Genetic characterization of mycobacterial L,D-transpeptidases

Integration of biophysical and biological approaches to validate fragment-like compounds targeting l,d-transpeptidases from Mycobacterium tuberculosis

Tuberculosis is one of the major causes of death worldwide; more than a million people die every year because of this infection. The constant emergency of Mycobacterium tuberculosis resistant strains against the most used treatments also contributes to the burden caused by this disease. Consequently, the development of new alternative therapies against this disease is constantly required. In recent years, only a few molecules have reached the market as new antituberculosis agents. The mycobacterial cell wall biosynthesis is for a longstanding considered an important target for drug development. Particularly, in M. tuberculosis, the peptidoglycan cross-links are predominantly formed by nonclassical bridges between the third residues of adjacent tetrapeptides. The responsible enzymes for these reactions are ld-transpeptidases (Ldts), for which M. tuberculosis has five paralogues. Although these enzymes are distinct from the penicillin-binding proteins (PBPs), they can also be inactivated by β-lactam antibiotics, but since M. tuberculosis has a chromosomal β-lactamase, most of the antibiotics of these classes can be degraded. Thus, to identify alternative scaffolds for the development of new antimicrobials against tuberculosis, we have integrated several fragment-based drug discovery techniques. Based on that, we identified and validated a number of small molecules that could be the starting point in the synthesis of more potent inhibitors against at least two Ldts from M. tuberculosis, LdtMt2 and LdtMt3. Eight identified molecules inhibited the Ldts activity in at least 20%, and three of them have antimycobacterial activity. The cell ultrastructural analysis suggested that one of the best compounds induced severe effects on the septum and cell wall morphologies, which corroborates our target-based approach to identifying new Ldts hits.

An update on Mycobacterium tuberculosis lipoproteins

Almost 3% of the proteins of Mycobacterium tuberculosis (M. tuberculosis), the main causative agent of human tuberculosis, are lipoproteins. These lipoproteins are characteristic of the mycobacterial cell envelope and participate in many mechanisms involved in the pathogenesis of M. tuberculosis. In this review, the authors provide an updated analysis of M. tuberculosis lipoproteins and categorize them according to their demonstrated or predicted functions, including transport of compounds to and from the cytoplasm, biosynthesis of the mycobacterial cell envelope, defense and resistance mechanisms, enzymatic activities and signaling pathways. In addition, this updated analysis revealed that at least 40% of M. tuberculosis lipoproteins are glycosylated.

A Metabolic-Tag-Based Method for Assessing the Permeation of Small Molecules Across the Mycomembrane in Live Mycobacteria

The general lack of permeability of small molecules observed for Mycobacterium tuberculosis (Mtb) is most ascribed to its unique cell envelope. More specifically, the outer mycomembrane is hypothesized to be the principal determinant for access of antibiotics to their molecular targets. We describe a novel assay that combines metabolic tagging of the peptidoglycan, which sits directly beneath the mycomembrane, click chemistry of test molecules, and a fluorescent labeling chase step, to measure the permeation of small molecules. We showed that the assay workflow was robust and compatible with high-throughput analysis in mycobacteria by testing a small panel of azide-tagged molecules. The general trend is similar across the two types of mycobacteria with some notable exceptions. We anticipate that this assay platform will lay the foundation for medicinal chemistry efforts to understand and improve uptake of both existing drugs and newly-discovered compounds into mycobacteria.

Identification of Genes Required for Long-Term Survival of Legionella pneumophila in Water

