A cytoplasmic peptidoglycan amidase homologue controls mycobacterial cell wall synthesis

MmpL3, Wag31 and PlrA are involved in coordinating polar growth with peptidoglycan metabolism and nutrient availability

Cell growth in mycobacteria involves cell wall expansion that is restricted to the cell poles. The DivIVA homolog Wag31 is required for this process, but the molecular mechanism and protein partners of Wag31 have not been described. In this study of Mycobacterium smegmatis, we identify a connection between wag31 and trehalose monomycolate (TMM) transporter mmpl3 in a suppressor screen, and show that Wag31 and polar regulator PlrA are required for MmpL3's polar localization. In addition, the localization of PlrA and MmpL3 are responsive to nutrient and energy deprivation and inhibition of peptidoglycan metabolism. We show that inhibition of MmpL3 causes delocalized cell wall metabolism, but does not delocalize MmpL3 itself. We found that cells with an MmpL3 C-terminal truncation, which is defective for localization, have only minor defects in polar growth, but are impaired in their ability to downregulate cell wall metabolism under stress. Our work suggests that, in addition to its established function in TMM transport, MmpL3 has a second function in regulating global cell wall metabolism in response to stress. Our data are consistent with a model in which the presence of TMMs in the periplasm stimulates polar elongation, and in which the connection between Wag31, PlrA and the C-terminus of MmpL3 is involved in detecting and responding to stress in order to coordinate synthesis of the different layers of the mycobacterial cell wall in changing conditions.

MSMEG_0311 is a conserved essential polar protein involved in mycobacterium cell wall metabolism

Cell wall synthesis and cell division are two closely linked pathways in a bacterial cell which distinctly influence the growth and survival of a bacterium. This requires an appreciable coordination between the two processes, more so, in case of mycobacteria with an intricate multi-layered cell wall structure. In this study, we investigated a conserved gene cluster using CRISPR-Cas12 based gene silencing technology to show that knockdown of most of the genes in this cluster leads to growth defects. Investigating conserved genes is important as they likely perform vital cellular functions and the functional insights on such genes can be extended to other mycobacterial species. We characterised one of the genes in the locus, MSMEG_0311. The repression of this gene not only imparts severe growth defect but also changes colony morphology. We demonstrate that the protein preferentially localises to the polar region and investigate its influence on the polar growth of the bacillus. A combination of permeability and drug susceptibility assay strongly suggests a cell wall associated function of this gene which is also corroborated by transcriptomic analysis of the knockdown where a number of cell wall associated genes, particularly iniA and sigF regulon get altered. Considering the gene is highly conserved across mycobacterial species and appears to be essential for growth, it may serve as a potential drug target.

Secrets of getting started: Regulation of the first committed step of peptidoglycan synthesis by protein phosphorylation in Enterococcus and other Gram-positive bacteria

Regulation of the first committed step of peptidoglycan precursor synthesis by MurA-enzyme homologs has recently taken center stage in many different bacteria. In different low-GC Gram-positive bacteria, regulation of this step has been shown to be regulated by phosphorylation of homologs of the IreB/ReoM regulatory protein by PASTA-domain Ser/Thr-protein kinases. In this issue, Mascari, Little, and Kristich determine this regulatory pathway and its links to resistance to cephalosporin β-lactam antibiotics in the major human pathogen, Enterococcus faecalis (Efa). Unbiased genetic selections identified MurAA (MurA-family homolog) as the downstream target of IreB regulation in the absence of the IreK Ser/Thr-protein kinase. Physiological and biochemical approaches, including determination of MICs to ceftriaxone, Western blotting of MurAA cellular amounts, isotope incorporation into peptidoglycan sacculi, and thermal-shift binding assays of purified proteins, demonstrated that unphosphorylated IreB, together with proteins MurAB (MurZ-family homolog), and ReoY(Efa) negatively regulate MurAA stability and cellular amount by the ClpCP protease. Importantly, this paper supports the idea that ceftriaxone stimulates phosphorylation of IreB, which leads to increased cellular MurAA amount and precursor pathway flux required for E. faecalis cephalosporin resistance. Overall, findings in this paper significantly contribute to understanding variations of this central regulatory pathway in other low-GC Gram-positive bacteria.

Negative regulation of MurZ and MurA underlies the essentiality of GpsB- and StkP-mediated protein phosphorylation in Streptococcus pneumoniae D39

GpsB links peptidoglycan synthases to other proteins that determine the shape of the respiratory pathogen Streptococcus pneumoniae (pneumococcus; Spn) and other low-GC Gram-positive bacteria. GpsB is also required for phosphorylation of proteins by the essential StkP(Spn) Ser/Thr protein kinase. Here we report three classes of frequently arising chromosomal duplications (≈21-176 genes) containing murZ (MurZ-family homolog of MurA) or murA that suppress ΔgpsB or ΔstkP. These duplications arose from three different repeated sequences and demonstrate the facility of pneumococcus to modulate gene dosage of numerous genes. Overproduction of MurZ or MurA alone or overproduction of MurZ caused by ΔkhpAB mutations suppressed ΔgpsB or ΔstkP phenotypes to varying extents. ΔgpsB and ΔstkP were also suppressed by MurZ amino-acid changes distant from the active site, including one in commonly studied laboratory strains, and by truncation or deletion of the homolog of IreB(ReoM). Unlike in other Gram-positive bacteria, MurZ is predominant to MurA in pneumococcal cells. However, ΔgpsB and ΔstkP were not suppressed by ΔclpCP, which did not alter MurZ or MurA amounts. These results support a model in which regulation of MurZ and MurA activity, likely by IreB(Spn), is the only essential requirement for StkP-mediated protein phosphorylation in exponentially growing D39 pneumococcal cells.

MtrA modulates Mycobacterium tuberculosis cell division in host microenvironments to mediate intrinsic resistance and drug tolerance

The success of Mycobacterium tuberculosis (Mtb) is largely attributed to its ability to physiologically adapt and withstand diverse localized stresses within host microenvironments. Here, we present a data-driven model (EGRIN 2.0) that captures the dynamic interplay of environmental cues and genome-encoded regulatory programs in Mtb. Analysis of EGRIN 2.0 shows how modulation of the MtrAB two-component signaling system tunes Mtb growth in response to related host microenvironmental cues. Disruption of MtrAB by tunable CRISPR interference confirms that the signaling system regulates multiple peptidoglycan hydrolases, among other targets, that are important for cell division. Further, MtrA decreases the effectiveness of antibiotics by mechanisms of both intrinsic resistance and drug tolerance. Together, the model-enabled dissection of complex MtrA regulation highlights its importance as a drug target and illustrates how EGRIN 2.0 facilitates discovery and mechanistic characterization of Mtb adaptation to specific host microenvironments within the host.

Chromosomal Duplications of MurZ (MurA2) or MurA (MurA1), Amino Acid Substitutions in MurZ (MurA2), and Absence of KhpAB Obviate the Requirement for Protein Phosphorylation in Streptococcus pneumoniae D39

GpsB links peptidoglycan synthases to other proteins that determine the shape of the respiratory pathogen Streptococcus pneumoniae (pneumococcus; Spn ) and other low-GC Gram-positive bacteria. GpsB is also required for phosphorylation of proteins by the essential StkP( Spn ) Ser/Thr protein kinase. Here we report three classes of frequently arising chromosomal duplications (≈21-176 genes) containing murZ (MurZ-family homolog of MurA) or murA that suppress Δ gpsB or Δ stkP . These duplications arose from three different repeated sequences and demonstrate the facility of pneumococcus to modulate gene dosage of numerous genes. Overproduction of MurZ or MurA alone or overexpression of MurZ caused by Δ khpAB mutations suppressed Δ gpsB or Δ stkP phenotypes to varying extents. Δ gpsB and Δ stkP were also suppressed by MurZ amino-acid changes distant from the active site, including one in commonly studied laboratory strains, and by truncation or deletion of the homolog of IreB(ReoM). Unlike in other Gram-positive bacteria, MurZ is predominant to MurA in pneumococcal cells. However, Δ gpsB and Δ stkP were not suppressed by Δ clpCP , which did not alter MurZ or MurA amounts. These results support a model in which regulation of MurZ and MurA activity, likely by IreB( Spn ), is the only essential requirement for protein phosphorylation in exponentially growing D39 pneumococcal cells.

