Synthetic lethality of the lytE cwlO genotype in Bacillus subtilis is caused by lack of D,L-endopeptidase activity at the lateral cell wall

The characteristics of autolysins associated with cell separation in Bacillus subtilis

The peptidoglycan hydrolases responsible for the cell separation of Bacillus subtilis cells are collectively referred to as autolysins. However, the role of each autolysin in the cell separation of B. subtilis is not fully understood. In this study, we constructed a series of cell separation-associated autolysin deficient strains and strains overexpressing the transcription factors SlrR and SinR, and the morphological changes of these strains in liquid culture were observed. The results showed that the absence of D,L-endopeptidases CwlS and LytF only increased the cell chain length in the early exponential phase. The absence of D,L-endopeptidase LytE or N-acetylmuramyl-L-alanine amidase LytC can cause cells to form chains throughout the growth of B. subtilis, although the cell chain length was significantly shortened during the stationary phase. However, the absence of peptidoglycan N-acetylglucosaminidase LytD only caused minor defect in cell separation. Therefore, we concluded that LytE and LytC were the major autolysins that ensure the timely separation of B. subtilis daughter cells, whereas CwlS, LytF, and LytD were the minor autolysins. In addition, overexpression of the transcription factors SinR and SlrR in the cwlS lytF lytC lytE mutant enabled B. subtilis cells to form ultra-long chains in the vegetative phase, and its biomass level was basically the same as that of the wild type. This led to the conclusion that besides inhibiting the expression of lytC and lytF, the SinR-SlrR complex also has other potential mechanisms to inhibit cell separation.IMPORTANCEIn this study, the effects of CwlS, LytC, LytD, LytF, LytE, and SinR-SlrR complex on the cell separation of Bacillus subtilis at different growth phases were studied, and an ultra-long-chained B. subtilis strain was constructed. In microbial fermentation, due to its large cell size, this ultra-long-chained B. subtilis strain may be more likely to be precipitated or intercepted during the removal of bacterial process with centrifugation and membrane filtration as the main methods, which is crucial to improve the purity of the product.

Comprehensive double-mutant analysis of the Bacillus subtilis envelope using double-CRISPRi

Understanding bacterial gene function remains a major biological challenge. Double-mutant genetic interaction (GI) analysis addresses this challenge by uncovering the functional partners of targeted genes, allowing us to associate genes of unknown function with novel pathways and unravel connections between well-studied pathways, but is difficult to implement at the genome-scale. Here, we develop and use double-CRISPRi to systematically quantify genetic interactions at scale in the Bacillus subtilis envelope, including essential genes. We discover > 1000 known and novel genetic interactions. Our analysis pipeline and experimental follow-ups reveal the distinct roles of paralogous genes such as the mreB and mbl actin homologs, and identify new genes involved in the well-studied process of cell division. Overall, our study provides valuable insights into gene function and demonstrates the utility of double-CRISPRi for high-throughput dissection of bacterial gene networks, providing a blueprint for future studies in diverse bacterial species.

Structural analysis of the peptidoglycan DL-endopeptidase CwlO complexed with its inhibitory protein IseA

Peptidoglycan DL-endopeptidases locally cleave the peptide stem of peptidoglycan in the bacterial cell wall. This process facilitates bacterial growth and division by loosening the rigid peptidoglycan layer. IseA binds to the active site of multiple DL-endopeptidases and inhibits excessive peptidoglycan degradation that leads to cell lysis. To better understand how IseA inhibits DL-endopeptidase activity, we determined the crystal structure of the peptidoglycan DL-endopeptidase CwlO/IseA complex and compared it with that of the peptidoglycan DL-endopeptidase LytE/IseA complex. Structural analyses showed significant differences between the hydrophobic pocket-binding residues of the DL-endopeptidases (F361 of CwlO and W237 of LytE). Additionally, binding assays showed that the F361 mutation of CwlO to the bulkier hydrophobic residue, tryptophan, increased its binding affinity for IseA, whereas mutation to alanine reduced the affinity. These analyses revealed that the hydrophobic pocket-binding residue of DL-endopeptidases determines IseA-binding affinity and is required for substrate-mimetic inhibition by IseA.

A potential space-making role in cell wall biogenesis for SltB1and DacB revealed by a beta-lactamase induction phenotype in Pseudomonas aeruginosa

Pseudomonas aeruginosa encodes the beta-lactamase AmpC, which promotes resistance to beta-lactam antibiotics. Expression of ampC is induced by anhydro-muropeptides (AMPs) released from the peptidoglycan (PG) cell wall upon beta-lactam treatment. AmpC can also be induced via genetic inactivation of PG biogenesis factors such as the endopeptidase DacB that cleaves PG crosslinks. Mutants in dacB occur in beta-lactam-resistant clinical isolates of P. aeruginosa, but it has remained unclear why DacB inactivation promotes ampC induction. Similarly, the inactivation of lytic transglycosylase (LT) enzymes such as SltB1 that cut PG glycans has also been associated with ampC induction and beta-lactam resistance. Given that LT enzymes are capable of producing AMP products that serve as ampC inducers, this latter observation has been especially difficult to explain. Here, we show that ampC induction in sltB1 or dacB mutants requires another LT enzyme called MltG. In Escherichia coli, MltG has been implicated in the degradation of nascent PG strands produced upon beta-lactam treatment. Accordingly, in P. aeruginosa sltB1 and dacB mutants, we detected the MltG-dependent production of pentapeptide-containing AMP products that are signatures of nascent PG degradation. Our results therefore support a model in which SltB1 and DacB use their PG-cleaving activity to open space in the PG matrix for the insertion of new material. Thus, their inactivation mimics low-level beta-lactam treatment by reducing the efficiency of new PG insertion into the wall, causing the degradation of some nascent PG material by MltG to produce the ampC-inducing signal.

IMPORTANCE

Inducible beta-lactamases like the ampC system of Pseudomonas aeruginosa are a common determinant of beta-lactam resistance among gram-negative bacteria. The regulation of ampC is elegantly tuned to detect defects in cell wall synthesis caused by beta-lactam drugs. Studies of mutations causing ampC induction in the absence of drug therefore promise to reveal new insights into the process of cell wall biogenesis in addition to aiding our understanding of how resistance to beta-lactam antibiotics arises in the clinic. In this study, the ampC induction phenotype for mutants lacking a glycan-cleaving enzyme or an enzyme that cuts cell wall crosslinks was used to uncover a potential role for these enzymes in making space in the wall matrix for the insertion of new material during cell growth.

Crosslink cleaving enzymes: the smart autolysins that remodel the bacterial cell wall

Peptidoglycan (PG) is a protective mesh-like polymer in bacterial cell walls that enables their survival in almost every ecological niche. PG is formed by crosslinking of several glycan strands through short peptides, conferring a characteristic structure and elasticity, distinguishing it from other polymeric exoskeletons. The significance of PG crosslink formation has been known for decades, as some of the most widely used antibiotics, namely β-lactams, target the enzymes that catalyze this step. However, the importance of crosslink hydrolysis in PG biology remained largely underappreciated. Recent advances demonstrate the functions of crosslink cleavage in diverse physiological processes, including an indispensable role in PG expansion during the cell cycle, thereby making crosslink cleaving enzymes an untapped target for novel drugs. Here, we elaborate on the fundamental roles of crosslink-specific endopeptidases and their regulation across the bacterial kingdom.

Genetic interaction mapping reveals functional relationships between peptidoglycan endopeptidases and carboxypeptidases

Peptidoglycan (PG) is the main component of the bacterial cell wall; it maintains cell shape while protecting the cell from internal osmotic pressure and external environmental challenges. PG synthesis is essential for bacterial growth and survival, and a series of PG modifications are required to allow expansion of the sacculus. Endopeptidases (EPs), for example, cleave the crosslinks between adjacent PG strands to allow the incorporation of newly synthesized PG. EPs are collectively essential for bacterial growth and must likely be carefully regulated to prevent sacculus degradation and cell death. However, EP regulation mechanisms are poorly understood. Here, we used TnSeq to uncover novel EP regulators in Vibrio cholerae. This screen revealed that the carboxypeptidase DacA1 (PBP5) alleviates EP toxicity. dacA1 is essential for viability on LB medium, and this essentiality was suppressed by EP overexpression, revealing that EP toxicity both mitigates, and is mitigated by, a defect in dacA1. A subsequent suppressor screen to restore viability of ΔdacA1 in LB medium identified hypomorphic mutants in the PG synthesis pathway, as well as mutations that promote EP activation. Our data thus reveal a more complex role of DacA1 in maintaining PG homeostasis than previously assumed.

