subtilis and Escherichia coli are the two well-studied models for research on FtsZ, the leader protein of the cell division machinery. As representatives of gram-positive and gram-negative bacteria, respectively, these organisms have provided an extensive outlook into the process of cell division in rod-shaped bacteria. However, research on other shapes of bacteria, like cocci and ovococci, lags behind that of model rods. Even though most regions of FtsZ show sequence and structural conservation throughout bacteria, the differences in FtsZ functioning and interacting partners establish several different modes of division in different bacteria. In this review, we compare the features of FtsZ and cell division in the model rods B. subtilis and E. coli and the four pathogens: Staphylococcus aureus, Streptococcus pneumoniae, Mycobacterium tuberculosis, and Pseudomonas aeruginosa. Reviewing several recent articles on these pathogenic bacteria, we have highlighted the functioning of FtsZ, the unique roles of FtsZ-associated proteins, and the cell division processes in them. Further, we provide a detailed look at the anti-FtsZ compounds discovered and their target bacteria, emphasizing the need for elucidation of the anti-FtsZ mechanism of action in different bacteria. Current challenges and opportunities in the ongoing journey of identifying potent anti-FtsZ drugs have also been described.

The cell cycle of Staphylococcus aureus: An updated review

As bacteria proliferate, DNA replication, chromosome segregation, cell wall synthesis, and cytokinesis occur concomitantly and need to be tightly regulated and coordinated. Although these cell cycle processes have been studied for decades, several mechanisms remain elusive, specifically in coccus-shaped cells such as Staphylococcus aureus. In recent years, major progress has been made in our understanding of how staphylococci divide, including new, fundamental insights into the mechanisms of cell wall synthesis and division site selection. Furthermore, several novel proteins and mechanisms involved in the regulation of replication initiation or progression of the cell cycle have been identified and partially characterized. In this review, we will summarize our current understanding of the cell cycle processes in the spheroid model bacterium S. aureus, with a focus on recent advances in the understanding of how these processes are regulated.

The Staphylococcus aureus cell division protein, DivIC, interacts with the cell wall and controls its biosynthesis

Bacterial cell division is a complex, dynamic process that requires multiple protein components to orchestrate its progression. Many division proteins are highly conserved across bacterial species alluding to a common, basic mechanism. Central to division is a transmembrane trimeric complex involving DivIB, DivIC and FtsL in Gram-positives. Here, we show a distinct, essential role for DivIC in division and survival of Staphylococcus aureus. DivIC spatially regulates peptidoglycan synthesis, and consequently cell wall architecture, by influencing the recruitment to the division septum of the major peptidoglycan synthetases PBP2 and FtsW. Both the function of DivIC and its recruitment to the division site depend on its extracellular domain, which interacts with the cell wall via binding to wall teichoic acids. DivIC facilitates the spatial and temporal coordination of peptidoglycan synthesis with the developing architecture of the septum during cell division. A better understanding of the cell division mechanisms in S. aureus and other pathogenic microorganisms can provide possibilities for the development of new, more effective treatments for bacterial infections.

Cell wall synthesis and remodelling dynamics determine division site architecture and cell shape in Escherichia coli

The bacterial division apparatus catalyses the synthesis and remodelling of septal peptidoglycan (sPG) to build the cell wall layer that fortifies the daughter cell poles. Understanding of this essential process has been limited by the lack of native three-dimensional views of developing septa. Here, we apply state-of-the-art cryogenic electron tomography (cryo-ET) and fluorescence microscopy to visualize the division site architecture and sPG biogenesis dynamics of the Gram-negative bacterium Escherichia coli. We identify a wedge-like sPG structure that fortifies the ingrowing septum. Experiments with strains defective in sPG biogenesis revealed that the septal architecture and mode of division can be modified to more closely resemble that of other Gram-negative (Caulobacter crescentus) or Gram-positive (Staphylococcus aureus) bacteria, suggesting that a conserved mechanism underlies the formation of different septal morphologies. Finally, analysis of mutants impaired in amidase activation (ΔenvC ΔnlpD) showed that cell wall remodelling affects the placement and stability of the cytokinetic ring. Taken together, our results support a model in which competition between the cell elongation and division machineries determines the shape of cell constrictions and the poles they form. They also highlight how the activity of the division system can be modulated to help generate the diverse array of shapes observed in the bacterial domain.

Dysregulation of Cell Envelope Homeostasis in Staphylococcus aureus Exposed to Solvated Lignin

Lignin is an aromatic plant cell wall polymer that facilitates water transport through the vasculature of plants and is generated in large quantities as an inexpensive by-product of pulp and paper manufacturing and biorefineries. Although lignin's ability to reduce bacterial growth has been reported previously, its hydrophobicity complicates the ability to examine its biological effects on living cells in aqueous growth media. We recently described the ability to solvate lignin in Good's buffers with neutral pH, a breakthrough that allowed examination of lignin's antimicrobial effects against the human pathogen Staphylococcus aureus. These analyses showed that lignin damages the S. aureus cell membrane, causes increased cell clustering, and inhibits growth synergistically with tunicamycin, a teichoic acid synthesis inhibitor. In the present study, we examined the physiological and transcriptomic responses of S. aureus to lignin. Intriguingly, lignin restored the susceptibility of genetically resistant S. aureus isolates to penicillin and oxacillin, decreased intracellular pH, impaired normal cell division, and rendered cells more resistant to detergent-induced lysis. Additionally, transcriptome sequencing (RNA-Seq) differential expression (DE) analysis of lignin-treated cultures revealed significant gene expression changes (P < 0.05 with 5% false discovery rate [FDR]) related to the cell envelope, cell wall physiology, fatty acid metabolism, and stress resistance. Moreover, a pattern of concurrent up- and downregulation of genes within biochemical pathways involved in transmembrane transport and cell wall physiology was observed, which likely reflects an attempt to tolerate or compensate for lignin-induced damage. Together, these results represent the first comprehensive analysis of lignin's antibacterial activity against S. aureus. IMPORTANCE S. aureus is a leading cause of skin and soft tissue infections. The ability of S. aureus to acquire genetic resistance to antibiotics further compounds its ability to cause life-threatening infections. While the historical response to antibiotic resistance has been to develop new antibiotics, bacterial pathogens are notorious for rapidly acquiring genetic resistance mechanisms. As such, the development of adjuvants represents a viable way of extending the life span of current antibiotics to which pathogens may already be resistant. Here, we describe the phenotypic and transcriptomic response of S. aureus to treatment with lignin. Our results demonstrate that lignin extracted from sugarcane and sorghum bagasse restores S. aureus susceptibility to β-lactams, providing a premise for repurposing these antibiotics in treatment of resistant S. aureus strains, possibly in the form of topical lignin/β-lactam formulations.