Identification and characterization of a novel lytic peptidoglycan transglycosylase (MltC) in Shigella dysenteriae

Shigellosis remains a worldwide health problem due to the lack of vaccines and the emergence of antibiotic-resistant strains. Shigella (S.) dysenteriae has rigid peptidoglycan (PG), and its tight regulation of biosynthesis and remodeling is essential for bacterial integrity. Lytic transglycosylases are highly conserved PG autolysins in bacteria that play essential roles in bacterial growth. However, their precise functions are obscure. We aimed to identify, clone, and express MltC, a unique autolysin in Escherichia (E.) coli C41 strain. The purification of recombinant MltC protein was performed using affinity chromatography and size-exclusion chromatography methods. The PG enzymatic activity of MltC was investigated using Zymogram and Fluorescein isothiocyanate (FITC)-labeled PG assays. Also, we aimed to detect its localization in bacterial fractions (cytoplasm and membrane) by western blot using specific polyclonal anti-MltC antibodies and its probable partners using immunoprecipitation and mass spectrometry applications. Purified MltC showed autolysin activity. Native MltC showed various locations in S. dysenteriae cells during different growth phases. In the Lag and early stationary phases, MltC was not found in cytoplasm and membrane fractions. However, it was detected in cytoplasm and membrane fractions during the exponential phase. In the late stationary phase, MltC was expressed in the membrane fraction only. Different candidate protein partners of MltC were identified that could be essential for bacterial growth and pathogenicity. This is the first study to suggest that MltC is indeed autolysin and could be a new drug target for the treatment of shigellosis by understanding its biological functions.

Cyclic AMP is a critical mediator of intrinsic drug resistance and fatty acid metabolism in M. tuberculosis

Cyclic AMP (cAMP) is a ubiquitous second messenger that transduces signals from cellular receptors to downstream effectors. Mycobacterium tuberculosis (Mtb), the etiological agent of tuberculosis, devotes a considerable amount of coding capacity to produce, sense, and degrade cAMP. Despite this fact, our understanding of how cAMP regulates Mtb physiology remains limited. Here, we took a genetic approach to investigate the function of the sole essential adenylate cyclase in Mtb H37Rv, Rv3645. We found that a lack of rv3645 resulted in increased sensitivity to numerous antibiotics by a mechanism independent of substantial increases in envelope permeability. We made the unexpected observation that rv3645 is conditionally essential for Mtb growth only in the presence of long-chain fatty acids, a host-relevant carbon source. A suppressor screen further identified mutations in the atypical cAMP phosphodiesterase rv1339 that suppress both fatty acid and drug sensitivity phenotypes in strains lacking rv3645. Using mass spectrometry, we found that Rv3645 is the dominant source of cAMP under standard laboratory growth conditions, that cAMP production is the essential function of Rv3645 in the presence of long-chain fatty acids, and that reduced cAMP levels result in increased long-chain fatty acid uptake and metabolism and increased antibiotic susceptibility. Our work defines rv3645 and cAMP as central mediators of intrinsic multidrug resistance and fatty acid metabolism in Mtb and highlights the potential utility of small molecule modulators of cAMP signaling.

Arginine methylation sites on SepIVA help balance elongation and septation in Mycobacterium smegmatis

The growth of mycobacterial cells requires successful coordination between elongation and septation. However, it is not clear which factors mediate this coordination. Here, we studied the function and post-translational modification of an essential division factor, SepIVA, in Mycobacterium smegmatis. We find that SepIVA is arginine methylated, and that alteration of its methylation sites affects both septation and polar elongation of Msmeg. Furthermore, we show that SepIVA regulates the localization of MurG and that this regulation may impact polar elongation. Finally, we map SepIVA's two regulatory functions to different ends of the protein: the N-terminus regulates elongation while the C-terminus regulates division. These results establish SepIVA as a regulator of both elongation and division and characterize a physiological role for protein arginine methylation sites for the first time in mycobacteria.

The Bacterial MtrAB Two-Component System Regulates the Cell Wall Homeostasis Responding to Environmental Alkaline Stress

Throughout the course of evolution, bacteria have developed signal transduction tools such as two-component systems (TCSs) to meet their demands to thrive even under the most challenging environmental conditions. One TCS called MtrAB is commonly found in Actinobacteria and is implicated in cell wall metabolism, osmoprotection, cell proliferation, antigen secretion, and biosynthesis of secondary metabolites. However, precisely how the MtrAB TCS regulates the bacterial responses to external environments remains unclear. Here, we report that the MtrAB TCS regulates the cell envelope response of alkali-tolerant bacterium Dietzia sp. strain DQ12-45-1b to extreme alkaline stimuli. We found that under alkaline conditions, an mtrAB mutant exhibited both reduced growth and abnormal morphology compared to the wild-type strain. Electrophoretic mobility shift assay analysis showed that MtrA binds the promoter of the mraZ gene critical for cell wall homeostasis, suggesting that MtrA directly controls transcription of this regulator. In conclusion, our findings demonstrate that MtrAB TCS is involved in controlling the bacterial response to alkaline stimuli by regulating the expression of the cell wall homeostasis regulator MraZ in Dietzia sp. DQ12-45-1b, providing novel details critical for a mechanistic understanding of how cell wall homeostasis is controlled. IMPORTANCE Microorganisms can be found in most extreme environments, and they have to adapt to a wide range of environmental stresses. The two-component systems (TCSs) found in bacteria detect environmental stimuli and regulate physiological pathways for survival. The MtrAB TCS conserved in Corynebacterineae is critical for maintaining the metabolism of the cell wall components that protects bacteria from diverse environmental stresses. However, how the MtrAB TCS regulates cell wall homeostasis and adaptation under stress conditions is unclear. Here, we report that the MtrAB TCS in Dietzia sp. DQ12-45-1b plays a critical role in alkaline resistance by modulating the cell wall homeostasis through the MtrAB-MraZ pathway. Thus, our work provides a novel regulatory pathway used by bacteria for adaptation and survival under extreme alkaline stresses.

The Role of Phosphorylation and Acylation in the Regulation of Drug Resistance in Mycobacterium tuberculosis

Tuberculosis is a chronic and lethal infectious disease caused by Mycobacterium tuberculosis. In previous decades, most studies in this area focused on the pathogenesis and drug targets for disease treatments. However, the emergence of drug-resistant strains has increased the difficulty of clinical trials over time. Now, more post-translational modified proteins in Mycobacterium tuberculosis have been discovered. Evidence suggests that these proteins have the ability to influence tuberculosis drug resistance. Hence, this paper systematically summarizes updated research on the impacts of protein acylation and phosphorylation on the acquisition of drug resistance in Mycobacterium tuberculosis through acylation and phosphorylation protein regulating processes. This provides us with a better understanding of the mechanism of antituberculosis drugs and may contribute to a reduction the harm that tuberculosis brings to society, as well as aiding in the discovery of new drug targets and therapeutic regimen adjustments in the future.