Long-term survival of Legionella pneumophila in aquatic environments is thought to be important for facilitating epidemic outbreaks. Eliminating bacterial colonization in plumbing systems is the primary strategy that depletes this reservoir and prevents disease. To uncover L. pneumophila determinants facilitating survival in water, a Tn-seq strategy was used to identify survival-defective mutants during 50-day starvation in tap water at 42°C. The mutants with the most drastic survival defects carried insertions in electron transport chain genes, indicating that membrane energy charge and/or ATP synthesis requires the generation of a proton gradient by the respiratory chain to maintain survival in the presence of water stress. In addition, periplasmically localized proteins that are known (EnhC) or hypothesized (lpg1697) to stabilize the cell wall against turnover were essential for water survival. To test that the identified mutations disrupted water survival, candidate genes were knocked down by CRISPRi. The vast majority of knockdown strains with verified transcript depletion showed remarkably low viability after 50-day incubations. To demonstrate that maintenance of cell wall integrity was an important survival determinant, a deletion mutation in lpg1697, in a gene encoding a predicted l,d-transpeptidase domain, was analyzed. The loss of this gene resulted in increased osmolar sensitivity and carbenicillin hypersensitivity relative to the wild type, as predicted for loss of an l,d-transpeptidase. These results indicate that the L. pneumophila envelope has been evolutionarily selected to allow survival under conditions in which the bacteria are subjected to long-term exposure to starvation and low osmolar conditions. IMPORTANCE Water is the primary vector for transmission of L. pneumophila to humans, and the pathogen is adapted to persist in this environment for extended periods of time. Preventing survival of L. pneumophila in water is therefore critical for prevention of Legionnaires' disease. We analyzed dense transposon mutation pools for strains with severe survival defects during a 50-day water incubation at 42°C. By tracking the associated transposon insertion sites in the genome, we defined a distinct essential gene set for water survival and demonstrate that a predicted peptidoglycan cross-linking enzyme, lpg1697, and components of the electron transport chain are required to ensure survival of the pathogen. Our results indicate that select characteristics of the cell wall and components of the respiratory chain of L. pneumophila are primary evolutionary targets being shaped to promote its survival in water.

LdtC Is a Key l,d-Transpeptidase for Peptidoglycan Assembly in Mycobacterium smegmatis

The peptidoglycan of mycobacteria has two types of direct cross-links, classical 4-3 cross-links that occur between diaminopimelate (DAP) and alanine residues, and nonclassical 3-3 cross-links that occur between DAP residues on adjacent peptides. The 3-3 cross-links are synthesized by the concerted action of d,d-carboxypeptidases and l,d-transpeptidases (Ldts). Mycobacterial genomes encode several Ldt proteins that can be classified into six classes based upon sequence identity. As a group, the Ldt enzymes are resistant to most β-lactam antibiotics but are susceptible to carbapenem antibiotics, with the exception of LdtC, a class 5 enzyme. In previous work, we showed that loss of LdtC has the greatest effect on the carbapenem susceptibility phenotype of Mycobacterium smegmatis (also known as Mycolicibacterium smegmatis) compared to other ldt deletion mutants. In this work, we show that a M. smegmatis mutant lacking the five ldt genes other than ldtC has a wild-type phenotype with the exception of increased susceptibility to rifampin. In contrast, a mutant lacking all six ldt genes has pleiotropic cell envelope defects, is temperature sensitive, and has increased susceptibility to a variety of antibiotics. These results indicate that LdtC is capable of functioning as the sole l,d-transpeptidase in M. smegmatis and suggest that it may represent a carbapenem-resistant pathway for peptidoglycan biosynthesis. IMPORTANCE Mycobacteria have several enzymes to catalyze nonclassical 3-3 linkages in the cell wall peptidoglycan. Understanding the biology of these cross-links is important for the development of antibiotic therapies to target peptidoglycan biosynthesis. Our work provides evidence that LdtC can function as the sole enzyme for 3-3 cross-link formation in M. smegmatis and suggests that LdtC may be part of a carbapenem-resistant l,d-transpeptidase pathway.

Identification of drivers of mycobacterial resistance to peptidoglycan synthesis inhibitors