GpsB Coordinates Cell Division and Cell Surface Decoration by Wall Teichoic Acids in Staphylococcus aureus

Bacterial cell division is a complex and highly regulated process requiring the coordination of many different proteins. Despite substantial work in model organisms, our understanding of the systems regulating cell division in noncanonical organisms, including critical human pathogens, is far from complete. One such organism is Staphylococcus aureus, a spherical bacterium that lacks known cell division regulatory proteins. Recent studies on GpsB, a protein conserved within the Firmicutes phylum, have provided insight into cell division regulation in S. aureus and other related organisms. It has been revealed that GpsB coordinates cell division and cell wall synthesis in multiple species. In S. aureus, we have previously shown that GpsB directly regulates FtsZ polymerization. In this study, using Bacillus subtilis as a tool, we isolated spontaneous suppressors that abrogate the lethality of S. aureus GpsB overproduction in B. subtilis. Through characterization, we identified several residues important for the function of GpsB. Furthermore, we discovered an additional role for GpsB in wall teichoic acid (WTA) biosynthesis in S. aureus. Specifically, we show that GpsB directly interacts with the WTA export protein TarG. We also identified a region in GpsB that is crucial for this interaction. Analysis of TarG localization in S. aureus suggests that WTA machinery is part of the divisome complex. Taken together, this research illustrates how GpsB performs an essential function in S. aureus by directly linking the tightly regulated cell cycle processes of cell division and WTA-mediated cell surface decoration. IMPORTANCE Cytokinesis in bacteria involves an intricate orchestration of several key cell division proteins and other factors involved in building a robust cell envelope. Presence of teichoic acids is a signature characteristic of the Gram-positive cell wall. By characterizing the role of Staphylococcus aureus GpsB, an essential cell division protein in this organism, we have uncovered an additional role for GpsB in wall teichoic acid (WTA) biosynthesis. We show that GpsB directly interacts with TarG of the WTA export complex. We also show that this function of GpsB may be conserved in other GpsB homologs as GpsB and the WTA exporter complex follow similar localization patterns. It has been suggested that WTA acts as a molecular signal to control the activity of autolytic enzymes, especially during the separation of conjoined daughter cells. Thus, our results reveal that GpsB, in addition to playing a role in cell division, may also help coordinate WTA biogenesis.

Staphylococcus aureus cell wall maintenance - the multifaceted roles of peptidoglycan hydrolases in bacterial growth, fitness, and virulence

Staphylococcus aureus is an important human and livestock pathogen that is well-protected against environmental insults by a thick cell wall. Accordingly, the wall is a major target of present-day antimicrobial therapy. Unfortunately, S. aureus has mastered the art of antimicrobial resistance, as underscored by the global spread of methicillin-resistant S. aureus (MRSA). The major cell wall component is peptidoglycan. Importantly, the peptidoglycan network is not only vital for cell wall function, but it also represents a bacterial Achilles' heel. In particular, this network is continuously opened by no less than 18 different peptidoglycan hydrolases (PGHs) encoded by the S. aureus core genome, which facilitate bacterial growth and division. This focuses attention on the specific functions executed by these enzymes, their subcellular localization, their control at the transcriptional and post-transcriptional levels, their contributions to staphylococcal virulence and their overall importance in bacterial homeostasis. As highlighted in the present review, our understanding of the different aspects of PGH function in S. aureus has been substantially increased over recent years. This is important because it opens up new possibilities to exploit PGHs as innovative targets for next-generation antimicrobials, passive or active immunization strategies, or even to engineer them into effective antimicrobial agents.

Molecular basis for cell-wall recycling regulation by transcriptional repressor MurR in Escherichia coli

The cell-wall recycling process is important for bacterial survival in nutrient-limited conditions and, in certain cases, is directly involved in antibiotic resistance. In the sophisticated cell-wall recycling process in Escherichia coli, the transcriptional repressor MurR controls the expression of murP and murQ, which are involved in transporting and metabolizing N-acetylmuramic acid (MurNAc), generating N-acetylmuramic acid-6-phosphate (MurNAc-6-P) and N-acetylglucosamine-6-phosphate (GlcNAc-6-P). Here, we report that both MurNAc-6-P and GlcNAc-6-P can bind to MurR and weaken the DNA binding ability of MurR. Structural characterizations of MurR in complex with MurNAc-6-P or GlcNAc-6-P as well as in the apo form revealed the detailed ligand recognition chemistries. Further studies showed that only MurNAc-6-P, but not GlcNAc-6-P, is capable of derepressing the expression of murQP controlled by MurR in cells and clarified the substrate specificity through the identification of key residues responsible for ligand binding in the complex structures. In summary, this study deciphered the molecular mechanism of the cell wall recycling process regulated by MurR in E. coli.

β-Lactams against the Fortress of the Gram-Positive Staphylococcus aureus Bacterium

The biological diversity of the unicellular bacteria-whether assessed by shape, food, metabolism, or ecological niche-surely rivals (if not exceeds) that of the multicellular eukaryotes. The relationship between bacteria whose ecological niche is the eukaryote, and the eukaryote, is often symbiosis or stasis. Some bacteria, however, seek advantage in this relationship. One of the most successful-to the disadvantage of the eukaryote-is the small (less than 1 μm diameter) and nearly spherical Staphylococcus aureus bacterium. For decades, successful clinical control of its infection has been accomplished using β-lactam antibiotics such as the penicillins and the cephalosporins. Over these same decades S. aureus has perfected resistance mechanisms against these antibiotics, which are then countered by new generations of β-lactam structure. This review addresses the current breadth of biochemical and microbiological efforts to preserve the future of the β-lactam antibiotics through a better understanding of how S. aureus protects the enzyme targets of the β-lactams, the penicillin-binding proteins. The penicillin-binding proteins are essential enzyme catalysts for the biosynthesis of the cell wall, and understanding how this cell wall is integrated into the protective cell envelope of the bacterium may identify new antibacterials and new adjuvants that preserve the efficacy of the β-lactams.