Dynamics of cell wall-binding proteins at a single molecule level: B. subtilis autolysins show different kinds of motion

The bacterial cell wall is a meshwork of crosslinked peptidoglycan strands, with a thickness of up to 50 nm in Firmicutes. Little is known about how proteins move through the cell wall to find sites of enzymatic activity. Cell wall synthesis for cell elongation involves the integration of new peptidoglycan strands by integral membrane proteins, as well as the degradation of existing strands by so-called autolysins, soluble proteins that are secreted through the cell membrane. Autolysins comprise different classes of proteases and glucanases and mostly contain cell-wall binding domains in addition to their catalytic domain. We have studied dynamics of Bacillus subtilis autolysins LytC, a major endopeptidase required for lateral cell wall growth, and LytF, a peptidase acting at the newly formed division site in order to achieve separation of daughter cells. We show that both proteins, fused to moxVenus are present as three distinct populations of different diffusion constants. The fastest population is compatible with free diffusion in a crowded liquid environment, that is similar to that of cytosolic enzymes, likely reflecting autolysins diffusing through the periplasm. The medium mobile fraction can be explained by constrained motion through a polymeric substance, indicating mobility of autolysins through the wall similar to that of DNA-binding proteins within the nucleoid. The slow-mobile fraction are most likely autolysins bound to their specific substrate sites. We show that LytF is more static during exponential phase, while LytC appears to be more active during the transition to stationary phase. Both autolysins became more static in backgrounds lacking redundant other autolysins, suggesting stochastic competition for binding sites. On the other hand, lack of inhibitor IseA or autolysin CwlS lead to an altered preference for polar localization of LytF within the cell wall, revealing that inhibitors and autolysins also affect each other's pattern of localization, in addition to their activity.

Reducing Cellular Autolysis of Bacillus subtilis to Improve Keratinase Production

Bacillus subtilis has been shown to be an excellent expression host for keratinases due to its powerful secretion system. However, cellular autolysis limits its production capacity. Here, we selected seven genes with significantly upregulated transcript levels from 15 genes associated with cellular autolysis as knockout targets by qRT-PCR and constructed a total of 127 strains to reduce cellular autolysis. Among them, the biomass of B. subtilis BSΔXLPC-ker deficient in xpf, lytC, pcfA, and cwlC increased by 57%. This was confirmed by cell staining, green fluorescent protein imaging, and extracellular nucleic acid leakage assay. Keratinase activity was increased by 1.46-fold in the 5 L fermenter. In addition, the activities of nattokinase and subtilisin E were also increased by 1.50-fold and 1.43-fold, respectively, in the modified chassis cells, which further confirms the generalizability of the strategy. Thus, reducing cellular autolysis to increase the ability of B. subtilis to produce subtilisins is promising.

Novel role for peptidoglycan carboxypeptidases in maintaining the balance between bacterial cell wall synthesis and degradation

Peptidoglycan (PG) is the main component of the bacterial cell wall; it maintains cell shape while protecting the cell from internal osmotic pressure and external environmental challenges. PG synthesis is essential for bacterial growth and survival, and a series of PG modifications are required to allow expansion of the sacculus. Endopeptidases (EPs), for example, cleave the crosslinks between adjacent PG strands to allow the incorporation of newly synthesized PG. EPs are collectively essential for bacterial growth and must likely be carefully regulated to prevent sacculus degradation and cell death. However, EP regulation mechanisms are poorly understood. Here, we used TnSeq to uncover novel EP regulation factors in Vibrio cholerae. This screen revealed that the carboxypeptidase DacA1 (PBP5) alleviates EP toxicity. dacA1 is essential for viability on LB medium, and this essentiality was suppressed by EP overexpression, revealing that EP toxicity both mitigates, and is mitigated by, a defect in dacA1. A subsequent suppressor screen to restore viability of ΔdacA1 in LB medium was answered by hypomorphic mutants in the PG synthesis pathway, as well as mutations that promote PG degradation. Our data thus reveal a key role of DacA1 in maintaining the balance between PG synthesis and degradation.

DipM controls multiple autolysins and mediates a regulatory feedback loop promoting cell constriction in Caulobacter crescentus

Proteins with a catalytically inactive LytM-type endopeptidase domain are important regulators of cell wall-degrading enzymes in bacteria. Here, we study their representative DipM, a factor promoting cell division in Caulobacter crescentus. We show that the LytM domain of DipM interacts with multiple autolysins, including the soluble lytic transglycosylases SdpA and SdpB, the amidase AmiC and the putative carboxypeptidase CrbA, and stimulates the activities of SdpA and AmiC. Its crystal structure reveals a conserved groove, which is predicted to represent the docking site for autolysins by modeling studies. Mutations in this groove indeed abolish the function of DipM in vivo and its interaction with AmiC and SdpA in vitro. Notably, DipM and its targets SdpA and SdpB stimulate each other's recruitment to midcell, establishing a self-reinforcing cycle that gradually increases autolytic activity as cytokinesis progresses. DipM thus coordinates different peptidoglycan-remodeling pathways to ensure proper cell constriction and daughter cell separation.

Dissecting the roles of peptidoglycan synthetic and autolytic activities in the walled to L-form bacterial transition

Bacterial cells are surrounded by a peptidoglycan (PG) wall, which is a crucial target for antibiotics. It is well known that treatment with cell wall-active antibiotics occasionally converts bacteria to a non-walled "L-form" state that requires the loss of cell wall integrity. L-forms may have an important role in antibiotic resistance and recurrent infection. Recent work has revealed that inhibition of de novo PG precursor synthesis efficiently induces the L-form conversion in a wide range of bacteria, but the molecular mechanisms remain poorly understood. Growth of walled bacteria requires the orderly expansion of the PG layer, which involves the concerted action not just of synthases but also degradative enzymes called autolysins. Most rod-shaped bacteria have two complementary systems for PG insertion, the Rod and aPBP systems. Bacillus subtilis has two major autolysins, called LytE and CwlO, which are thought to have partially redundant functions. We have dissected the functions of autolysins, relative to the Rod and aPBP systems, during the switch to L-form state. Our results suggest that when de novo PG precursor synthesis is inhibited, residual PG synthesis occurs specifically via the aPBP pathway, and that this is required for continued autolytic activity by LytE/CwlO, resulting in cell bulging and efficient L-form emergence. The failure of L-form generation in cells lacking aPBPs was rescued by enhancing the Rod system and in this case, emergence specifically required LytE but was not associated with cell bulging. Our results suggest that two distinct pathways of L-form emergence exist depending on whether PG synthesis is being supported by the aPBP or RodA PG synthases. This work provides new insights into mechanisms of L-form generation, and specialisation in the roles of essential autolysins in relation to the recently recognised dual PG synthetic systems of bacteria.

Peptidoglycan NlpC/P60 peptidases in bacterial physiology and host interactions

The bacterial cell wall is composed of a highly crosslinked matrix of glycopeptide polymers known as peptidoglycan that dictates bacterial cell morphology and protects against environmental stresses. Regulation of peptidoglycan turnover is therefore crucial for bacterial survival and growth and is mediated by key protein complexes and enzyme families. Here, we review the prevalence, structure, and activity of NlpC/P60 peptidases, a family of peptidoglycan hydrolases that are crucial for cell wall turnover and division as well as interactions with antibiotics and different hosts. Understanding the molecular functions of NlpC/P60 peptidases should provide important insight into bacterial physiology, their interactions with different kingdoms of life, and the development of new therapeutic approaches.