Mycobacterial serine/threonine phosphatase PstP is phosphoregulated and localized to mediate control of cell wall metabolism

The mycobacterial cell wall is profoundly regulated in response to environmental stresses, and this regulation contributes to antibiotic tolerance. The reversible phosphorylation of different cell wall regulatory proteins is a major mechanism of cell wall regulation. Eleven serine/threonine protein kinases phosphorylate many critical cell wall-related proteins in mycobacteria. PstP is the sole serine/ threonine phosphatase, but few proteins have been verified as PstP substrates. PstP is itself phosphorylated, but the role of its phosphorylation in regulating its activity has been unclear. In this study, we aim to discover novel substrates of PstP in Mycobacterium tuberculosis (Mtb). We show in vitro that PstP dephosphorylates two regulators of peptidoglycan in Mtb, FhaA, and Wag31. We also show that a phosphomimetic mutation of T137 on PstP negatively regulates its catalytic activity against the cell wall regulators FhaA, Wag31, CwlM, PknB, and PknA, and that the corresponding mutation in Mycobacterium smegmatis causes misregulation of peptidoglycan in vivo. We show that PstP is localized to the septum, which likely restricts its access to certain substrates. These findings on the regulation of PstP provide insight into the control of cell wall metabolism in mycobacteria.

Transposon mutagenesis in Mycobacterium abscessus identifies an essential penicillin-binding protein involved in septal peptidoglycan synthesis and antibiotic sensitivity

Mycobacterium abscessus (Mab) is a rapidly growing non-tuberculous mycobacterium (NTM) that causes a wide range of infections. Treatment of Mab infections is difficult because the bacterium is intrinsically resistant to many classes of antibiotics. Developing new and effective treatments against Mab requires a better understanding of the unique vulnerabilities that can be targeted for future drug development. To achieve this, we identified essential genes in Mab by conducting transposon sequencing (TnSeq) on the reference Mab strain ATCC 19977. We generated ~51,000 unique transposon mutants and used this high-density library to identify 362 essential genes for in vitro growth. To investigate species-specific vulnerabilities in Mab, we further characterized MAB_3167c, a predicted penicillin-binding protein and hypothetical lipoprotein (PBP-lipo) that is essential in Mab and non-essential in Mycobacterium tuberculosis (Mtb). We found that PBP-lipo primarily localizes to the subpolar region and later to the septum as cells prepare to divide. Depletion of Mab PBP-lipo causes cells to elongate, develop ectopic branches, and form multiple septa. Knockdown of PBP-lipo along with PbpB, DacB1, and a carboxypeptidase, MAB_0519 lead to synergistic growth arrest. In contrast, these genetic interactions were absent in the Mtb model organism, Mycobacterium smegmatis, indicating that the PBP-lipo homologs in the two species exist in distinct genetic networks. Finally, repressing PBP-lipo sensitized the reference strain and 11 Mab clinical isolates to several classes of antibiotics, including the β-lactams, ampicillin, and amoxicillin by greater than 128-fold. Altogether, this study presents PBP-lipo as a key enzyme to study Mab-specific processes in cell wall synthesis and importantly positions PBP-lipo as an attractive drug target to treat Mab infections.

CRISPR Inhibition of Essential Peptidoglycan Biosynthesis Genes in Mycobacterium abscessus and Its Impact on β-Lactam Susceptibility

We utilized a CRISPR interference (CRISPRi) assay to control the gene expressions of two predicted essential peptidoglycan biosynthesis genes, pbpB and cwIM, in Mycobacterium abscessus and to evaluate their contribution to β-lactam susceptibility. Our results showed that CRISPR inhibition of each gene led to a significant 3-log₁₀ reduction in CFU in the presence of imipenem but not for cefoxitin. These results demonstrate that CRISPRi provides an experimental approach to study drug/target interactions in M. abscessus.

Recent advances in bacterial signaling by serine/threonine protein kinases

It has been nearly three decades since the discovery of the first bacterial serine/threonine protein kinase (STPK). Since then, a blend of technological advances has led to the characterization of a multitude of STPKs and phosphorylation substrates in several bacterial species that finely regulate intricate signaling cascades. Years of intense research from several laboratories have demonstrated unexpected roles for serine/threonine phosphorylation, regulating not only bacterial growth and cell division but also antibiotic persistence, virulence and infection, metabolism, chromosomal biology, and cellular differentiation. This review aims to provide an account of the most recent and significant developments in this up and growing field in microbiology.

Characterisation of a putative M23-domain containing protein in Mycobacterium tuberculosis

Mycobacterium tuberculosis, the causative agent of tuberculosis remains a global health concern, further compounded by the high rates of HIV-TB co-infection and emergence of multi- and extensive drug resistant TB, all of which have hampered efforts to eradicate this disease. As a result, novel anti-tubercular interventions are urgently required, with the peptidoglycan component of the M. tuberculosis cell wall emerging as an attractive drug target. Peptidoglycan M23 endopeptidases can function as active cell wall hydrolases or degenerate activators of hydrolases in a variety of bacteria, contributing to important processes such as bacterial growth, division and virulence. Herein, we investigate the function of the Rv0950-encoded putative M23 endopeptidase in M. tuberculosis. In silico analysis revealed that this protein is conserved in mycobacteria, with a zinc-binding catalytic site predictive of hydrolytic activity. Transcript analysis indicated that expression of Rv0950c was elevated during lag and log phases of growth and reduced in stationary phase. Deletion of Rv0950c yielded no defects in growth, colony morphology, antibiotic susceptibility or intracellular survival but caused a reduction in cell length. Staining with a monopeptide-derived fluorescent D-amino acid, which spatially reports on sites of active PG biosynthesis or repair, revealed an overall reduction in uptake of the probe in ΔRv0950c. When stained with a dipeptide probe in the presence of cell wall damaging agents, the ΔRv0950c mutant displayed reduced sidewall labelling. As bacterial peptidoglycan metabolism is important for survival and pathogenesis, the role of Rv0950c and other putative M23 endopeptidases in M. tuberculosis should be explored further.

PASTA kinase-dependent control of peptidoglycan synthesis via ReoM is required for cell wall stress responses, cytosolic survival, and virulence in Listeria monocytogenes

Pathogenic bacteria rely on protein phosphorylation to adapt quickly to stress, including that imposed by the host during infection. Penicillin-binding protein and serine/threonine-associated (PASTA) kinases are signal transduction systems that sense cell wall integrity and modulate multiple facets of bacterial physiology in response to cell envelope stress. The PASTA kinase in the cytosolic pathogen Listeria monocytogenes, PrkA, is required for cell wall stress responses, cytosolic survival, and virulence, yet its substrates and downstream signaling pathways remain incompletely defined. We combined orthogonal phosphoproteomic and genetic analyses in the presence of a β-lactam antibiotic to define PrkA phosphotargets and pathways modulated by PrkA. These analyses synergistically highlighted ReoM, which was recently identified as a PrkA target that influences peptidoglycan (PG) synthesis, as an important phosphosubstrate during cell wall stress. We find that deletion of reoM restores cell wall stress sensitivities and cytosolic survival defects of a ΔprkA mutant to nearly wild-type levels. While a ΔprkA mutant is defective for PG synthesis during cell wall stress, a double ΔreoM ΔprkA mutant synthesizes PG at rates similar to wild type. In a mouse model of systemic listeriosis, deletion of reoM in a ΔprkA background almost fully restored virulence to wild-type levels. However, loss of reoM alone also resulted in attenuated virulence, suggesting ReoM is critical at some points during pathogenesis. Finally, we demonstrate that the PASTA kinase/ReoM cell wall stress response pathway is conserved in a related pathogen, methicillin-resistant Staphylococcus aureus. Taken together, our phosphoproteomic analysis provides a comprehensive overview of the PASTA kinase targets of an important model pathogen and suggests that a critical role of PrkA in vivo is modulating PG synthesis through regulation of ReoM to facilitate cytosolic survival and virulence.