Mini-Review: Bioactivities of Bacterial Cell Envelopes in the Central Nervous System

During acute bacterial meningitis, recognition of the bacterial envelope by immune cells of the central nervous system (CNS) generates a robust response that is essential to clear bacteria. This response is further amplified during treatment when lytic antibiotics, required for cure, also generate a burst of highly inflammatory cell envelope debris. Different peptidoglycan (PG) subcomponents interact with neurons, glia, and the blood brain barrier resulting in the entire symptom complex of meningitis. Recently, this CNS-cell envelope signaling axis has been extended to non-inflammatory recognition of cell wall components circulating from endogenous bacteria to the brain resulting in both benefit and chronic damage. This review will describe the molecular details of a broad array of cell envelope-induced responses in the CNS and what current strategies can be implemented to improve clinical outcome.

Structural Visualization of Septum Formation in Staphylococcus warneri Using Atomic Force Microscopy

Cell division of Staphylococcus adopts a "popping" mechanism that mediates extremely rapid separation of the septum. Elucidating the structure of the septum is crucial for understanding this exceptional bacterial cell division mechanism. Here, the septum structure of Staphylococcus warneri was extensively characterized using high-speed time-lapse confocal microscopy, atomic force microscopy, and electron microscopy. The cells of S. warneri divide in a fast popping manner on a millisecond timescale. Our results show that the septum is composed of two separable layers, providing a structural basis for the ultrafast daughter cell separation. The septum is formed progressively toward the center with nonuniform thickness of the septal disk in radial directions. The peptidoglycan on the inner surface of double-layered septa is organized into concentric rings, which are generated along with septum formation. Moreover, this study signifies the importance of new septum formation in initiating new cell cycles. This work unravels the structural basis underlying the popping mechanism that drives S. warneri cell division and reveals a generic structure of the bacterial cell.IMPORTANCE This work shows that the septum of Staphylococcus warneri is composed of two layers and that the peptidoglycan on the inner surface of the double-layered septum is organized into concentric rings. Moreover, new cell cycles of S. warneri can be initiated before the previous cell cycle is complete. This work advances our knowledge about a basic structure of bacterial cell and provides information on the double-layered structure of the septum for bacteria that divide with the "popping" mechanism.

The Sle1 Cell Wall Amidase Is Essential for β-Lactam Resistance in Community-Acquired Methicillin-Resistant Staphylococcus aureus USA300

Most clinically relevant methicillin-resistant Staphylococcus aureus (MRSA) strains have become resistant to β-lactams antibiotics through horizontal acquisition of the mecA gene encoding PBP2a, a peptidoglycan transpeptidase with low affinity for β-lactams. The level of resistance conferred by mecA is, however, strain dependent, and the mechanisms underlying this phenomenon remain poorly understood. We show here that β-lactam resistance correlates to expression of the Sle1 cell wall amidase in the fast-spreading and highly virulent community-acquired MRSA USA300 clone. Sle1 is a substrate of the ClpXP protease, and while the high Sle1 levels in cells lacking ClpXP activity confer β-lactam hyper-resistance, USA300 cells lacking Sle1 are as susceptible to β-lactams as cells lacking mecA This finding prompted us to assess the cellular roles of Sle1 in more detail, and we demonstrate that high Sle1 levels accelerate the onset of daughter cells splitting and decrease cell size. Vice versa, oxacillin decreases the Sle1 level and imposes a cell separation defect that is antagonized by high Sle1 levels, suggesting that high Sle1 levels increase tolerance to oxacillin by promoting cell separation. In contrast, increased oxacillin sensitivity of sle1 cells appears linked to a synthetic lethal effect on septum synthesis. In conclusion, this study demonstrates that Sle1 is a key factor in resistance to β-lactam antibiotics in the JE2 USA300 model strain and that PBP2a is required for the expression of Sle1 in JE2 cells exposed to oxacillin.

The Staphylococcal Cell Wall

Dating back to the 1960s, initial studies on the staphylococcal cell wall were driven by the need to clarify the mode of action of the first antibiotics and the resistance mechanisms developed by the bacteria. During the following decades, the elucidation of the biosynthetic path and primary composition of staphylococcal cell walls was propelled by advances in microbial cell biology, specifically, the introduction of high-resolution analytical techniques and molecular genetic approaches. The field of staphylococcal cell wall gradually gained its own significance as the complexity of its chemical structure and involvement in numerous cellular processes became evident, namely its versatile role in host interactions, coordination of cell division and environmental stress signaling.This chapter includes an updated description of the anatomy of staphylococcal cell walls, paying particular attention to information from the last decade, under four headings: high-resolution analysis of the Staphylococcus aureus peptidoglycan; variations in peptidoglycan composition; genetic determinants and enzymes in cell wall synthesis; and complex functions of cell walls. The latest contributions to a more precise picture of the staphylococcal cell envelope were possible due to recently developed state-of-the-art microscopy and spectroscopy techniques and to a wide combination of -omics approaches, that are allowing to obtain a more integrative view of this highly dynamic structure.

Extracellular electron transfer features of Gram-positive bacteria

Electroactive microorganisms possess the unique ability to transfer electrons to or from solid phase electron conductors, e.g., electrodes or minerals, through various physiological mechanisms. The processes are commonly known as extracellular electron transfer and broadly harnessed in microbial electrochemical systems, such as microbial biosensors, microbial electrosynthesis, or microbial fuel cells. Apart from a few model microorganisms, the nature of the microbe-electrode conductive interaction is poorly understood for most of the electroactive species. The interaction determines the efficiency and a potential scaling up of bioelectrochemical systems. Gram-positive bacteria generally have a thick electron non-conductive cell wall and are believed to exhibit weak extracellular electron shuttling activity. This review highlights reported research accomplishments on electroactive Gram-positive bacteria. The use of electron-conducting polymers as mediators is considered as one promising strategy to enhance the electron transfer efficiency up to application scale. In view of the recent progress in understanding the molecular aspects of the extracellular electron transfer mechanisms of Enterococcus faecalis, the electron transfer properties of this bacterium are especially focused on. Fundamental knowledge on the nature of microbial extracellular electron transfer and its possibilities can provide insight in interspecies electron transfer and biogeochemical cycling of elements in nature. Additionally, a comprehensive understanding of cell-electrode interactions may help in overcoming insufficient electron transfer and restricted operational performance of various bioelectrochemical systems and facilitate their practical applications.