Recent Advances in Peptidoglycan Synthesis and Regulation in Bacteria

Bacteria must synthesize their cell wall and membrane during their cell cycle, with peptidoglycan being the primary component of the cell wall in most bacteria. Peptidoglycan is a three-dimensional polymer that enables bacteria to resist cytoplasmic osmotic pressure, maintain their cell shape and protect themselves from environmental threats. Numerous antibiotics that are currently used target enzymes involved in the synthesis of the cell wall, particularly peptidoglycan synthases. In this review, we highlight recent progress in our understanding of peptidoglycan synthesis, remodeling, repair, and regulation in two model bacteria: the Gram-negative Escherichia coli and the Gram-positive Bacillus subtilis. By summarizing the latest findings in this field, we hope to provide a comprehensive overview of peptidoglycan biology, which is critical for our understanding of bacterial adaptation and antibiotic resistance.

Regulation of peptidoglycan hydrolases: localization, abundance, and activity

Most bacteria are surrounded by a cell wall composed of peptidoglycan (PG) that specifies shape and protects the cell from osmotic rupture. Growth, division, and morphogenesis are intimately linked to the synthesis of this exoskeleton but also its hydrolysis. The enzymes that cleave the PG meshwork require careful control to prevent aberrant hydrolysis and loss of envelope integrity. Bacteria employ diverse mechanisms to control the activity, localization, and abundance of these potentially autolytic enzymes. Here, we discuss four examples of how cells integrate these control mechanisms to finely tune cell wall hydrolysis. We highlight recent advances and exciting avenues for future investigation.

Masters of Misdirection: Peptidoglycan Glycosidases in Bacterial Growth

The dynamic composition of the peptidoglycan cell wall has been the subject of intense research for decades, yet how bacteria coordinate the synthesis of new peptidoglycan with the turnover and remodeling of existing peptidoglycan remains elusive. Diversity and redundancy within peptidoglycan synthases and peptidoglycan autolysins, enzymes that degrade peptidoglycan, have often made it challenging to assign physiological roles to individual enzymes and determine how those activities are regulated. For these reasons, peptidoglycan glycosidases, which cleave within the glycan strands of peptidoglycan, have proven veritable masters of misdirection over the years. Unlike many of the broadly conserved peptidoglycan synthetic complexes, diverse bacteria can employ unrelated glycosidases to achieve the same physiological outcome. Additionally, although the mechanisms of action for many individual enzymes have been characterized, apparent conserved homologs in other organisms can exhibit an entirely different biochemistry. This flexibility has been recently demonstrated in the context of three functions critical to vegetative growth: (i) release of newly synthesized peptidoglycan strands from their membrane anchors, (ii) processing of peptidoglycan turned over during cell wall expansion, and (iii) removal of peptidoglycan fragments that interfere with daughter cell separation during cell division. Finally, the regulation of glycosidase activity during these cell processes may be a cumulation of many factors, including protein-protein interactions, intrinsic substrate preferences, substrate availability, and subcellular localization. Understanding the true scope of peptidoglycan glycosidase activity will require the exploration of enzymes from diverse organisms with equally diverse growth and division strategies.

Structural insights into the regulation of peptidoglycan DL-endopeptidases by inhibitory protein IseA

Peptidoglycan, a physical barrier that protects bacteria from the environment, is constantly degraded and resynthesized for remodeling during cell growth and division. Because excessive or insufficient peptidoglycan hydrolysis affects bacterial homeostasis and viability, peptidoglycan degradation must be precisely regulated. In Bacillus subtilis, DL-endopeptidases play an essential role in peptidoglycan remodeling, and their activity is regulated by IseA. Here, we report the crystal structure of peptidoglycan DL-endopeptidase LytE complexed with IseA. In the crystal structure, the inhibitory loop connecting the two lobes of IseA blocks the active site of LytE by mimicking its substrate. Consistently, mutations in the inhibitory loop resulted in the loss of IseA activity. The structure also shows that conformational rearrangements in both LytE and IseA restrict access of the inhibitory loop to the LytE catalytic site. These results reveal an inhibition mechanism of peptidoglycan DL-endopeptidase in which the inhibitory protein mimics the substrate but is not degraded.

Insights into the Roles of Lipoteichoic Acids and MprF in Bacillus subtilis

Gram-positive bacterial cells are protected from the environment by a cell envelope that is comprised of a thick layer of peptidoglycan that maintains cell shape and teichoic acid polymers whose biological function remains unclear. In Bacillus subtilis, the loss of all class A penicillin-binding proteins (aPBPs), which function in peptidoglycan synthesis, is conditionally lethal. Here, we show that this lethality is associated with an alteration of lipoteichoic acids (LTAs) and the accumulation of the major autolysin LytE in the cell wall. Our analysis provides further evidence that the length and abundance of LTAs act to regulate the cellular level and activity of autolytic enzymes, specifically LytE. Importantly, we identify a novel function for the aminoacyl-phosphatidylglycerol synthase MprF in the modulation of LTA biosynthesis in both B. subtilis and Staphylococcus aureus. This finding has implications for our understanding of antimicrobial resistance (particularly to daptomycin) in clinically relevant bacteria and the involvement of MprF in the virulence of pathogens such as methicillin-resistant S. aureus (MRSA). IMPORTANCE In Gram-positive bacteria such as Bacillus subtilis and Staphylococcus aureus, the cell envelope is a structure that protects the cells from the environment but is also dynamic in that it must be modified in a controlled way to allow cell growth. In this study, we show that lipoteichoic acids (LTAs), which are anionic polymers attached to the membrane, have a direct role in modulating the cellular abundance of cell wall-degrading enzymes. We also find that the apparent length of the LTA is modulated by the activity of the enzyme MprF, previously implicated in modifications of the cell membrane leading to resistance to antimicrobial peptides. These findings are important contributions to our understanding of how bacteria balance cell wall synthesis and degradation to permit controlled growth and division. These results also have implications for the interpretation of antibiotic resistance, particularly for the clinical treatment of MRSA infections.

Mechanistic Insights into the Activities of Major Families of Enzymes in Bacterial Peptidoglycan Assembly and Breakdown

Serving as an exoskeletal scaffold, peptidoglycan is a polymeric macromolecule that is essential and conserved across all bacteria, yet is absent in mammalian cells; this has made bacterial peptidoglycan a well-established excellent antibiotic target. In addition, soluble peptidoglycan fragments derived from bacteria are increasingly recognised as key signalling molecules in mediating diverse intra- and inter-species communication in nature, including in gut microbiota-host crosstalk. Each bacterial species encodes multiple redundant enzymes for key enzymatic activities involved in peptidoglycan assembly and breakdown. In this review, we discuss recent findings on the biochemical activities of major peptidoglycan enzymes, including peptidoglycan glycosyltransferases (PGT) and transpeptidases (TPs) in the final stage of peptidoglycan assembly, as well as peptidoglycan glycosidases, lytic transglycosylase (LTs), amidases, endopeptidases (EPs) and carboxypeptidases (CPs) in peptidoglycan turnover and metabolism. Biochemical characterisation of these enzymes provides valuable insights into their substrate specificity, regulation mechanisms and potential modes of inhibition.

Imbalance of peptidoglycan biosynthesis alters the cell surface charge of Listeria monocytogenes

The bacterial cell wall is composed of a thick layer of peptidoglycan and cell wall polymers, which are either embedded in the membrane or linked to the peptidoglycan backbone and referred to as lipoteichoic acid (LTA) and wall teichoic acid (WTA), respectively. Modifications of the peptidoglycan or WTA backbone can alter the susceptibility of the bacterial cell towards cationic antimicrobials and lysozyme. The human pathogen Listeria monocytogenes is intrinsically resistant towards lysozyme, mainly due to deacetylation and O-acetylation of the peptidoglycan backbone via PgdA and OatA. Recent studies identified additional factors, which contribute to the lysozyme resistance of this pathogen. One of these is the predicted ABC transporter, EslABC. An eslB mutant is hyper-sensitive towards lysozyme, likely due to the production of thinner and less O-acetylated peptidoglycan. Using a suppressor screen, we show here that suppression of eslB phenotypes could be achieved by enhancing peptidoglycan biosynthesis, reducing peptidoglycan hydrolysis or alterations in WTA biosynthesis and modification. The lack of EslB also leads to a higher negative surface charge, which likely stimulates the activity of peptidoglycan hydrolases and lysozyme. Based on our results, we hypothesize that the portion of cell surface exposed WTA is increased in the eslB mutant due to the thinner peptidoglycan layer and that latter one could be caused by an impairment in UDP-N-acetylglucosamine (UDP-GlcNAc) production or distribution.