Many antibiotics target bacterial cell wall biosynthesis, justifying continued study of this process and the ways bacteria respond to cell wall insults during infection. Penicillin-binding protein and serine/threonine-associated (PASTA) kinases are master regulators of cell wall stress responses in bacteria and are conserved in several major pathogens, including Listeria monocytogenes, Staphylococcus aureus, and Mycobacterium tuberculosis. We previously showed that the PASTA kinase in L. monocytogenes, PrkA, is essential for the response to cell wall stress and for virulence. In this work, we combined proteomic and genetic approaches to identify PrkA substrates in L. monocytogenes. We show that regulation of one candidate from both screens, ReoM, increases synthesis of the cell wall component peptidoglycan and that this regulation is required for pathogenesis. We also demonstrate that the PASTA kinase-ReoM pathway regulates cell wall stress responses in another significant pathogen, methicillin-resistant S. aureus. Additionally, we uncover a PrkA-independent role for ReoM in vivo in L. monocytogenes, suggesting a need for nuanced modulation of peptidoglycan synthesis during infection. Cumulatively, this study provides new insight into how bacterial pathogens control cell wall synthesis during infection.

The surprising structural and mechanistic dichotomy of membrane-associated phosphoglycosyl transferases

Phosphoglycosyl transferases (PGTs) play a pivotal role at the inception of complex glycoconjugate biosynthesis pathways across all domains of life. PGTs promote the first membrane-committed step in the en bloc biosynthetic strategy by catalyzing the transfer of a phospho-sugar from a nucleoside diphospho-sugar to a membrane-resident polyprenol phosphate. Studies on the PGTs have been hampered because they are integral membrane proteins, and often prove to be recalcitrant to expression, purification and analysis. However, in recent years exciting new information has been derived on the structures and the mechanisms of PGTs, revealing the existence of two unique superfamilies of PGT enzymes that enact catalysis at the membrane interface. Genome neighborhood analysis shows that these superfamilies, the polytopic PGT (polyPGT) and monotopic PGT (monoPGT), may initiate different pathways within the same organism. Moreover, the same fundamental two-substrate reaction is enacted through two different chemical mechanisms with distinct modes of catalysis. This review highlights the structural and mechanistic divergence between the PGT enzyme superfamilies and how this is reflected in differences in regulation in their varied glycoconjugate biosynthesis pathways.

The Mur Enzymes Chink in the Armour of Mycobacterium tuberculosis cell wall

TUBERCULOSIS: (TB) transmitted by Mycobacterium tuberculosis (Mtb) is one of the top 10 causes of death globally. Currently, the widespread occurrence of resistance toward Mtb strains is becoming a significant concern to public health. This scenario exaggerated the need for the discovery of novel targets and their inhibitors. Targeting the "Mtb cell wall peptidoglycan synthesis" is an attractive strategy to overcome drug resistance. Mur enzymes (MurA-MurF) play essential roles in the peptidoglycan synthesis by catalyzing the ligation of key amino acid residues to the stem peptide. These enzymes are unique and confined to the eubacteria and are absent in humans, representing potential targets for anti-tubercular drug discovery. Mtb Mur ligases with the same catalytic mechanism share conserved amino acid regions and structural features that can conceivably exploit for the designing of the inhibitors, which can simultaneously target more than one isoforms (MurC-MurF) of the enzyme. In light of these findings in the current review, we have discussed the recent advances in medicinal chemistry of Mtb Mur enzymes (MurA-MurF) and their inhibitors, offering attractive multi-targeted strategies to combat the problem of drug-resistant in M. tuberculosis.

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.

Deletion of N-acetylmuramyl-L-alanine amidases alters the host immune response to Mycobacterium tuberculosis infection

Peptidoglycan (PG), a heteropolysaccharide component of the mycobacterial cell wall can be shed during tuberculosis infection with immunomodulatory consequences. As such, changes in PG structure are expected to have important implications on disease progression and host responses during infection with Mycobacterium tuberculosis. Mycobacterial amidases have important roles in remodeling of PG during cell division and are implicated in susceptibility to antibiotics. However, their role in modulating host immunity remains unknown. We assessed the bacterial burden and host immune responses to M. tuberculosis mutants defective for either one of two PG N-acetylmuramyl-L-alanine amidases, Ami1 and Ami4, in bone marrow-derived macrophages (BMDM) and C57BL/6 mice. In infected BMDM, the single deletion of both genes resulted in increased proinflammatory cytokine responses. In mice, infection with the Δami1 mutant led to differential induction of pro-inflammatory cytokines and chemokines, decreased cellular recruitment and reduced lung pathology during the acute phase of the infection. While increased proinflammatory cytokines production was observed in BMDM infected with the Δami4 mutant, these effects did not prevail in mice. Infection using the Δami1 and Δami4 Mtb mutants showed that these genes are dispensable for intracellular mycobacterial growth in macrophages and mycobacterial burden in mice. These findings suggest that both Ami1 and Ami4 in M. tuberculosis are not essential for mycobacterial growth within the host. In summary, we show that amidases are important for modulating host immunity during Mtb infection in murine macrophages and mice.

Resource sharing between central metabolism and cell envelope synthesis

Synthesis of the bacterial cell envelope requires a regulated partitioning of resources from central metabolism. Here, we consider the key metabolic junctions that provide the precursors needed to assemble the cell envelope. Peptidoglycan synthesis requires redirection of a glycolytic intermediate, fructose-6-phosphate, into aminosugar biosynthesis by the highly regulated branchpoint enzyme GlmS. MurA directs the downstream product, UDP-GlcNAc, specifically into peptidoglycan synthesis. Other shared resources required for cell envelope synthesis include the isoprenoid carrier lipid undecaprenyl phosphate and amino acids required for peptidoglycan cross-bridges. Assembly of the envelope requires a sharing of limited resources between competing cellular pathways and may additionally benefit from scavenging of metabolites released from neighboring cells or the formation of symbiotic relationships with a host.

Phosphorylation on PstP Regulates Cell Wall Metabolism and Antibiotic Tolerance in Mycobacterium smegmatis

Mycobacterium tuberculosis and its relatives, like many bacteria, have dynamic cell walls that respond to environmental stresses. Modulation of cell wall metabolism in stress is thought to be responsible for decreased permeability and increased tolerance to antibiotics. The signaling systems that control cell wall metabolism under stress, however, are poorly understood. Here, we examine the cell wall regulatory function of a key cell wall regulator, the serine/threonine phosphatase PstP, in the model organism Mycobacterium smegmatis We show that the peptidoglycan regulator CwlM is a substrate of PstP. We find that a phosphomimetic mutation, pstP T171E, slows growth, misregulates both mycolic acid and peptidoglycan metabolism in different conditions, and interferes with antibiotic tolerance. These data suggest that phosphorylation on PstP affects its activity against various substrates and is important in the transition between growth and stasis.IMPORTANCE Regulation of cell wall assembly is essential for bacterial survival and contributes to pathogenesis and antibiotic tolerance in mycobacteria, including pathogens such as Mycobacterium tuberculosis However, little is known about how the cell wall is regulated in stress. We describe a pathway of cell wall modulation in Mycobacterium smegmatis through the only essential Ser/Thr phosphatase, PstP. We showed that phosphorylation on PstP is important in regulating peptidoglycan metabolism in the transition to stasis and mycolic acid metabolism in growth. This regulation also affects antibiotic tolerance in growth and stasis. This work helps us to better understand the phosphorylation-mediated cell wall regulation circuitry in Mycobacteria.