Direct observation of the cell-wall remodeling in adhering Staphylococcus aureus 27217: An AFM study supported by SEM and TEM

We took benefit from Atomic Force Microscopy (AFM) in the force spectroscopy mode to describe the time evolution – over 24 h – of the surface nanotopography and mechanical properties of the strain Staphylococcus aureus 27217 from bacterial adhesion to the first stage of biofilm genesis. In addition, Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) experiments allowed identifying two types of self-adhering subpopulations (the so-called “bald” and “hairy” cells) and revealed changes in their relative populations with the bacterial culture age and the protocol of preparation. We indeed observed a dramatic evanescing of the “hairy” subpopulation for samples that underwent centrifugation and resuspension processes. When examined by AFM, the “hairy” cell surface resembled to a herringbone structure characterized by upper structural units with lateral dimensions of ∼70 nm and a high Young modulus value (∼2.3 MPa), a mean depth of the trough between them of ∼15 nm and a resulting roughness of ∼5 nm. By contrast, the “bald” cells appeared much softer (∼0.35 MPa) with a roughness one order of magnitude lower. We observed too the gradual detachment of the herringbone patterns from the “hairy” bacterial envelope of cell harvested from a 16 h old culture and their progressive accumulation between the bacteria in the form of globular clusters. The secretion of a soft extracellular polymeric substance was also identified that, in addition to the globular clusters, may contribute to the initiation of the biofilm spatial organization.

Antibiotic sensitivity reveals that wall teichoic acids mediate DNA binding during competence in Bacillus subtilis

Despite decades of investigation of genetic transformation in the model Gram-positive bacterium Bacillus subtilis, the factors responsible for exogenous DNA binding at the surface of competent cells remain to be identified. Here, we report that wall teichoic acids (WTAs), cell wall-anchored anionic glycopolymers associated to numerous critical functions in Gram-positive bacteria, are involved in this initial step of transformation. Using a combination of cell wall-targeting antibiotics and fluorescence microscopy, we show that competence-specific WTAs are produced and specifically localized in the competent cells to mediate DNA binding at the proximity of the transformation apparatus. Furthermore, we propose that TuaH, a putative glycosyl transferase induced during competence, modifies competence-induced WTAs in order to promote (directly or indirectly) DNA binding. On the basis of our results and previous knowledge in the field, we propose a model for DNA binding and transport during genetic transformation in B. subtilis.

Natural genetic transformation in bacteria requires DNA binding at the surface of competent cells. Here, Mirouze et al. show that wall teichoic acids are specifically produced or modified during competence in Bacillus subtilis and promote (directly or indirectly) DNA binding at the cell surface.

Lysostaphin Lysibody Leads to Effective Opsonization and Killing of Methicillin-Resistant Staphylococcus aureus in a Murine Model

The cell wall of Gram-positive bacteria contains abundant surface-exposed carbohydrate structures that are highly conserved. While these properties make surface carbohydrates ideal targets for immunotherapy, carbohydrates elicit a poor immune response that results primarily in low-affinity IgM antibodies. In a previous publication, we introduced the lysibody approach to address this shortcoming. Lysibodies are engineered molecules that combine a high-affinity carbohydrate-binding domain of bacterial or bacteriophage origin and an Fc effector portion of a human IgG antibody, thus directing effective immunity to conserved bacterial surface carbohydrates. Here, we describe the first example of a lysibody containing the binding domain from a bacteriocin, lysostaphin. We also describe the creation of five lysibodies with binding domains derived from phage lysins, directed against Staphylococcus aureus The lysostaphin and LysK lysibodies showed the most promise and were further characterized. Both lysibodies bound a range of clinically important staphylococcal strains, fixed complement on the staphylococcal surface, and induced phagocytosis of S. aureus by macrophages and human neutrophils. The lysostaphin lysibody had superior in vitro activity compared to that of the LysK lysibody, as well as that of the previously characterized ClyS lysibody, and it effectively protected mice in a kidney abscess/bacteremia model. These results further demonstrate that the lysibody approach is a reproducible means of creating antibacterial antibodies that cannot be produced by conventional means. Lysibodies therefore are a promising solution for opsonic antibodies that may be used passively to both treat and prevent infection by drug-resistant pathogens.

Inroads through the bacterial cell envelope: seeing is believing

A singular feature of all prokaryotic cells is the presence of a cell envelope composed of a cytoplasmic membrane and a cell wall. The introduction of bacterial cell fractionation techniques in the 1950s and 1960s along with developments in procedures for electron microscopy opened the window towards an understanding of the chemical composition and architecture of the cell envelope. This review traces the contribution of Terry Beveridge in these endeavours, beginning with his doctoral studies in the 1970s on the structure of paracrystalline surface arrays (S-layers), followed by an exploration of cryogenic methods for preserving bacteria for ultrastructural analyses. His insights are reflected in a current example of the contribution of cryo-electron microscopy to S-layer studies — the structure and assembly of the surface array of Caulobacter crescentus. The review then focuses on Terry’s contributions to imaging the ultrastructure of bacterial cell envelopes and to the development of cryo-electron microscopy techniques, including the use of CEMOVIS (Cryo-electron Microscopy of Vitreous Sections) to “see” the ultrastructure of the Gram-positive cell envelope — his last scientific endeavour.