Mirubactin C rescues the lethal effect of cell wall biosynthesis mutations in Bacillus subtilis

Growth of most rod-shaped bacteria is accompanied by the insertion of new peptidoglycan into the cylindrical cell wall. This insertion, which helps maintain and determine the shape of the cell, is guided by a protein machine called the rod complex or elongasome. Although most of the proteins in this complex are essential under normal growth conditions, cell viability can be rescued, for reasons that are not understood, by the presence of a high (mM) Mg2+ concentration. We screened for natural product compounds that could rescue the growth of mutants affected in rod-complex function. By screening > 2,000 extracts from a diverse collection of actinobacteria, we identified a compound, mirubactin C, related to the known iron siderophore mirubactin A, which rescued growth in the low micromolar range, and this activity was confirmed using synthetic mirubactin C. The compound also displayed toxicity at higher concentrations, and this effect appears related to iron homeostasis. However, several lines of evidence suggest that the mirubactin C rescuing activity is not due simply to iron sequestration. The results support an emerging view that the functions of bacterial siderophores extend well beyond simply iron binding and uptake.

Mechanism of Action of Ribosomally Synthesized and Post-Translationally Modified Peptides

Ribosomally synthesized and post-translationally modified peptides (RiPPs) are a natural product class that has undergone significant expansion due to the rapid growth in genome sequencing data and recognition that they are made by biosynthetic pathways that share many characteristic features. Their mode of actions cover a wide range of biological processes and include binding to membranes, receptors, enzymes, lipids, RNA, and metals as well as use as cofactors and signaling molecules. This review covers the currently known modes of action (MOA) of RiPPs. In turn, the mechanisms by which these molecules interact with their natural targets provide a rich set of molecular paradigms that can be used for the design or evolution of new or improved activities given the relative ease of engineering RiPPs. In this review, coverage is limited to RiPPs originating from bacteria.

Intrinsically disordered protein regions are required for cell wall homeostasis in Bacillus subtilis

Intrinsically disordered protein regions (IDRs) have been implicated in diverse nuclear and cytoplasmic functions in eukaryotes, but their roles in bacteria are less clear. Here, we report that extracytoplasmic IDRs in Bacillus subtilis are required for cell wall homeostasis. The B. subtilis σ^I transcription factor is activated in response to envelope stress through regulated intramembrane proteolysis (RIP) of its membrane-anchored anti-σ factor, RsgI. Unlike canonical RIP pathways, we show that ectodomain (site-1) cleavage of RsgI is constitutive, but the two cleavage products remain stably associated, preventing intramembrane (site-2) proteolysis. The regulated step in this pathway is their dissociation, which is triggered by impaired cell wall synthesis and requires RsgI's extracytoplasmic IDR. Intriguingly, the major peptidoglycan polymerase PBP1 also contains an extracytoplasmic IDR, and we show that this region is important for its function. Disparate IDRs can replace the native IDRs on both RsgI and PBP1, arguing that these unstructured regions function similarly. Our data support a model in which the RsgI-σ^I signaling system and PBP1 represent complementary pathways to repair gaps in the PG meshwork. The IDR on RsgI senses these gaps and activates σ^I, while the IDR on PBP1 directs the synthase to these sites to fortify them.

Remodeling Bacillus amyloliquefaciens Cell Wall Rigidity to Reduce Cell Lysis and Increase the Yield of Heterologous Proteins

Bacillus amyloliquefaciens has great potential as a host for heterologous protein production, but its severe autolytic behavior has precluded its industrial application to date. Because d,l-endopeptidase activity-guided cell wall rigidity is considered essential for autolysis resistance, we investigated the effects of d,l-endopeptidase genes lytE, lytF, cwlO, and cwlS play on the growth, lysis, and morphology remodeling of B. amyloliquefaciens strain TCCC11018. Individual and combinatorial deletion of lytE, lytF, and cwlS enhanced the cell growth and delayed cell lysis. For the best mutant with the lytF and cwlS double deletion, the viable cell number at 24 h increased by 11.90% and the cell wall thickness at 6 h increased by 25.87%. Transcriptomic and proteomic analyses indicated that the improvement was caused by enhanced peptidoglycan synthesis. With the lytF and cwlS double deletion, the extracellular green fluorescent protein and phospholipase D expression levels increased by 113 and 55.89%, respectively. This work broadens our understanding of the relationship between d,l-endopeptidases and B. amyloliquefaciens cell characteristics, which provides an effective strategy to improve the heterologous protein expression in B. amyloliquefaciens-based cell factories.

A Cell Wall Hydrolase MepH Is Negatively Regulated by Proteolysis Involving Prc and NlpI in Escherichia coli

Cell wall assembly of Gram-negative bacteria requires DD-endopeptidase activity that cleaves peptidoglycan (PG) crosslinks in addition to PG synthetic activity, and the activity of DD-endopeptidases needs to be tightly regulated to maintain cell wall integrity during PG expansion. Among the major DD-endopeptidases functioning for PG assembly in Escherichia coli, MepS and MepM have been shown to be negatively controlled by the periplasmic protease Prc. In this study, we performed a genetic selection using the synthetic lethality between the mepS and mepM mutations in rich medium to uncover regulatory mechanisms controlling the activity of DD-endopeptidases other than MepS and MepM. This selection revealed mutations in prc and nlpI as suppressors. Gene deletion analyses revealed that MepH is required for suppression of the MepS^— MepM^— growth defect by the prc or nlpI mutation. We also discovered that MepH is directly degraded by Prc and that this degradation is further promoted by NlpI. Thus, our study showed that all three DD-endopeptidases which play major roles in PG assembly of E. coli under normal physiological conditions are controlled by a common periplasmic protease.

Magnesium rescues the morphology of Bacillus subtilis mreB mutants through its inhibitory effect on peptidoglycan hydrolases

Cell wall homeostasis in bacteria is tightly regulated by balanced synthesis and degradation of peptidoglycan (PG), allowing cells to expand their sacculus during growth while maintaining physical integrity. In rod-shaped bacteria, actin-like MreB proteins are key players of the PG elongation machinery known as the Rod complex. In the Gram-positive model bacterium Bacillus subtilis depletion of the essential MreB leads to loss of rod shape and cell lysis. However, millimolar concentrations of magnesium in the growth medium rescue the viability and morphological defects of mreB mutants by an unknown mechanism. Here, we used a combination of cytological, biochemical and biophysical approaches to investigate the cell surface properties of mreB null mutant cells and the interactions of Mg²⁺ with the cell wall of B. subtilis. We show that ∆mreB cells have rougher and softer surfaces, and changes in PG composition indicative of increased DL- and DD-endopeptidase activities as well as increased deacetylation of the sugar moieties. Increase in DL-endopeptidase activity is mitigated by excess Mg²⁺ while DD-endopeptidase activity remains high. Visualization of PG degradation in pulse-chase experiments showed anisotropic PG hydrolase activity along the sidewalls of ∆mreB cells, in particular at the sites of increased cell width and bulging, while PG synthesis remained isotropic. Overall, our data support a model in which divalent cations maintain rod shape in ∆mreB cells by inhibiting PG hydrolases, possibly through the formation of crosslinks with carboxyl groups of the PG meshwork that affect the capacity of PG hydrolases to act on their substrate.