Trehalose Recycling Promotes Energy-Efficient Biosynthesis of the Mycobacterial Cell Envelope

The mycomembrane layer of the mycobacterial cell envelope is a barrier to environmental, immune, and antibiotic insults. There is considerable evidence of mycomembrane plasticity during infection and in response to host-mimicking stresses.

The mycomembrane layer of the mycobacterial cell envelope is a barrier to environmental, immune, and antibiotic insults. There is considerable evidence of mycomembrane plasticity during infection and in response to host-mimicking stresses. Since mycobacteria are resource and energy limited under these conditions, it is likely that remodeling has distinct requirements from those of the well-characterized biosynthetic program that operates during unrestricted growth. Unexpectedly, we found that mycomembrane remodeling in nutrient-starved, nonreplicating mycobacteria includes synthesis in addition to turnover. Mycomembrane synthesis under these conditions occurs along the cell periphery, in contrast to the polar assembly of actively growing cells, and both liberates and relies on the nonmammalian disaccharide trehalose. In the absence of trehalose recycling, de novo trehalose synthesis fuels mycomembrane remodeling. However, mycobacteria experience ATP depletion, enhanced respiration, and redox stress, hallmarks of futile cycling and the collateral dysfunction elicited by some bactericidal antibiotics. Inefficient energy metabolism compromises the survival of trehalose recycling mutants in macrophages. Our data suggest that trehalose recycling alleviates the energetic burden of mycomembrane remodeling under stress. Cell envelope recycling pathways are emerging targets for sensitizing resource-limited bacterial pathogens to host and antibiotic pressure.

Cell Surface Biosynthesis and Remodeling Pathways in Mycobacteria Reveal New Drug Targets

Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis (TB), remains the leading cause of death from an infectious bacterium and is responsible for 1.8 million deaths annually. The emergence of drug resistance, together with the need for a shorter more effective regimen, has prompted the drive to identify novel therapeutics with the bacterial cell surface emerging as a tractable area for drug development. Mtb assembles a unique, waxy, and complex cell envelope comprised of the mycolyl-arabinogalactan-peptidoglycan complex and an outer capsule like layer, which are collectively essential for growth and pathogenicity while serving as an inherent barrier against antibiotics. A detailed understanding of the biosynthetic pathways required to assemble the polymers that comprise the cell surface will enable the identification of novel drug targets as these structures provide a diversity of biochemical reactions that can be targeted. Herein, we provide an overview of recently described mycobacterial cell wall targeting compounds, novel drug combinations and their modes of action. We anticipate that this summary will enable prioritization of the best pathways to target and triage of the most promising molecules to progress for clinical assessment.

Functional Characterization of the N-Acetylmuramyl-l-Alanine Amidase, Ami1, from Mycobacterium abscessus

Peptidoglycan (PG) is made of a polymer of disaccharides organized as a three-dimensional mesh-like network connected together by peptidic cross-links. PG is a dynamic structure that is essential for resistance to environmental stressors. Remodeling of PG occurs throughout the bacterial life cycle, particularly during bacterial division and separation into daughter cells. Numerous autolysins with various substrate specificities participate in PG remodeling. Expression of these enzymes must be tightly regulated, as an excess of hydrolytic activity can be detrimental for the bacteria. In non-tuberculous mycobacteria such as Mycobacterium abscessus, the function of PG-modifying enzymes has been poorly investigated. In this study, we characterized the function of the PG amidase, Ami1 from M. abscessus. An ami1 deletion mutant was generated and the phenotypes of the mutant were evaluated with respect to susceptibility to antibiotics and virulence in human macrophages and zebrafish. The capacity of purified Ami1 to hydrolyze muramyl-dipeptide was demonstrated in vitro. In addition, the screening of a 9200 compounds library led to the selection of three compounds inhibiting Ami1 in vitro. We also report the structural characterization of Ami1 which, combined with in silico docking studies, allows us to propose a mode of action for these inhibitors.

Role of post-translational modifications in the acquisition of drug resistance in Mycobacterium tuberculosis

Tuberculosis (TB) is one of the primary causes of deaths due to infectious diseases. The current TB regimen is long and complex, failing of which leads to relapse and/or the emergence of drug resistance. There is a critical need to understand the mechanisms of resistance development. With increasing drug pressure, Mycobacterium tuberculosis (Mtb) activates various pathways to counter drug-related toxicity. Signaling modules steer the evolution of Mtb to a variant that can survive, persist, adapt, and emerge as a form that is resistant to one or more drugs. Recent studies reveal that about 1/3rd of the annotated Mtb proteome is modified post-translationally, with a large number of these proteins being essential for mycobacterial survival. Post-translational modifications (PTMs) such as phosphorylation, acetylation, and pupylation play a salient role in mycobacterial virulence, pathogenesis, and metabolism. The role of many other PTMs is still emerging. Understanding the signaling pathways and PTMs may assist clinical strategies and drug development for Mtb. In this review, we explore the contribution of PTMs to mycobacterial physiology, describe the related cellular processes, and discuss how these processes are linked to drug resistance. A significant number of drug targets, InhA, RpoB, EmbR, and KatG, are modified at multiple residues via PTMs. A better understanding of drug-resistance regulons and associated PTMs will aid in developing effective drugs against TB.

Alterations in the Cell Wall of Rhodococcus biphenylivorans Under Norfloxacin Stress

Many microorganisms can enter a viable but non-culturable (VBNC) state under various environmental stresses, while they can also resuscitate when the surroundings turn to suitable conditions. Cell walls play a vital role in maintaining cellular integrity and protecting cells from ambient threats. Here, we investigated the alterations in the cell wall of Rhodococcus biphenylivorans TG9 at VBNC state under norfloxacin stress and then at resuscitated state in fresh lysogeny broth medium. Electron microscopy analyses presented that TG9 in the VBNC state had a thicker and rougher cell wall than that in exponential phase or resuscitated state. Meanwhile, the results from infrared spectroscopy also showed that its VBNC state has different peptidoglycan structures in the cell wall. Moreover, in the VBNC cells the gene expressions related to cell wall synthesis and remodeling maintain a relatively high level. It indicates that the morphological variations of TG9 at the VBNC state might result from kinetic changes in the cell wall synthesis and remodeling. As a consequence, the alterations in the cell wall of VBNC TG9 may somewhat account for its tolerance mechanisms to antibiotic treatment.

Modulation of the M. tuberculosis cell envelope between replicating and non-replicating persistent bacteria

The success of Mycobacterium tuberculosis as a human pathogen depends on the bacterium's ability to persist in a quiescent form in oxygen and nutrient-poor host environments. In vitro studies have demonstrated that these restricting environments induce a shift from bacterial replication to non-replicating persistence (NRP). Entry into NRP involves changes in bacterial metabolism and remodeling of the cell envelope. Findings consistent with the phenotypes observed in vitro have been observed in patient and animal model samples. This review focuses on the cell envelope differences seen between replicating and NRP M. tuberculosis and summarizes the ways in which serine/threonine protein kinases (STPKs) may mediate this process.

Coupling of Peptidoglycan Synthesis to Central Metabolism in Mycobacteria: Post-transcriptional Control of CwlM by Aconitase

Mycobacterium tuberculosis causes human tuberculosis, and a better understanding of its biology is required to identify vulnerabilities that might be exploited in developing new therapeutics. The iron-sulfur cluster of the essential M. tuberculosis central metabolic enzyme, aconitase (AcnA), disassembles when exposed to oxidative/nitrosative stress or iron chelators. The catalytically inactive apo-AcnA interacts with a sequence resembling an iron-responsive element (IRE) located within the transcript of another essential protein, CwlM, a regulator of peptidoglycan synthesis. A Mycobacterium smegmatis cwlM conditional mutant complemented with M. tuberculosis cwlM with a disrupted IRE is unable to recover from combinations of oxidative, nitrosative, and iron starvation stresses. An equivalent M. tuberculosis cwlM conditional mutant complemented with the cwlM gene lacking a functional IRE exhibits a growth defect in THP-1 macrophages. It appears that AcnA acts to couple peptidoglycan synthesis and central metabolism, and disruption of this coupling potentially leaves mycobacteria vulnerable to attack by macrophages.