Structome analysis of Escherichia coli cells by serial ultrathin sectioning reveals the precise cell profiles and the ribosome density

Structome analysis, the quantitative three-dimensional structural analysis of whole cells at the electron microscopic level, of Exophiala dermatitidis (black yeast), Saccharomyces cerevisiae, Mycobacterium tuberculosis (MTB) and Myojin spiral bacteria (MSB) have already been reported. Here, the results of the structome analysis of Escherichia coli cells based on transmission electron microscope observation of serial ultrathin sections was reported, and compared with the data obtained from phase contrast microscopy and scanning electron microscopy. On average, the cells had 0.89 μm in diameter, 2.47 μm in length and 1.16 fl (μm3) in cell volume in the structome analysis. Furthermore, E. coli cells had 26 100 ribosomes per whole cell with density of 2840 per 0.1 fl cytoplasm. The total ribosome number per cell was 15 times larger than that of MTB and about one-eighth of those of the yeast cells above. On the other hand, the ribosome density of E. coli cells are more than 13 times, 4 times, 2.5-times and 1.5-times higher than MSB, MTB, E. dermatitidis and S. cerevisiae, respectively. Finally, our ribosome enumeration data were compared between the structome-analyzed species and the relationship between the ribosome density and the growth rate among these species was discussed.

Fracture mechanics modeling of popping event during daughter cell separation

Most bacteria cells divide by binary fission which is part of a bacteria cell cycle and requires tight regulations and precise coordination. Fast separation of Staphylococcus Aureus (S. Aureus) daughter cells, named as popping event, has been observed in recent experiments. The popping event was proposed to be driven by mechanical crack propagation in the peripheral ring which connected two daughter cells before their separation. It has also been shown that after the fast separation, a small portion of the peripheral ring was left as a hinge. In the article, we develop a fracture mechanics model for the crack growth in the peripheral ring during S. Aureus daughter cell separation. In particular, using finite element analysis, we calculate the energy release rate associated with the crack growth in the peripheral ring, when daughter cells are inflated by a uniform turgor pressure inside. Our results show that with a fixed inflation of daughter cells, the energy release rate depends on the crack length non-monotonically. The energy release rate reaches a maximum value for a crack of an intermediate length. The non-monotonic relationship between the energy release rate and crack length clearly indicates that the crack propagation in the peripheral ring can be unstable. The computed energy release rate as a function of crack length can also be used to explain the existence of a small portion of peripheral ring remained as hinge after the popping event.

Molecular coordination of Staphylococcus aureus cell division

The bacterial cell wall is essential for viability, but despite its ability to withstand internal turgor must remain dynamic to permit growth and division. Peptidoglycan is the major cell wall structural polymer, whose synthesis requires multiple interacting components. The human pathogen Staphylococcus aureus is a prolate spheroid that divides in three orthogonal planes. Here, we have integrated cellular morphology during division with molecular level resolution imaging of peptidoglycan synthesis and the components responsible. Synthesis occurs across the developing septal surface in a diffuse pattern, a necessity of the observed septal geometry, that is matched by variegated division component distribution. Synthesis continues after septal annulus completion, where the core division component FtsZ remains. The novel molecular level information requires re-evaluation of the growth and division processes leading to a new conceptual model, whereby the cell cycle is expedited by a set of functionally connected but not regularly distributed components.

How bacterial cell division might cheat turgor pressure - a unified mechanism of septal division in Gram-positive and Gram-negative bacteria

An important question for bacterial cell division is how the invaginating septum can overcome the turgor force generated by the high osmolarity of the cytoplasm. I suggest that it may not need to. Several studies in Gram-negative bacteria have shown that the periplasm is isoosmolar with the cytoplasm. Indirect evidence suggests that this is also true for Gram-positive bacteria. In this case the invagination of the septum takes place within the uniformly high osmotic pressure environment, and does not have to fight turgor pressure. A related question is how the V-shaped constriction of Gram-negative bacteria relates to the plate-like septum of Gram-positive bacteria. I collected evidence that Gram-negative bacteria have a latent capability of forming plate-like septa, and present a model in which septal division is the basic mechanism in both Gram-positive and Gram-negative bacteria.

The Human Pathogen Streptococcus pyogenes Releases Lipoproteins as Lipoprotein-rich Membrane Vesicles

Bacterial lipoproteins are attractive vaccine candidates because they represent a major class of cell surface-exposed proteins in many bacteria and are considered as potential pathogen-associated molecular patterns sensed by Toll-like receptors with built-in adjuvanticity. Although Gram-negative lipoproteins have been extensively characterized, little is known about Gram-positive lipoproteins. We isolated from Streptococcus pyogenes a large amount of lipoproteins organized in vesicles. These vesicles were obtained by weakening the bacterial cell wall with a sublethal concentration of penicillin. Lipid and proteomic analysis of the vesicles revealed that they were enriched in phosphatidylglycerol and almost exclusively composed of lipoproteins. In association with lipoproteins, a few hypothetical proteins, penicillin-binding proteins, and several members of the ExPortal, a membrane microdomain responsible for the maturation of secreted proteins, were identified. The typical lipidic moiety was apparently not necessary for lipoprotein insertion in the vesicle bilayer because they were also recovered from the isogenic diacylglyceryl transferase deletion mutant. The vesicles were not able to activate specific Toll-like receptor 2, indicating that lipoproteins organized in these vesicular structures do not act as pathogen-associated molecular patterns. In light of these findings, we propose to name these new structures Lipoprotein-rich Membrane Vesicles.

Bacterial division. Mechanical crack propagation drives millisecond daughter cell separation in Staphylococcus aureus

When Staphylococcus aureus undergoes cytokinesis, it builds a septum, generating two hemispherical daughters whose cell walls are only connected via a narrow peripheral ring. We found that resolution of this ring occurred within milliseconds ("popping"), without detectable changes in cell volume. The likelihood of popping depended on cell-wall stress, and the separating cells split open asymmetrically, leaving the daughters connected by a hinge. An elastostatic model of the wall indicated high circumferential stress in the peripheral ring before popping. Last, we observed small perforations in the peripheral ring that are likely initial points of mechanical failure. Thus, the ultrafast daughter cell separation in S. aureus appears to be driven by accumulation of stress in the peripheral ring and exhibits hallmarks of mechanical crack propagation.