Growth and Division of the Peptidoglycan Matrix

Most bacteria are surrounded by a peptidoglycan cell wall that defines their shape and protects them from osmotic lysis. The expansion and division of this structure therefore plays an integral role in bacterial growth and division. Additionally, the biogenesis of the peptidoglycan layer is the target of many of our most effective antibiotics. Thus, a better understanding of how the cell wall is built will enable the development of new therapies to combat the rise of drug-resistant bacterial infections. This review covers recent advances in defining the mechanisms involved in assembling the peptidoglycan layer with an emphasis on discoveries related to the function and regulation of the cell elongation and division machineries in the model organisms Escherichia coli and Bacillus subtilis. Expected final online publication date for the Annual Review of Microbiology, Volume 75 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Understanding tolerance to cell wall-active antibiotics

Antibiotic tolerance-the ability of bacteria to survive for an extended time in the presence of bactericidal antibiotics-is an understudied contributor to antibiotic treatment failure. Herein, I review the manifestations, mechanisms, and clinical relevance of tolerance to cell wall-active (CWA) antibiotics, one of the most important groups of antibiotics at the forefront of clinical use. I discuss definitions of tolerance and assays for tolerance detection, comprehensively discuss the mechanism of action of β-lactams and other CWA antibiotics, and then provide an overview of how cells mitigate the potentially lethal effects of CWA antibiotic-induced cell damage to become tolerant. Lastly, I discuss evidence for a role of CWA antibiotic tolerance in clinical antibiotic treatment failure.

Ser/Thr Kinase-Dependent Phosphorylation of the Peptidoglycan Hydrolase CwlA Controls Its Export and Modulates Cell Division in Clostridioides difficile

Cell growth and division require a balance between synthesis and hydrolysis of the peptidoglycan (PG). Inhibition of PG synthesis or uncontrolled PG hydrolysis can be lethal for the cells, making it imperative to control peptidoglycan hydrolase (PGH) activity. The synthesis or activity of several key enzymes along the PG biosynthetic pathway is controlled by the Hanks-type serine/threonine kinases (STKs). In Gram-positive bacteria, inactivation of genes encoding STKs is associated with a range of phenotypes, including cell division defects and changes in cell wall metabolism, but only a few kinase substrates and associated mechanisms have been identified. We previously demonstrated that STK-PrkC plays an important role in cell division, cell wall metabolism, and resistance to antimicrobial compounds in the human enteropathogen Clostridioides difficile In this work, we characterized a PG hydrolase, CwlA, which belongs to the NlpC/P60 family of endopeptidases and hydrolyses cross-linked PG between daughter cells to allow cell separation. We identified CwlA as the first PrkC substrate in C. difficile We demonstrated that PrkC-dependent phosphorylation inhibits CwlA export, thereby controlling hydrolytic activity in the cell wall. High levels of CwlA at the cell surface led to cell elongation, whereas low levels caused cell separation defects. Thus, we provided evidence that the STK signaling pathway regulates PGH homeostasis to precisely control PG hydrolysis during cell division.IMPORTANCE Bacterial cells are encased in a PG exoskeleton that helps to maintain cell shape and confers physical protection. To allow bacterial growth and cell separation, PG needs to be continuously remodeled by hydrolytic enzymes that cleave PG at critical sites. How these enzymes are regulated remains poorly understood. We identify a new PG hydrolase involved in cell division, CwlA, in the enteropathogen C. difficile Lack or accumulation of CwlA at the bacterial surface is responsible for a division defect, while its accumulation in the absence of PrkC also increases susceptibility to antimicrobial compounds targeting the cell wall. CwlA is a substrate of the kinase PrkC in C. difficile PrkC-dependent phosphorylation controls the export of CwlA, modulating its levels and, consequently, its activity in the cell wall. This work provides a novel regulatory mechanism by STK in tightly controlling protein export.

CwlQ Is Required for Swarming Motility but Not Flagellar Assembly in Bacillus subtilis

Lytic enzymes play an essential role in the remodeling of bacterial peptidoglycan (PG), an extracellular mesh-like structure that retains the membrane in the context of high internal osmotic pressure. Peptidoglycan must be unfailingly stable to preserve cell integrity, but must also be dynamically remodeled for the cell to grow, divide, and insert macromolecular machines. The flagellum is one such macromolecular machine that transits the PG, and flagellar insertion is aided by localized activity of a dedicated PG lyase in Gram-negative bacteria. To date, there is no known dedicated lyase in Gram-positive bacteria for the insertion of flagella. Here, we take a reverse-genetic candidate-gene approach and find that cells mutated for the lytic transglycosylase CwlQ exhibit a severe defect in flagellum-dependent swarming motility. We further show that CwlQ is expressed by the motility sigma factor SigD and is secreted by the type III secretion system housed inside the flagellum. Nonetheless, cells with mutations of CwlQ remain proficient for flagellar biosynthesis even when mutated in combination with four other lyases related to motility (LytC, LytD, LytF, and CwlO). The PG lyase (or lyases) essential for flagellar synthesis in B. subtilis, if any, remains unknown.IMPORTANCE Bacteria are surrounded by a wall of peptidoglycan and early work in Bacillus subtilis was the first to suggest that bacteria needed to enzymatically remodel the wall to permit insertion of the flagellum. No PG remodeling enzyme alone or in combination, however, has been found to be essential for flagellar assembly in B. subtilis Here, we take a reverse-genetic candidate-gene approach and find that the PG lytic transglycosylase CwlQ is required for swarming motility. Subsequent characterization determined that while CwlQ was coexpressed with motility genes and is secreted by the flagellar secretion apparatus, it was not required for flagellar synthesis. The PG lyase needed for flagellar assembly in B. subtilis remains unknown.

Class A Penicillin-Binding Protein-Mediated Cell Wall Synthesis Promotes Structural Integrity during Peptidoglycan Endopeptidase Insufficiency in Vibrio cholerae

The bacterial cell wall is composed primarily of peptidoglycan (PG), a poly-aminosugar that is essential to sustain cell shape, growth, and structural integrity. PG is synthesized by class A/B penicillin-binding proteins (a/bPBPs) and shape, elongation, division, and sporulation (SEDS) proteins like RodA (as part of the Rod system cell elongation machinery) and degraded by "autolytic" enzymes to accommodate growth processes. It is thought that autolysins (particularly endopeptidases [EPs]) are required for PG synthesis and incorporation by creating gaps that are patched and paved by PG synthases, but the exact relationship between autolysins and PG synthesis remains incompletely understood. Here, we have probed the consequences of EP depletion for PG synthesis in the diarrheal pathogen Vibrio cholerae We found that EP depletion resulted in severe morphological and division defects, but these cells continued to increase in mass and aberrantly incorporated new cell wall material. Mass increase proceeded in the presence of Rod system inhibitors, but cells lysed upon inhibition of aPBPs, suggesting that aPBPs are required for structural integrity under these conditions. The Rod system, although not essential for the observed mass increase, remained functional even after prolonged EP depletion. Last, heterologous expression of an EP from Neisseria gonorrhoeae fully complemented growth and morphology of an EP-insufficient V. cholerae, highlighting the possibility that the PG synthases may not necessarily function via direct interaction with EPs. Overall, our findings suggest that during EP insufficiency in V. cholerae, aPBPs become essential for structural integrity while the Rod system is unable to promote proper cell expansion.IMPORTANCE Synthesis and turnover of the bacterial cell wall must be tightly coordinated to avoid structural integrity failure and cell death. Details of this coordination are poorly understood, particularly if and how cell wall turnover enzymes are required for the activity of the different cell wall synthesis machines, the aPBPs and the Rod system. Our results suggest that in Vibrio cholerae, one class of turnover enzymes, the endopeptidases, are necessary for proper cell elongation and division. aPBPs become essential for maintaining structural integrity during EP insufficiency, while the Rod system remains active but contributes little to cell expansion under these conditions. Our results suggest that aPBPs are more versatile than the Rod system in their ability to recognize cell wall gaps formed by autolysins other than the major endopeptidases, adding to our understanding of the coordination between autolysins and cell wall synthases. A detailed understanding of autolysin biology may promote the development of antibiotics that target these essential turnover processes.