Cell wall peptidoglycan in Mycobacterium tuberculosis: An Achilles' heel for the TB-causing pathogen

Tuberculosis (TB), caused by the intracellular pathogen Mycobacterium tuberculosis, remains one of the leading causes of mortality across the world. There is an urgent requirement to build a robust arsenal of effective antimicrobials, targeting novel molecular mechanisms to overcome the challenges posed by the increase of antibiotic resistance in TB. Mycobacterium tuberculosis has a unique cell envelope structure and composition, containing a peptidoglycan layer that is essential for maintaining cellular integrity and for virulence. The enzymes involved in the biosynthesis, degradation, remodelling and recycling of peptidoglycan have resurfaced as attractive targets for anti-infective drug discovery. Here, we review the importance of peptidoglycan, including the structure, function and regulation of key enzymes involved in its metabolism. We also discuss known inhibitors of ATP-dependent Mur ligases, and discuss the potential for the development of pan-enzyme inhibitors targeting multiple Mur ligases.

Mycobacterium smegmatis HtrA Blocks the Toxic Activity of a Putative Cell Wall Amidase

Mycobacterium tuberculosis, the causative agent of tuberculosis, withstands diverse environmental stresses in the host. The periplasmic protease HtrA is required only to survive extreme conditions in most bacteria but is predicted to be essential for normal growth in mycobacteria. We confirm that HtrA is indeed essential in Mycobacterium smegmatis and interacts with another essential protein of unknown function, LppZ. However, the loss of any of three unlinked genes, including those encoding Ami3, a peptidoglycan muramidase, and Pmt, a mannosyltransferase, suppresses the essentiality of both HtrA and LppZ, indicating the functional relevance of these genes' protein products. Our data indicate that HtrA-LppZ is required to counteract the accumulation of active Ami3, which is toxic under the stabilizing influence of Pmt-based mannosylation. This suggests that HtrA-LppZ blocks the toxicity of a cell wall enzyme to maintain mycobacterial homeostasis.

Importance of protein Ser/Thr/Tyr phosphorylation for bacterial pathogenesis

Protein phosphorylation regulates a large variety of biological processes in all living cells. In pathogenic bacteria, the study of serine, threonine, and tyrosine (Ser/Thr/Tyr) phosphorylation has shed light on the course of infectious diseases, from adherence to host cells to pathogen virulence, replication, and persistence. Mass spectrometry (MS)-based phosphoproteomics has provided global maps of Ser/Thr/Tyr phosphosites in bacterial pathogens. Despite recent developments, a quantitative and dynamic view of phosphorylation events that occur during bacterial pathogenesis is currently lacking. Temporal, spatial, and subpopulation resolution of phosphorylation data is required to identify key regulatory nodes underlying bacterial pathogenesis. Herein, we discuss how technological improvements in sample handling, MS instrumentation, data processing, and machine learning should improve bacterial phosphoproteomic datasets and the information extracted from them. Such information is expected to significantly extend the current knowledge of Ser/Thr/Tyr phosphorylation in pathogenic bacteria and should ultimately contribute to the design of novel strategies to combat bacterial infections.

New tuberculosis drug targets, their inhibitors, and potential therapeutic impact

The current tuberculosis (TB) predicament poses numerous challenges and therefore every incremental scientific work and all positive socio-political engagements, are steps taken in the right direction to eradicate TB. Progression of the late stage TB-drug pipeline into the clinics is an immediate deliverable of this global effort. At the same time, fueling basic research and pursuing early discovery work must be sustained to maintain a healthy TB-drug pipeline. This review encompasses a broad analysis of chemotherapeutic strategies that target the DNA replication, protein synthesis, cell wall biosynthesis, energy metabolism and proteolysis of Mycobacterium tuberculosis (Mtb). It includes a status check of the current TB-drug pipeline with a focus on the associated biology, emerging targets, and their promising chemical inhibitors. Potential synergies and/or gaps within or across different chemotherapeutic strategies are systematically reviewed as well.

PrkA controls peptidoglycan biosynthesis through the essential phosphorylation of ReoM

Peptidoglycan (PG) is the main component of bacterial cell walls and the target for many antibiotics. PG biosynthesis is tightly coordinated with cell wall growth and turnover, and many of these control activities depend upon PASTA-domain containing eukaryotic-like serine/threonine protein kinases (PASTA-eSTK) that sense PG fragments. However, only a few PG biosynthetic enzymes are direct kinase substrates. Here, we identify the conserved ReoM protein as a novel PASTA-eSTK substrate in the Gram-positive pathogen Listeria monocytogenes. Our data show that the phosphorylation of ReoM is essential as it controls ClpCP-dependent proteolytic degradation of the essential enzyme MurA, which catalyses the first committed step in PG biosynthesis. We also identify ReoY as a second novel factor required for degradation of ClpCP substrates. Collectively, our data imply that the first committed step of PG biosynthesis is activated through control of ClpCP protease activity in response to signals of PG homeostasis imbalance.

Peptidoglycan biosynthesis and remodeling revisited

The bacterial peptidoglycan layer forms a complex mesh-like structure that surrounds the cell, imparting rigidity to withstand cytoplasmic turgor and the ability to tolerate stress. As peptidoglycan has been the target of numerous clinically successful antimicrobials such as penicillin, the biosynthesis, remodeling and recycling of this polymer has been the subject of much interest. Herein, we review recent advances in the understanding of peptidoglycan biosynthesis and remodeling in a variety of different organisms. In order for bacterial cells to grow and divide, remodeling of cross-linked peptidoglycan is essential hence, we also summarize the activity of important peptidoglycan hydrolases and how their functions differ in various species. There is a growing body of evidence highlighting complex regulatory mechanisms for peptidoglycan metabolism including protein interactions, phosphorylation and protein degradation and we summarize key recent findings in this regard. Finally, we provide an overview of peptidoglycan recycling and how components of this pathway mediate resistance to drugs. In the face of growing antimicrobial resistance, these recent advances are expected to uncover new drug targets in peptidoglycan metabolism, which can be used to develop novel therapies.

Growth promotion in Corynebacterium glutamicum by overexpression of the NCgl2986 gene encoding a protein homologous to peptidoglycan amidases

We previously reported the extracellular production of antibody fragment Fab by Corynebacterium glutamicum. In the course of searching for genes which improve the secretion efficiency of Fab, we coincidentally found that the final growth increased significantly when the NCgl2986 gene encoding an amidase-like protein was overexpressed. This effect was observed when cells were grown on the production medium MMTG, which contains high concentrations of glucose and neutralizing agent CaCO₃, but not on MMTG without CaCO₃ or Lennox medium. Not only turbidity but also dry cell weight was increased by NCgl2986 overexpression, although the growth rate was not affected. It was recently reported that the Mycobacterium tuberculosis homolog Rv3915 functions as an activator of MurA protein, which catalyzes the initial step of peptidoglycan synthesis. Growth promotion was also observed when the MurA protein was overproduced. His-tagged NCgl2986 protein was purified, but its peptidoglycan hydrolyzing activity could not be detected. These results suggest that NCgl2986 promotes cell growth by activating the peptidoglycan synthetic pathway.