The interplay between cell wall mechanical properties and the cell cycle in Staphylococcus aureus

The nanoscale mechanical properties of live Staphylococcus aureus cells during different phases of growth were studied by atomic force microscopy. Indentation to different depths provided access to both local cell wall mechanical properties and whole-cell properties, including a component related to cell turgor pressure. Local cell wall properties were found to change in a characteristic manner throughout the division cycle. Splitting of the cell into two daughter cells followed a local softening of the cell wall along the division circumference, with the cell wall on either side of the division circumference becoming stiffer. Once exposed, the newly formed septum was found to be stiffer than the surrounding, older cell wall. Deeper indentations, which were affected by cell turgor pressure, did not show a change in stiffness throughout the division cycle, implying that enzymatic cell wall remodeling and local variations in wall properties are responsible for the evolution of cell shape through division.

Ramoplanin at bactericidal concentrations induces bacterial membrane depolarization in Staphylococcus aureus

Ramoplanin is an actinomycetes-derived antibiotic with broad-spectrum activity against Gram-positive bacteria that has been evaluated in clinical trials for the treatment of gastrointestinal vancomycin-resistant enterococci (VRE) and Clostridium difficile infections. Recent studies have proposed that ramoplanin binds to bacterial membranes as a C2 symmetrical dimer that can sequester Lipid II, which causes inhibition of cell wall peptidoglycan biosynthesis and cell death. In this study, ramoplanin was shown to bind to anionic and zwitterionic membrane mimetics with a higher affinity for anionic membranes and to induce membrane depolarization of methicillin-susceptible Staphylococcus aureus (MSSA) ATCC 25923 at concentrations at or above the minimal bactericidal concentration (MBC). The ultrastructural effects of ramoplanin on S. aureus were also examined by transmission electron microscopy (TEM), and this showed dramatic changes to bacterial cell morphology. The correlation observed between membrane depolarization and bacterial cell viability suggests that this mechanism may contribute to the bactericidal activity of ramoplanin.

The medium is the message: interspecies and interkingdom signaling by peptidoglycan and related bacterial glycans

Peptidoglycan serves as a key structure of the bacterial cell by determining cell shape and providing resistance to internal turgor pressure. However, in addition to these essential and well-studied functions, bacterial signaling by peptidoglycan fragments, or muropeptides, has been demonstrated by recent work. Actively growing bacteria release muropeptides as a consequence of cell wall remodeling during elongation and division. Therefore, the presence of muropeptide synthesis is indicative of growth-promoting conditions and may serve as a broadly conserved signal for nongrowing cells to reinitiate growth. In addition, muropeptides serve as signals between bacteria and eukaryotic organisms during both pathogenic and symbiotic interactions. The increasingly appreciated role of the microbiota in metazoan organisms suggests that muropeptide signaling likely has important implications for homeostatic mammalian physiology.

Differential localization of LTA synthesis proteins and their interaction with the cell division machinery in Staphylococcus aureus

Lipoteichoic acid (LTA) is an important cell wall component of Gram-positive bacteria. In Staphylococcus aureus it consists of a polyglycerolphosphate-chain that is retained within the membrane via a glycolipid. Using an immunofluorescence approach, we show here that the LTA polymer is not surface exposed in S. aureus, as it can only be detected after digestion of the peptidoglycan layer. S. aureus mutants lacking LTA are enlarged and show aberrant positioning of septa, suggesting a link between LTA synthesis and the cell division process. Using a bacterial two-hybrid approach, we show that the three key LTA synthesis proteins, YpfP and LtaA, involved in glycolipid production, and LtaS, required for LTA backbone synthesis, interact with one another. All three proteins also interacted with numerous cell division and peptidoglycan synthesis proteins, suggesting the formation of a multi-enzyme complex and providing further evidence for the co-ordination of these processes. When assessed by fluorescence microscopy, YpfP and LtaA fluorescent protein fusions localized to the membrane while the LtaS enzyme accumulated at the cell division site. These data support a model whereby LTA backbone synthesis proceeds in S. aureus at the division site in co-ordination with cell division, while glycolipid synthesis takes place throughout the membrane.

Division site positioning in bacteria: one size does not fit all

Spatial regulation of cell division in bacteria has been a focus of research for decades. It has been well studied in two model rod-shaped organisms, Escherichia coli and Bacillus subtilis, with the general belief that division site positioning occurs as a result of the combination of two negative regulatory systems, Min and nucleoid occlusion. These systems influence division by preventing the cytokinetic Z ring from forming anywhere other than midcell. However, evidence is accumulating for the existence of additional mechanisms that are involved in controlling Z ring positioning both in these organisms and in several other bacteria. In some cases the decision of where to divide is solved by variations on a common evolutionary theme, and in others completely different proteins and mechanisms are involved. Here we review the different ways bacteria solve the problem of finding the right place to divide. It appears that a one-size-fits-all model does not apply, and that individual species have adapted a division-site positioning mechanism that best suits their lifestyle, environmental niche and mode of growth to ensure equal partitioning of DNA for survival of the next generation.

How to get (a)round: mechanisms controlling growth and division of coccoid bacteria

Bacteria come in a range of shapes, including round, rod-shaped, curved and spiral cells. This morphological diversity implies that different mechanisms exist to guide proper cell growth, division and chromosome segregation. Although the majority of studies on cell division have focused on rod-shaped cells, the development of new genetic and cell biology tools has provided mechanistic insight into the cell cycles of bacteria with different shapes, allowing us to appreciate the underlying molecular basis for their morphological diversity. In this Review, we discuss recent progress that has advanced our knowledge of the complex mechanisms for chromosome segregation and cell division in bacteria which have, deceptively, the simplest possible shape: the cocci.

Combination therapy with lysin CF-301 and antibiotic is superior to antibiotic alone for treating methicillin-resistant Staphylococcus aureus-induced murine bacteremia

Lysins are bacteriophage-derived enzymes that degrade bacterial peptidoglycans. Lysin CF-301 is being developed to treat Staphylococcus aureus because of its potent, specific, and rapid bacteriolytic effects. It also demonstrates activity on drug-resistant strains, has a low resistance profile, eradicates biofilms, and acts synergistically with antibiotics. CF-301 was bacteriolytic against 250 S. aureus strains tested including 120 methicillin-resistant S. aureus (MRSA) isolates. In time-kill studies with 62 strains, CF-301 reduced S. aureus by 3-log₁₀ within 30 minutes compared to 6–12 hours required by antibiotics. In bacteremia, CF-301 increased survival by reducing blood MRSA 100-fold within 1 hour. Combinations of CF-301 with vancomycin or daptomycin synergized in vitro and increased survival significantly in staphylococcal-induced bacteremia compared to treatment with antibiotics alone (P < .0001). Superiority of CF-301 combinations with antibiotics was confirmed in 26 independent bacteremia studies. Combinations including CF-301 and antibiotics represent an attractive alternative to antibiotic monotherapies currently used to treat S. aureus bacteremia.