Not Just Transporters: Alternative Functions of ABC Transporters in Bacillus subtilis and Listeria monocytogenes

ATP-binding cassette (ABC) transporters are usually involved in the translocation of their cognate substrates, which is driven by ATP hydrolysis. Typically, these transporters are required for the import or export of a wide range of substrates such as sugars, ions and complex organic molecules. ABC exporters can also be involved in the export of toxic compounds such as antibiotics. However, recent studies revealed alternative detoxification mechanisms of ABC transporters. For instance, the ABC transporter BceAB of Bacillus subtilis seems to confer resistance to bacitracin via target protection. In addition, several transporters with functions other than substrate export or import have been identified in the past. Here, we provide an overview of recent findings on ABC transporters of the Gram-positive organisms B. subtilis and Listeria monocytogenes with transport or regulatory functions affecting antibiotic resistance, cell wall biosynthesis, cell division and sporulation.

Peptidoglycan: Structure, Synthesis, and Regulation

Peptidoglycan is a defining feature of the bacterial cell wall. Initially identified as a target of the revolutionary beta-lactam antibiotics, peptidoglycan has become a subject of much interest for its biology, its potential for the discovery of novel antibiotic targets, and its role in infection. Peptidoglycan is a large polymer that forms a mesh-like scaffold around the bacterial cytoplasmic membrane. Peptidoglycan synthesis is vital at several stages of the bacterial cell cycle: for expansion of the scaffold during cell elongation and for formation of a septum during cell division. It is a complex multifactorial process that includes formation of monomeric precursors in the cytoplasm, their transport to the periplasm, and polymerization to form a functional peptidoglycan sacculus. These processes require spatio-temporal regulation for successful assembly of a robust sacculus to protect the cell from turgor and determine cell shape. A century of research has uncovered the fundamentals of peptidoglycan biology, and recent studies employing advanced technologies have shed new light on the molecular interactions that govern peptidoglycan synthesis. Here, we describe the peptidoglycan structure, synthesis, and regulation in rod-shaped bacteria, particularly Escherichia coli, with a few examples from Salmonella and other diverse organisms. We focus on the pathway of peptidoglycan sacculus elongation, with special emphasis on discoveries of the past decade that have shaped our understanding of peptidoglycan biology.

Cell morphology maintenance in Bacillus subtilis through balanced peptidoglycan synthesis and hydrolysis

The peptidoglycan layer is responsible for maintaining bacterial cell shape and permitting cell division. Cell wall growth is facilitated by peptidoglycan synthases and hydrolases and is potentially modulated by components of the central carbon metabolism. In Bacillus subtilis, UgtP synthesises the glucolipid precursor for lipoteichoic acid and has been suggested to function as a metabolic sensor governing cell size. Here we show that ugtP mutant cells have increased levels of cell wall precursors and changes in their peptidoglycan that suggest elevated DL-endopeptidase activity. The additional deletion of lytE, encoding a DL-endopeptidase important for cell elongation, in the ugtP mutant background produced cells with severe shape defects. Interestingly, the ugtP lytE mutant recovered normal rod-shape by acquiring mutations that decreased the expression of the peptidoglycan synthase PBP1. Together our results suggest that cells lacking ugtP must re-adjust the balance between peptidoglycan synthesis and hydrolysis to maintain proper cell morphology.

A regulatory pathway that selectively up-regulates elongasome function in the absence of class A PBPs

Bacteria surround themselves with peptidoglycan, an adaptable enclosure that contributes to cell shape and stability. Peptidoglycan assembly relies on penicillin-binding proteins (PBPs) acting in concert with SEDS-family transglycosylases RodA and FtsW, which support cell elongation and division respectively. In Bacillus subtilis, cells lacking all four PBPs with transglycosylase activity (aPBPs) are viable. Here, we show that the alternative sigma factor σ^I is essential in the absence of aPBPs. Defects in aPBP-dependent wall synthesis are compensated by σ^I-dependent upregulation of an MreB homolog, MreBH, which localizes the LytE autolysin to the RodA-containing elongasome complex. Suppressor analysis reveals that cells unable to activate this σ^I stress response acquire gain-of-function mutations in the essential histidine kinase WalK, which also elevates expression of sigI, mreBH and lytE. These results reveal compensatory mechanisms that balance the directional peptidoglycan synthesis arising from the elongasome complex with the more diffusive action of aPBPs.

Structural basis of peptidoglycan endopeptidase regulation

Most bacteria surround themselves with a cell wall, a strong meshwork consisting primarily of the polymerized aminosugar peptidoglycan (PG). PG is essential for structural maintenance of bacterial cells, and thus for viability. PG is also constantly synthesized and turned over; the latter process is mediated by PG cleavage enzymes, for example, the endopeptidases (EPs). EPs themselves are essential for growth but also promote lethal cell wall degradation after exposure to antibiotics that inhibit PG synthases (e.g., β-lactams). Thus, EPs are attractive targets for novel antibiotics and their adjuvants. However, we have a poor understanding of how these enzymes are regulated in vivo, depriving us of novel pathways for the development of such antibiotics. Here, we have solved crystal structures of the LysM/M23 family peptidase ShyA, the primary EP of the cholera pathogen Vibrio cholerae Our data suggest that ShyA assumes two drastically different conformations: a more open form that allows for substrate binding and a closed form, which we predicted to be catalytically inactive. Mutations expected to promote the open conformation caused enhanced activity in vitro and in vivo, and these results were recapitulated in EPs from the divergent pathogens Neisseria gonorrheae and Escherichia coli Our results suggest that LysM/M23 EPs are regulated via release of the inhibitory Domain 1 from the M23 active site, likely through conformational rearrangement in vivo.

Recent Advances in Recombinant Protein Production byBacillus subtilis

Bacillus subtilis has become a widely used microbial cell factory for the production of recombinant proteins, especially those associated with foods and food processing. Recent advances in genetic manipulation and proteomic analysis have been used to greatly improve protein production in B. subtilis. This review begins with a discussion of genome-editing technologies and application of the CRISPR–Cas9 system to B. subtilis. A summary of the characteristics of crucial legacy strains is followed by suggestions regarding the choice of origin strain for genetic manipulation. Finally, the review analyzes the genes and operons of B. subtilis that are important for the production of secretory proteins and provides suggestions and examples of how they can be altered to improve protein production. This review is intended to promote the engineering of this valuable microbial cell factory for better recombinant protein production.

Peptidoglycan Hydrolases RipA and Ami1 Are Critical for Replication and Persistence of Mycobacterium tuberculosis in the Host

Tuberculosis (TB) is a major global heath burden, with 1.6 million people succumbing to the disease every year. The search for new drugs to improve the current chemotherapeutic regimen is crucial to reducing this global health burden. The cell wall polymer peptidoglycan (PG) has emerged as a very successful drug target in bacterial pathogens, as many currently used antibiotics target the synthesis of this macromolecule. However, the multitude of genes encoding PG-synthesizing and PG-modifying enzymes with apparent redundant functions has hindered the identification of novel drug targets in PG synthesis in Mycobacterium tuberculosis. Here, we demonstrate that two PG-cleaving enzymes are important for virulence of M. tuberculosis. In particular, the d,l-endopeptidase RipA represents a potentially attractive drug target, as its depletion results in the clearance of M. tuberculosis from the host and renders the bacteria hypersusceptible to rifampin, a frontline TB drug, and to several cell wall-targeting antibiotics.

Synthesis and cleavage of the cell wall polymer peptidoglycan (PG) are carefully orchestrated processes and are essential for the growth and survival of bacteria. Yet, the function and importance of many enzymes that act on PG in Mycobacterium tuberculosis remain to be elucidated. We demonstrate that the activity of the N-acetylmuramyl-l-alanine amidase Ami1 is dispensable for cell division in M. tuberculosisin vitro yet contributes to the bacterium’s ability to persist during chronic infection in mice. Furthermore, the d,l-endopeptidase RipA, a predicted essential enzyme, is dispensable for the viability of M. tuberculosis but required for efficient cell division in vitro and in vivo. Depletion of RipA sensitizes M. tuberculosis to rifampin and to cell envelope-targeting antibiotics. Ami1 helps sustain residual cell division in cells lacking RipA, but the partial redundancy provided by Ami1 is not sufficient during infection, as depletion of RipA prevents M. tuberculosis from replicating in macrophages and leads to dramatic killing of the bacteria in mice. Notably, RipA is essential for persistence of M. tuberculosis in mice, suggesting that cell division is required during chronic mouse infection. Despite the multiplicity of enzymes acting on PG with redundant functions, we have identified two PG hydrolases that are important for M. tuberculosis to replicate and persist in the host.