Protein kinases PknA and PknB independently and coordinately regulate essential Mycobacterium tuberculosis physiologies and antimicrobial susceptibility

The Mycobacterium tuberculosis Ser/Thr protein kinases PknA and PknB are essential for growth and have been proposed as possible drug targets. We used a titratable conditional depletion system to investigate the functions of these kinases. Depletion of PknA or PknB or both kinases resulted in growth arrest, shortening of cells, and time-dependent loss of acid-fast staining with a concomitant decrease in mycolate synthesis and accumulation of trehalose monomycolate. Depletion of PknA and/or PknB resulted in markedly increased susceptibility to β-lactam antibiotics, and to the key tuberculosis drug rifampin. Phosphoproteomic analysis showed extensive changes in protein phosphorylation in response to PknA depletion and comparatively fewer changes with PknB depletion. These results identify candidate substrates of each kinase and suggest specific and coordinate roles for PknA and PknB in regulating multiple essential physiologies. These findings support these kinases as targets for new antituberculosis drugs and provide a valuable resource for targeted investigation of mechanisms by which protein phosphorylation regulates pathways required for growth and virulence in M. tuberculosis.

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.

Effect of peptidoglycan amidase MSMEG_6281 on fatty acid metabolism in Mycobacterium smegmatis

Mycobacterium smegmatis MSMEG_6281, a peptidoglycan (PG) amidase, is essential in maintaining cell wall integrity. To address the potential roles during the MSMEG_6281-mediated biological process, we compared proteomes from wild-type M.smegmatis and MSMEG_6281 gene knockout strain (M.sm-ΔM_6281) using LC-MS/MS analysis. Peptide analysis revealed that 851 proteins were differentially produced with at least 1.2-fold changes, including some proteins involved in fatty acid metabolism such as acyl-CoA synthase, acyl-CoA dehydrogenase, MCE-family proteins, ATP-binding cassette (ABC) transporters, and MmpL4. Some proteins related to fatty acid degradation were enriched through protein-protein interaction analysis. Therefore, proteomic data showed that a lack of MSMEG_6281 affected fatty acid metabolism. Mycobacteria can produce diverse lipid molecules ranging from single fatty acids to highly complex mycolic acids, and mycobacterial surface-exposed lipids may impact biofilm formation. In this study, we also assessed the effects of MSMEG_6281 on biofilm phenotype using semi-quantitative and morphology analysis methods. These results found that M.sm-ΔM_6281 exhibited a delayed biofilm phenotype compared to that of the wild-type M.smegmatis, and the changes were recovered when PG amidase was rescued in a ΔM_6281::Rv3717 strain. Our results demonstrated that MSMEG_6281 impacts fatty acid metabolism and further interferes with biofilm formation. These results provide a clue to study the effects of PG amidase on mycobacterial pathogenicity.

Identification of peptidomimetic compounds as potential inhibitors against MurA enzyme of Mycobacterium tuberculosis

Increasing prevalence of resistance to anti-tubercular drugs has become the foremost challenge now. According to WHO, over half a million of multidrug resistance cases (rifampicin, isoniazid, etc.) were reported in 2017, mostly emerging from countries such as China, India, and Russia. Therefore, developing new drugs or repurposing existing ones is need of the hour. The Mycobacterium cell wall biogenesis pathway offers many attractive targets for drug discovery against Tuberculosis (TB). MurA, a transferase enzyme that catalyzes the initial step of peptidoglycan (PG) biosynthesis, is one among them. A peptidoglycan layer resides over the plasma membrane and is an integral component of the bacterial cell wall. Therefore, disruption of their formation through inhibition of MurA enzyme should lead to deficiency in Mycobacterium cell synthesis. Based on this strategy, we have designed this study where two libraries of peptidomimetic compounds (Asinex & ChemDiv) were first screened against our modeled MurA structure and then validated through molecular dynamic simulations. From our virtual screening, top four compounds (ChemDiv: D675-0102, D675-0217; Asinex: BDE25373574, BDE 26717803) were selected based on their docking scores, binding energies, and interactions with catalytic site residues, for further evaluation. Results revealed stable ligand-MurA interactions throughout 50 ns of MD simulation and also druggability acceptable pharmacokinetic profile for all four compounds. Thus, based on our findings, these compounds could be considered as potential inhibitors of Mycobacterium MurA enzyme and hence be further tested for in vitro experimental validation as TB therapeutic drug candidate.Communicated by Ramaswamy H. Sarma.

The mycobacterial cell envelope - a moving target

Mycobacterium tuberculosis, the leading cause of death due to infection, has a dynamic and immunomodulatory cell envelope. The cell envelope structurally and functionally varies across the length of the cell and during the infection process. This variability allows the bacterium to manipulate the human immune system, tolerate antibiotic treatment and adapt to the variable host environment. Much of what we know about the mycobacterial cell envelope has been gleaned from model actinobacterial species, or model conditions such as growth in vitro, in macrophages and in the mouse. In this Review, we combine data from different experimental systems to build a model of the dynamics of the mycobacterial cell envelope across space and time. We describe the regulatory pathways that control metabolism of the cell wall and surface lipids in M. tuberculosis during growth and stasis, and speculate about how this regulation might affect antibiotic susceptibility and interactions with the immune system.

Protein kinase B controls Mycobacterium tuberculosis growth via phosphorylation of the transcriptional regulator Lsr2 at threonine 112

Mycobacterium tuberculosis (Mtb) is able to persist in the body through months of multi‐drug therapy. Mycobacteria possess a wide range of regulatory proteins, including the protein kinase B (PknB) which controls peptidoglycan biosynthesis during growth. Here, we observed that depletion of PknB resulted in specific transcriptional changes that are likely caused by reduced phosphorylation of the H‐NS‐like regulator Lsr2 at threonine 112. The activity of PknB towards this phosphosite was confirmed with purified proteins, and this site was required for adaptation of Mtb to hypoxic conditions, and growth on solid media. Like H‐NS, Lsr2 binds DNA in sequence‐dependent and non‐specific modes. PknB phosphorylation of Lsr2 reduced DNA binding, measured by fluorescence anisotropy and electrophoretic mobility shift assays, and our NMR structure of phosphomimetic T112D Lsr2 suggests that this may be due to increased dynamics of the DNA‐binding domain. Conversely, the phosphoablative T112A Lsr2 had increased binding to certain DNA sites in ChIP‐sequencing, and Mtb containing this variant showed transcriptional changes that correspond with the change in DNA binding. In summary, PknB controls Mtb growth and adaptations to the changing host environment by phosphorylating the global transcriptional regulator Lsr2.

Novel mechanistic insights into physiological signaling pathways mediated by mycobacterial Ser/Thr protein kinases

Protein phosphorylation is known to be one of the keystones of signal sensing and transduction in all living organisms. Once thought to be essentially confined to the eukaryotic kingdoms, reversible phosphorylation on serine, threonine and tyrosine residues, has now been shown to play a major role in many prokaryotes, where the number of Ser/Thr protein kinases (STPKs) equals or even exceeds that of two component systems. Mycobacterium tuberculosis, the etiological agent of tuberculosis, is one of the most studied organisms for the role of STPK-mediated signaling in bacteria. Driven by the interest and tractability of these enzymes as potential therapeutic targets, extensive studies revealed the remarkable conservation of protein kinases and their cognate phosphatases across evolution, and their involvement in bacterial physiology and virulence. Here, we present an overview of the current knowledge of mycobacterial STPKs structures and kinase activation mechanisms, and we then focus on PknB and PknG, two well-characterized STPKs that are essential for the intracellular survival of the bacillus. We summarize the mechanistic evidence that links PknB to the regulation of peptidoglycan synthesis in cell division and morphogenesis, and the major findings that establishes PknG as a master regulator of central carbon and nitrogen metabolism. Two decades after the discovery of STPKs in M. tuberculosis, the emerging landscape of O-phosphosignaling is starting to unveil how eukaryotic-like kinases can be engaged in unique, non-eukaryotic-like, signaling mechanisms in mycobacteria.