Critical cell wall hole size for lysis in Gram-positive bacteria

Gram-positive bacteria can transport molecules necessary for their survival through holes in their cell wall. The holes in cell walls need to be large enough to let critical nutrients pass through. However, the cell wall must also function to prevent the bacteria's membrane from protruding through a large hole into the environment and lysing the cell. As such, we hypothesize that there exists a range of cell wall hole sizes that allow for molecule transport but prevent membrane protrusion. Here, we develop and analyse a biophysical theory of the response of a Gram-positive cell's membrane to the formation of a hole in the cell wall. We predict a critical hole size in the range of 15-24 nm beyond which lysis occurs. To test our theory, we measured hole sizes in Streptococcus pyogenes cells undergoing enzymatic lysis via transmission electron microscopy. The measured hole sizes are in strong agreement with our theoretical prediction. Together, the theory and experiments provide a means to quantify the mechanisms of death of Gram-positive cells via enzymatically mediated lysis and provides insights into the range of cell wall hole sizes compatible with bacterial homeostasis.

Wall teichoic acids of gram-positive bacteria

The peptidoglycan layers of many gram-positive bacteria are densely functionalized with anionic glycopolymers known as wall teichoic acids (WTAs). These polymers play crucial roles in cell shape determination, regulation of cell division, and other fundamental aspects of gram-positive bacterial physiology. Additionally, WTAs are important in pathogenesis and play key roles in antibiotic resistance. We provide an overview of WTA structure and biosynthesis, review recent studies on the biological roles of these polymers, and highlight remaining questions. We also discuss prospects for exploiting WTA biosynthesis as a target for new therapies to overcome resistant infections.

The physiology of bacterial cell division

Bacterial cell division is facilitated by the divisome, a dynamic multiprotein assembly localizing at mid-cell to synthesize the stress-bearing peptidoglycan and to constrict all cell envelope layers. Divisome assembly occurs in two steps and involves multiple interactions between more than 20 essential and accessory cell division proteins. Well before constriction and while the cell is still elongating, the tubulin-like FtsZ and early cell division proteins form a ring-like structure at mid-cell. Cell division starts once certain peptidoglycan enzymes and their activators have moved to the FtsZ-ring. Gram-negative bacteria like Escherichia coli simultaneously synthesize and cleave the septum peptidoglycan during division leading to a constriction. The outer membrane constricts together with the peptidoglycan layer with the help of the transenvelope spanning Tol-Pal system.

An antibiotic that inhibits a late step in wall teichoic acid biosynthesis induces the cell wall stress stimulon in Staphylococcus aureus

Wall teichoic acids (WTAs) are phosphate-rich, sugar-based polymers attached to the cell walls of most Gram-positive bacteria. In Staphylococcus aureus, these anionic polymers regulate cell division, protect cells from osmotic stress, mediate host colonization, and mask enzymatically susceptible peptidoglycan bonds. Although WTAs are not required for survival in vitro, blocking the pathway at a late stage of synthesis is lethal. We recently discovered a novel antibiotic, targocil, that inhibits a late acting step in the WTA pathway. Its target is TarG, the transmembrane component of the ABC transporter (TarGH) that exports WTAs to the cell surface. We examined here the effects of targocil on S. aureus using transmission electron microscopy and gene expression profiling. We report that targocil treatment leads to multicellular clusters containing swollen cells displaying evidence of osmotic stress, strongly induces the cell wall stress stimulon, and reduces the expression of key virulence genes, including dltABCD and capsule genes. We conclude that WTA inhibitors that act at a late stage of the biosynthetic pathway may be useful as antibiotics, and we present evidence that they could be particularly useful in combination with beta-lactams.

Purification and activity testing of the full-length YycFGHI proteins of Staphylococcus aureus

Background

The YycFG two-component regulatory system (TCS) of Staphylococcus aureus represents the only essential TCS that is almost ubiquitously distributed in Gram-positive bacteria with a low G+C-content. YycG (WalK/VicK) is a sensor histidine-kinase and YycF (WalR/VicR) is the cognate response regulator. Both proteins play an important role in the biosynthesis of the cell envelope and mutations in these proteins have been involved in development of vancomycin and daptomycin resistance.

Methodology/Principal Findings

Here we present high yield expression and purification of the full-length YycG and YycF proteins as well as of the auxiliary proteins YycH and YycI of Staphylococcus aureus. Activity tests of the YycG kinase and a mutated version, that harbours an Y306N exchange in its cytoplasmic PAS domain, in a detergent-micelle-model and a phosholipid-liposome-model showed kinase activity (autophosphorylation and phosphoryl group transfer to YycF) only in the presence of elevated concentrations of alkali salts. A direct comparison of the activity of the kinases in the liposome-model indicated a higher activity of the mutated YycG kinase. Further experiments indicated that YycG responds to fluidity changes in its microenvironment.

Conclusions/Significance

The combination of high yield expression, purification and activity testing of membrane and membrane-associated proteins provides an excellent experimental basis for further protein-protein interaction studies and for identification of all signals received by the YycFGHI system.