Comparative Analysis of Structural Variations Due to Genome Shuffling of Bacillus Subtilis VS15 for Improved Cellulase Production

Cellulose is one of the most abundant and renewable biomass products used for the production of bioethanol. Cellulose can be efficiently hydrolyzed by Bacillus subtilis VS15, a strain isolate obtained from decomposing logs. A genome shuffling approach was implemented to improve the cellulase activity of Bacillus subtilis VS15. Mutant strains were created using ethyl methyl sulfonate (EMS), N-Methyl-N′ nitro-N-nitrosoguanidine (NTG), and ultraviolet light (UV) followed by recursive protoplast fusion. After two rounds of shuffling, the mutants Gb2, Gc8, and Gd7 were produced that had an increase in cellulase activity of 128%, 148%, and 167%, respectively, in comparison to the wild type VS15. The genetic diversity of the shuffled strain Gd7 and wild type VS15 was compared at whole genome level. Genomic-level comparisons identified a set of eight genes, consisting of cellulase and regulatory genes, of interest for further analyses. Various genes were identified with insertions and deletions that may be involved in improved celluase production in Gd7. Strain Gd7 maintained the capability of hydrolyzing wheatbran to glucose and converting glucose to ethanol by fermentation with Saccharomyces cerevisiae of the wild type VS17. This ability was further confirmed by the acidified potassium dichromate (K₂Cr₂O₇) method.

Uncovering the activities, biological roles, and regulation of bacterial cell wall hydrolases and tailoring enzymes

Bacteria account for 1000-fold more biomass than humans. They vary widely in shape and size. The morphological diversity of bacteria is due largely to the different peptidoglycan-based cell wall structures that encase bacterial cells. Although the basic structure of peptidoglycan is highly conserved, consisting of long glycan strands that are cross-linked by short peptide chains, the mature cell wall is chemically diverse. Peptidoglycan hydrolases and cell wall-tailoring enzymes that regulate glycan strand length, the degree of cross-linking, and the addition of other modifications to peptidoglycan are central in determining the final architecture of the bacterial cell wall. Historically, it has been difficult to biochemically characterize these enzymes that act on peptidoglycan because suitable peptidoglycan substrates were inaccessible. In this review, we discuss fundamental aspects of bacterial cell wall synthesis, describe the regulation and diverse biochemical and functional activities of peptidoglycan hydrolases, and highlight recently developed methods to make and label defined peptidoglycan substrates. We also review how access to these substrates has now enabled biochemical studies that deepen our understanding of how bacterial cell wall enzymes cooperate to build a mature cell wall. Such improved understanding is critical to the development of new antibiotics that disrupt cell wall biogenesis, a process essential to the survival of bacteria.

Homeostatic control of cell wall hydrolysis by the WalRK two-component signaling pathway in Bacillus subtilis

Bacterial cells are encased in a peptidoglycan (PG) exoskeleton that protects them from osmotic lysis and specifies their distinct shapes. Cell wall hydrolases are required to enlarge this covalently closed macromolecule during growth, but how these autolytic enzymes are regulated remains poorly understood. Bacillus subtilis encodes two functionally redundant D,L-endopeptidases (CwlO and LytE) that cleave peptide crosslinks to allow expansion of the PG meshwork during growth. Here, we provide evidence that the essential and broadly conserved WalR-WalK two component regulatory system continuously monitors changes in the activity of these hydrolases by sensing the cleavage products generated by these enzymes and modulating their levels and activity in response. The WalR-WalK pathway is conserved among many Gram-positive pathogens where it controls transcription of distinct sets of PG hydrolases. Cell wall remodeling in these bacteria may be subject to homeostatic control mechanisms similar to the one reported here.

SweC and SweD are essential co-factors of the FtsEX-CwlO cell wall hydrolase complex in Bacillus subtilis

The peptidoglycan (PG) sacculus is composed of long glycan strands cross-linked together by short peptides forming a covalently closed meshwork that protects the bacterial cell from osmotic lysis and specifies its shape. PG hydrolases play essential roles in remodeling this three-dimensional network during growth and division but how these autolytic enzymes are regulated remains poorly understood. The FtsEX ABC transporter-like complex has emerged as a broadly conserved regulatory module in controlling cell wall hydrolases in diverse bacterial species. In most characterized examples, this complex regulates distinct PG hydrolases involved in cell division and is intimately associated with the cytokinetic machinery called the divisome. However, in the gram-positive bacterium Bacillus subtilis the FtsEX complex is required for cell wall elongation where it regulates the PG hydrolase CwlO that acts along the lateral cell wall. To investigate whether additional factors are required for FtsEX function outside the divisome, we performed a synthetic lethal screen taking advantage of the conditional essentiality of CwlO. This screen identified two uncharacterized factors (SweD and SweC) that are required for CwlO activity. We demonstrate that these proteins reside in a membrane complex with FtsX and that amino acid substitutions in residues adjacent to the ATPase domain of FtsE partially bypass the requirement for them. Collectively our data indicate that SweD and SweC function as essential co-factors of FtsEX in controlling CwlO during cell wall elongation. We propose that factors analogous to SweDC function to support FtsEX activity outside the divisome in other bacteria.

Distinct and Specific Role of NlpC/P60 Endopeptidases LytA and LytB in Cell Elongation and Division of Lactobacillus plantarum

Peptidoglycan (PG) is an essential lattice of the bacterial cell wall that needs to be continuously remodeled to allow growth. This task is ensured by the concerted action of PG synthases that insert new material in the pre-existing structure and PG hydrolases (PGHs) that cleave the PG meshwork at critical sites for its processing. Contrasting with Bacillus subtilis that contains more than 35 PGHs, Lactobacillus plantarum is a non-sporulating rod-shaped bacterium that is predicted to possess a minimal set of 12 PGHs. Their role in morphogenesis and cell cycle remains mostly unexplored, except for the involvement of the glucosaminidase Acm2 in cell separation and the NlpC/P60 D, L-endopeptidase LytA in cell shape maintenance. Besides LytA, L. plantarum encodes three additional NlpC/P60 endopeptidases (i.e., LytB, LytC and LytD). The in silico analysis of these four endopeptidases suggests that they could have redundant functions based on their modular organization, forming two pairs of paralogous enzymes. In this work, we investigate the role of each Lyt endopeptidase in cell morphogenesis in order to evaluate their distinct or redundant functions, and eventually their synthetic lethality. We show that the paralogous LytC and LytD enzymes are not required for cell shape maintenance, which may indicate an accessory role such as in PG recycling. In contrast, LytA and LytB appear to be key players of the cell cycle. We show here that LytA is required for cell elongation while LytB is involved in the spatio-temporal regulation of cell division. In addition, both PGHs are involved in the proper positioning of the division site. The absence of LytA activity is responsible for the asymmetrical positioning of septa in round cells while the lack of LytB results in a lateral misplacement of division planes in rod-shaped cells. Finally, we show that the co-inactivation of LytA and LytB is synthetically affecting cell growth, which confirms the key roles played by both enzymes in PG remodeling during the cell cycle of L. plantarum. Based on the large distribution of NlpC/P60 endopeptidases in low-GC Gram-positive bacteria, these enzymes are attractive targets for the discovery of novel antimicrobial compounds.