Mycobacterial phosphatase PstP regulates global serine threonine phosphorylation and cell division

Protein phosphatase PstP is conserved throughout the Actinobacteria in a genetic locus related to cell wall synthesis and cell division. In many Actinobacteria it is the sole annotated serine threonine protein phosphatase to counter the activity of multiple serine threonine protein kinases. We used transcriptional knockdown, electron microscopy and comparative phosphoproteomics to investigate the putative dual functions of PstP as a specific regulator of cell division and as a global regulator of protein phosphorylation. Comparative phosphoproteomics in the early stages of PstP depletion showed hyperphosphorylation of protein kinases and their substrates, confirming PstP as a negative regulator of kinase activity and global serine and threonine phosphorylation. Analysis of the 838 phosphorylation sites that changed significantly, suggested that PstP may regulate diverse phosphoproteins, preferentially at phosphothreonine near acidic residues, near the protein termini, and within membrane associated proteins. Increased phosphorylation of the activation loop of protein kinase B (PknB) and of the essential PknB substrate CwlM offer possible explanations for the requirement for pstP for growth and for cell wall defects when PstP was depleted.

The Dream of a Mycobacterium

How do mycobacteria divide? Cell division has been studied extensively in the model rod-shaped bacteria Escherichia coli and Bacillus subtilis, but much less is understood about cell division in mycobacteria, a genus that includes the major human pathogens M. tuberculosis and M. leprae. In general, bacterial cell division requires the concerted effort of many proteins in both space and time to elongate the cell, replicate and segregate the chromosome, and construct and destruct the septum - processes which result in the creation of two new daughter cells. Here, we describe these distinct stages of cell division in B. subtilis and follow with the current knowledge in mycobacteria. As will become apparent, there are many differences between mycobacteria and B. subtilis in terms of both the broad outline of cell division and the molecular details. So, while the fundamental challenge of spatially and temporally organizing cell division is shared between these rod-shaped bacteria, they have solved these challenges in often vastly different ways.

Lack of MSMEG_6281, a peptidoglycan amidase, affects cell wall integrity and virulence of Mycobacterium smegmatis

Mycolyl-arabinogalactan-peptidoglycan (mAGP) is the major content of the mycobacterium cell wall structure and essential for mycobacterial survival. Peptidoglycan (PG) plays an important role in maintenance of cell division, cell wall integrity and pathogenesis. Mycobacterium smegmatis MSMEG_6281, a peptidoglycan amidase, is vital for mycobacterial cell division. However, the effects of MSMEG_6281on cell wall integrity and mycobacterial virulence remain unknown. In the current study, we demonstrate that MSMEG_6281gene knockout in M.smegmatis alters the microbiological characteristics. Our results revealed that MSMEG_6281gene knockout bacteria (M. sm-ΔM_6281) lost their acid-fastness, increased their sensitivity to lipophilic compounds and presented an abnormal morphology. Our results revealed that MSMEG_6281was related to maintaining the cell wall integrity. Furthermore, we investigated the effects of MSMEG_6281 inactivation on mycobacterial virulence using mice models infected by different M.smegmatis strains. MSMEG_6281 inactivation in the M sm-ΔM_6281 infected group caused less mycobacterial colonization, reduced pathological signs, decreased the anti-microbial enzymes production including iNOS and β-defensins in mouse lungs. Moreover, IL-1β and TLR2 expression were significantly down-regulated, while the production of IFN-γ and TNF-α was up-regulated. These findings indicated the diversity of host immune responses induced by different strains of M.smegmatis, suggesting that MSMEG_6281 inactivation impact mycobacterial virulence. In conclusion, the MSMEG_6281 protein plays important roles in maintaining cell wall integrity and mycobacterial virulence.

Cell Walls and Membranes of Actinobacteria

Actinobacteria is a group of diverse bacteria. Most species in this class of bacteria are filamentous aerobes found in soil, including the genus Streptomyces perhaps best known for their fascinating capabilities of producing antibiotics. These bacteria typically have a Gram-positive cell envelope, comprised of a plasma membrane and a thick peptidoglycan layer. However, there is a notable exception of the Corynebacteriales order, which has evolved a unique type of outer membrane likely as a consequence of convergent evolution. In this chapter, we will focus on the unique cell envelope of this order. This cell envelope features the peptidoglycan layer that is covalently modified by an additional layer of arabinogalactan . Furthermore, the arabinogalactan layer provides the platform for the covalent attachment of mycolic acids , some of the longest natural fatty acids that can contain ~100 carbon atoms per molecule. Mycolic acids are thought to be the main component of the outer membrane, which is composed of many additional lipids including trehalose dimycolate, also known as the cord factor. Importantly, a subset of bacteria in the Corynebacteriales order are pathogens of human and domestic animals, including Mycobacterium tuberculosis. The surface coat of these pathogens are the first point of contact with the host immune system, and we now know a number of host receptors specific to molecular patterns exposed on the pathogen's surface, highlighting the importance of understanding how the cell envelope of Actinobacteria is structured and constructed. This chapter describes the main structural and biosynthetic features of major components found in the actinobacterial cell envelopes and highlights the key differences between them.

Maturing Mycobacterium smegmatis peptidoglycan requires non-canonical crosslinks to maintain shape

In most well-studied rod-shaped bacteria, peptidoglycan is primarily crosslinked by penicillin-binding proteins (PBPs). However, in mycobacteria, crosslinks formed by L,D-transpeptidases (LDTs) are highly abundant. To elucidate the role of these unusual crosslinks, we characterized Mycobacterium smegmatis cells lacking all LDTs. We find that crosslinks generate by LDTs are required for rod shape maintenance specifically at sites of aging cell wall, a byproduct of polar elongation. Asymmetric polar growth leads to a non-uniform distribution of these two types of crosslinks in a single cell. Consequently, in the absence of LDT-mediated crosslinks, PBP-catalyzed crosslinks become more important. Because of this, Mycobacterium tuberculosis (Mtb) is more rapidly killed using a combination of drugs capable of PBP- and LDT- inhibition. Thus, knowledge about the spatial and genetic relationship between drug targets can be exploited to more effectively treat this pathogen.

Most bacteria have a cell wall that protects them and maintains their shape. Many of these organisms make their cell walls from fibers of proteins and sugars, called peptidoglycan. As bacteria grow, peptidoglycan is constantly broken down and reassembled, and in many species, new units of peptidoglycan are added into the sidewall. However, in a group of bacteria called mycobacteria, which cause tuberculosis and other diseases, the units are added at the tips.

The peptidoglycan layer is often a successful target for antibiotic treatments. But, drugs that treat tuberculosis do not attack this layer, partly because we know very little about the cell walls of mycobacteria.

Here, Baranowski et al. used genetic manipulation and microscopy to study how mycobacteria build their cell wall. The results showed that these bacteria link peptidoglycan units together in an unusual way. In most bacteria, peptidoglycan units are connected by chemical links known as 4-3 crosslinks. This is initially the same in mycobacteria, but as the cell grows and the cell wall expands, these bonds break and so-called 3-3 crosslinks form. In genetically modified bacteria that could not form these 3-3 bonds, the cell wall became brittle and weak, and the bacteria eventually died.

These findings could be important for developing new drugs that treat infections caused by mycobacteria. Baranowski et al. demonstrate that a combination of drugs blocking both 4-3 and 3-3 crosslinks is particularly effective at killing the bacterium that causes tuberculosis.