Synthetic lethal compound combinations reveal a fundamental connection between wall teichoic acid and peptidoglycan biosyntheses in Staphylococcus aureus

Methicillin resistance in Staphylococcus aureus depends on the production of mecA, which encodes penicillin-binding protein 2A (PBP2A), an acquired peptidoglycan transpeptidase (TP) with reduced susceptibility to β-lactam antibiotics. PBP2A cross-links nascent peptidoglycan when the native TPs are inhibited by β-lactams. Although mecA expression is essential for β-lactam resistance, it is not sufficient. Here we show that blocking the expression of wall teichoic acids (WTAs) by inhibiting the first enzyme in the pathway, TarO, sensitizes methicillin-resistant S. aureus (MRSA) strains to β-lactams even though the β-lactam-resistant transpeptidase, PBP2A, is still expressed. The dramatic synergy between TarO inhibitors and β-lactams is noteworthy not simply because strategies to overcome MRSA are desperately needed but because neither TarO nor the activities of the native TPs are essential in MRSA strains. The "synthetic lethality" of inhibiting TarO and the native TPs suggests a functional connection between ongoing WTA expression and peptidoglycan assembly in S. aureus. Indeed, transmission electron microscopy shows that S. aureus cells blocked in WTA synthesis have extensive defects in septation and cell separation, indicating dysregulated cell wall assembly and degradation. Our studies imply that WTAs play a fundamental role in S. aureus cell division and raise the possibility that synthetic lethal compound combinations may have therapeutic utility for overcoming antibiotic-resistant bacterial infections.

In vitro analysis of the Staphylococcus aureus lipoteichoic acid synthase enzyme using fluorescently labeled lipids

Lipoteichoic acid (LTA) is an important cell wall component of Gram-positive bacteria. The key enzyme responsible for polyglycerolphosphate lipoteichoic acid synthesis in the Gram-positive pathogen Staphylococcus aureus is the membrane-embedded lipoteichoic acid synthase enzyme, LtaS. It is presumed that LtaS hydrolyzes the glycerolphosphate head group of the membrane lipid phosphatidylglycerol (PG) and catalyzes the formation of the polyglycerolphosphate LTA backbone chain. Here we describe an in vitro assay for this new class of enzyme using PG with a fluorescently labeled fatty acid chain (NBD-PG) as the substrate and the recombinant soluble C-terminal enzymatic domain of LtaS (eLtaS). Thin-layer chromatography and mass spectrometry analysis of the lipid reaction products revealed that eLtaS is sufficient to cleave the glycerolphosphate head group from NBD-PG, resulting in the formation of NBD-diacylglycerol. An excess of soluble glycerolphosphate could not compete with the hydrolysis of the fluorescently labeled PG lipid substrate, in contrast to the addition of unlabeled PG. This indicates that the enzyme recognizes and binds other parts of the lipid substrate, besides the glycerolphosphate head group. Furthermore, eLtaS activity was Mn(2+) ion dependent; Mg(2+) and Ca(2+) supported only weak enzyme activity. Addition of Zn(2+) or EDTA inhibited enzyme activity even in the presence of Mn(2+). The pH optimum of the enzyme was 6.5, characteristic for an enzyme that functions extracellularly. Lastly, we show that the in vitro assay can be used to study the enzyme activities of other members of the lipoteichoic acid synthase enzyme family.

Peptidoglycan architecture can specify division planes in Staphylococcus aureus

Division in Staphylococci occurs equatorially and on specific sequentially orthogonal planes in three dimensions, resulting, after incomplete cell separation, in the 'bunch of grapes' cluster organization that defines the genus. The shape of Staphylococci is principally maintained by peptidoglycan. In this study, we use Atomic Force Microscopy (AFM) and fluorescence microscopy with vancomycin labelling to examine purified peptidoglycan architecture and its dynamics in Staphylococcus aureus and correlate these with the cell cycle. At the presumptive septum, cells were found to form a large belt of peptidoglycan in the division plane before the centripetal formation of the septal disc; this often had a 'piecrust' texture. After division, the structures remain as orthogonal ribs, encoding the location of past division planes in the cell wall. We propose that this epigenetic information is used to enable S. aureus to divide in sequentially orthogonal planes, explaining how a spherical organism can maintain division plane localization with fidelity over many generations.

Cell wall synthesis is necessary for membrane dynamics during sporulation of Bacillus subtilis

During Bacillus subtilis sporulation, an endocytic-like process called engulfment results in one cell being entirely encased in the cytoplasm of another cell. The driving force underlying this process of membrane movement has remained unclear, although components of the machinery have been characterized. Here we provide evidence that synthesis of peptidoglycan, the rigid, strength bearing extracellular polymer of bacteria, is a key part of the missing force-generating mechanism for engulfment. We observed that sites of peptidoglycan synthesis initially coincide with the engulfing membrane and later with the site of engulfment membrane fission. Furthermore, compounds that block muropeptide synthesis or polymerization prevented membrane migration in cells lacking a component of the engulfment machinery (SpoIIQ), and blocked the membrane fission event at the completion of engulfment in all cells. In addition, these compounds inhibited bulge and vesicle formation that occur in spoIID mutant cells unable to initiate engulfment, as did genetic ablation of a protein that polymerizes muropeptides. This is the first report to our knowledge that peptidoglycan synthesis is necessary for membrane movements in bacterial cells and has implications for the mechanism of force generation during cytokinesis.

Disulfide bond formation and cysteine exclusion in gram-positive bacteria

Most secretion pathways in bacteria and eukaryotic cells are challenged by the requirement for their substrate proteins to mature after they traverse a membrane barrier and enter a reactive oxidizing environment. For Gram-positive bacteria, the mechanisms that protect their exported proteins from misoxidation during their post-translocation maturation are poorly understood. To address this, we separated numerous bacterial species according to their tolerance for oxygen and divided their proteomes based on the predicted subcellular localization of their proteins. We then applied a previously established computational approach that utilizes cysteine incorporation patterns in proteins as an indicator of enzymatic systems that may exist in each species. The Sec-dependent exported proteins from aerobic Gram-positive Actinobacteria were found to encode cysteines in an even-biased pattern indicative of a functional disulfide bond formation system. In contrast, aerobic Gram-positive Firmicutes favor the exclusion of cysteines from both their cytoplasmic proteins and their substantially longer exported proteins. Supporting these findings, we show that Firmicutes, but not Actinobacteria, tolerate growth in reductant. We further demonstrate that the actinobacterium Corynebacterium glutamicum possesses disulfide-bonded proteins and two dimeric Dsb-like enzymes that can efficiently catalyze the formation of disulfide bonds. Our results suggest that cysteine exclusion is an important adaptive strategy against the challenges presented by oxidative environments.