Peptidoglycan hydrolase of an unusual cross-link cleavage specificity contributes to bacterial cell wall synthesis

Bacteria are surrounded by a protective exoskeleton, peptidoglycan (PG), a cross-linked mesh-like macromolecule consisting of glycan strands interlinked by short peptides. Because PG completely encases the cytoplasmic membrane, cleavage of peptide cross-links is a prerequisite to make space for incorporation of nascent glycan strands for its successful expansion during cell growth. In most bacteria, the peptides consist of l-alanine, d-glutamate, meso-diaminopimelic acid (mDAP) and d-alanine (d-Ala) with cross-links occurring either between d-Ala and mDAP or two mDAP residues. In Escherichia coli, the d-Ala-mDAP cross-links whose cleavage by specialized endopeptidases is crucial for expansion of PG predominate. However, a small proportion of mDAP-mDAP cross-links also exist, yet their role in the context of PG expansion or the hydrolase(s) capable of catalyzing their cleavage is not known. Here, we identified an ORF of unknown function, YcbK (renamed MepK), as an mDAP-mDAP cross-link cleaving endopeptidase working in conjunction with other elongation-specific endopeptidases to make space for efficient incorporation of nascent PG strands into the sacculus. E. coli mutants lacking mepK and another d-Ala-mDAP-specific endopeptidase (mepS) were synthetic sick, and the defects were abrogated by lack of l,d-transpeptidases, enzymes catalyzing the formation of mDAP cross-links. Purified MepK was able to cleave the mDAP cross-links of soluble muropeptides and of intact PG sacculi. Overall, this study describes a PG hydrolytic enzyme with a hitherto unknown substrate specificity that contributes to expansion of the PG sacculus, emphasizing the fundamental importance of cross-link cleavage in bacterial peptidoglycan synthesis.

Endopeptidase Regulation as a Novel Function of the Zur-Dependent Zinc Starvation Response

Bacteria encode a variety of adaptations that enable them to survive during zinc starvation, a condition which is encountered both in natural environments and inside the human host. In Vibrio cholerae, the causative agent of the diarrheal disease cholera, we have identified a novel member of this zinc starvation response, a cell wall hydrolase that retains function and is conditionally essential for cell growth in low-zinc environments. Other Gram-negative bacteria contain homologs that appear to be under similar regulatory control. These findings are significant because they represent, to our knowledge, the first evidence that zinc homeostasis influences cell wall turnover. Anti-infective therapies commonly target the bacterial cell wall; therefore, an improved understanding of how the cell wall adapts to host-induced zinc starvation could lead to new antibiotic development. Such therapeutic interventions are required to combat the rising threat of drug-resistant infections.

The cell wall is a strong, yet flexible, meshwork of peptidoglycan (PG) that gives a bacterium structural integrity. To accommodate a growing cell, the wall is remodeled by both PG synthesis and degradation. Vibrio cholerae encodes a group of three nearly identical zinc-dependent endopeptidases (EPs) that are predicted to hydrolyze PG to facilitate cell growth. Two of these (ShyA and ShyC) are conditionally essential housekeeping EPs, while the third (ShyB) is not expressed under standard laboratory conditions. To investigate the role of ShyB, we conducted a transposon screen to identify mutations that activate shyB transcription. We found that shyB is induced as part of the Zur-mediated zinc starvation response, a mode of regulation not previously reported for cell wall lytic enzymes. In vivo, ShyB alone was sufficient to sustain cell growth in low-zinc environments. In vitro, ShyB retained its d,d-endopeptidase activity against purified sacculi in the presence of the metal chelator EDTA at concentrations that inhibit ShyA and ShyC. This insensitivity to metal chelation is likely what enables ShyB to substitute for other EPs during zinc starvation. Our survey of transcriptomic data from diverse bacteria identified other candidate Zur-regulated EPs, suggesting that this adaptation to zinc starvation is employed by other Gram-negative bacteria.

σI from Bacillus subtilis: Impact on Gene Expression and Characterization of σI-Dependent Transcription That Requires New Types of Promoters with Extended -35 and -10 Elements

The σI sigma factor from Bacillus subtilis is a σ factor associated with RNA polymerase (RNAP) that was previously implicated in adaptation of the cell to elevated temperature. Here, we provide a comprehensive characterization of this transcriptional regulator. By transcriptome sequencing (RNA-seq) of wild-type (wt) and σI-null strains at 37°C and 52°C, we identified ∼130 genes affected by the absence of σI Further analysis revealed that the majority of these genes were affected indirectly by σI The σI regulon, i.e., the genes directly regulated by σI, consists of 16 genes, of which eight (the dhb and yku operons) are involved in iron metabolism. The involvement of σI in iron metabolism was confirmed phenotypically. Next, we set up an in vitro transcription system and defined and experimentally validated the promoter sequence logo that, in addition to -35 and -10 regions, also contains extended -35 and -10 motifs. Thus, σI-dependent promoters are relatively information rich in comparison with most other promoters. In summary, this study supplies information about the least-explored σ factor from the industrially important model organism B. subtilisIMPORTANCE In bacteria, σ factors are essential for transcription initiation. Knowledge about their regulons (i.e., genes transcribed from promoters dependent on these σ factors) is the key for understanding how bacteria cope with the changing environment and could be instrumental for biotechnologically motivated rewiring of gene expression. Here, we characterize the σI regulon from the industrially important model Gram-positive bacterium Bacillus subtilis We reveal that σI affects expression of ∼130 genes, of which 16 are directly regulated by σI, including genes encoding proteins involved in iron homeostasis. Detailed analysis of promoter elements then identifies unique sequences important for σI-dependent transcription. This study thus provides a comprehensive view on this underexplored component of the B. subtilis transcription machinery.

Recent advances in understanding how rod-like bacteria stably maintain their cell shapes

Cell shape and cell volume are important for many bacterial functions. In recent years, we have seen a range of experimental and theoretical work that led to a better understanding of the determinants of cell shape and size. The roles of different molecular machineries for cell-wall expansion have been detailed and partially redefined, mechanical forces have been shown to influence cell shape, and new connections between metabolism and cell shape have been proposed. Yet the fundamental determinants of the different cellular dimensions remain to be identified. Here, we highlight some of the recent developments and focus on the determinants of rod-like cell shape and size in the well-studied model organisms Escherichia coli and Bacillus subtilis.

Lysozyme Counteracts β-Lactam Antibiotics by Promoting the Emergence of L-Form Bacteria

β-lactam antibiotics inhibit bacterial cell wall assembly and, under classical microbiological culture conditions that are generally hypotonic, induce explosive cell death. Here, we show that under more physiological, osmoprotective conditions, for various Gram-positive bacteria, lysis is delayed or abolished, apparently because inhibition of class A penicillin-binding protein leads to a block in autolytic activity. Although these cells still then die by other mechanisms, exogenous lytic enzymes, such as lysozyme, can rescue viability by enabling the escape of cell wall-deficient "L-form" bacteria. This protective L-form conversion was also observed in macrophages and in an animal model, presumably due to the production of host lytic activities, including lysozyme. Our results demonstrate the potential for L-form switching in the host environment and highlight the unexpected effects of innate immune effectors, such as lysozyme, on antibiotic activity. Unlike previously described dormant persisters, L-forms can continue to proliferate in the presence of antibiotic.

Essentiality of WalRK for growth in Bacillus subtilis and its role during heat stress

WalRK is an essential two-component signal transduction system that plays a central role in coordinating cell wall synthesis and cell growth in Bacillus subtilis. However, the physiological role of WalRK and its essentiality for growth have not been elucidated. We investigated the behaviour of WalRK during heat stress and its essentiality for cell proliferation. We determined that the inactivation of the walHI genes which encode the negative modulator of WalK, resulted in growth defects and eventual cell lysis at high temperatures. Screening of suppressor mutations revealed that the inactivation of LytE, an dl-endopeptidase, restored the growth of the ΔwalHI mutant at high temperatures. Suppressor mutations that reduced heat induction arising from the walRK regulon were also mapped to the walK ORF. Therefore, we hypothesized that overactivation of LytE affects the phenotype of the ΔwalHI mutant. This hypothesis was corroborated by the overexpression of the negative regulator of LytE, IseA and PdaC, which rescued the growth of the ΔwalHI mutant at high temperatures. Elucidating the cause of the temperature sensitivity of the ΔwalHI mutant could explain the essentiality of WalRK. We proved that the constitutive expression of lytE or cwlO using a synthetic promoter uncouples these expressions from WalRK, and renders WalRK nonessential in the pdaC and iseA mutant backgrounds. We propose that the essentiality of WalRK is derived from the coordination of cell wall metabolism with cell growth by regulating dl-endopeptidase activity under various growth conditions.