Beta-lactams have been excluded from tuberculosis therapy due to the intrinsic resistance of Mycobacterium tuberculosis (Mtb) to this antibiotic class, usually attributed to a potent beta-lactamase, BlaC, and to an unusually complex cell wall. In this pathogen, the peptidoglycan is cross-linked by penicillin-binding proteins (PBPs) and L,D-transpeptidases, the latter resistant to inhibition by most beta-lactams. However, recent studies have shown encouraging results of beta-lactam/beta-lactamase inhibitor combinations in clinical strains. Additional research on the mechanisms of action and resistance to these antibiotics and other inhibitors of peptidoglycan synthesis, such as the glycopeptides, is crucial to ascertain their place in alternative regimens against drug-resistant strains. Within this scope, we applied selective pressure to generate mutants resistant to amoxicillin, meropenem or vancomycin in Mtb H37Rv or Mycolicibacterium smegmatis (Msm) mc2-155. These were phenotypically characterized, and whole-genome sequencing was performed. Mutations in promising targets or orthologue genes were inspected in Mtb clinical strains to establish potential associations between altered susceptibility to beta-lactams and the presence of key genomic signatures. The obtained isolates had substantial increases in the minimum inhibitory concentration of the selection antibiotic, and beta-lactam cross-resistance was detected in Mtb. Mutations in L,D-transpeptidases and major PBPs, canonical targets, or BlaC were not found. The transcriptional regulator PhoP (Rv0757) emerged as a common denominator for Mtb resistance to both amoxicillin and meropenem, while Rv2864c, a lipoprotein with PBP activity, appears to be specifically involved in decreased susceptibility to the carbapenem. Nonetheless, the mutational pattern detected in meropenem-resistant mutants was different from the yielded by amoxicillin-or vancomycin-selected isolates, suggesting that distinct pathways may participate in increased resistance to peptidoglycan inhibitors, including at the level of beta-lactam subclasses. Cross-resistance between beta-lactams and antimycobacterials was mostly unnoticed, and Msm meropenem-resistant mutants from parental strains with previous resistance to isoniazid or ethambutol were isolated at a lower frequency. Although cell-associated nitrocefin hydrolysis was increased in some of the isolates, our findings suggest that traditional assumptions of Mtb resistance relying largely in beta-lactamase activity and impaired access of hydrophilic molecules through lipid-rich outer layers should be challenged. Moreover, the therapeutical potential of the identified Mtb targets should be explored.

T405, a New Penem, Exhibits In Vivo Efficacy against M. abscessus and Synergy with β-Lactams Imipenem and Cefditoren

Mycobacteroides abscessus (Mab) is an emerging environmental microbe that causes chronic lung disease in patients with compromised lung function such as cystic fibrosis and bronchiectasis. It is intrinsically resistant to most antibiotics, therefore there are only few antibiotics that can be repurposed to treat Mab disease. Although current recommendations require daily intake of multiple antibiotics for more than a year, cure rate is low and often associated with significant adverse events. Here, we describe in vivo efficacy of T405, a recently discovered β-lactam antibiotic of the penem subclass, in a mouse model of pulmonary Mab infection. Imipenem, one of the standard-of-care drugs to treat Mab disease, and also a β-lactam antibiotic from a chemical class similar to T405, was included as a comparator. Probenecid was included with both T405 and imipenem to reduce the rate of their renal clearance. T405 exhibited bactericidal activity against Mab from the onset of treatment and reduced Mab lung burden at a rate similar to that exhibited by imipenem. The MIC of T405 against Mab was unaltered after 4 weeks of exposure to T405 in the lungs of mice. Using an in vitro assay, we also demonstrate that T405 in combination with imipenem, cefditoren or avibactam exhibits synergism against Mab. Additionally, we describe a scheme for synthesis and purification of T405 on an industrial scale. These attributes make T405 a promising candidate for further preclinical assessment to treat Mab disease.

Allosteric cooperation in ß-lactam binding to a non-classical transpeptidase

L,D-transpeptidase function predominates in atypical 3®3 transpeptide networking of peptidoglycan (PG) layer in Mycobacterium tuberculosis. Prior studies of L,D-transpeptidases have identified only the catalytic site that binds to peptide moiety of the PG substrate or ß-lactam antibiotics. This insight was leveraged to develop mechanism of its activity and inhibition by ß-lactams. Here we report identification of an allosteric site at a distance of 21 Å from the catalytic site that binds the sugar moiety of PG substrates (hereafter referred to as the S-pocket). This site also binds a second ß-lactam molecule and influences binding at the catalytic site. We provide evidence that two ß-lactam molecules bind co-operatively to this enzyme, one non-covalently at the S-pocket and one covalently at the catalytic site. This dual ß-lactam binding phenomenon is previously unknown and is an observation that may offer novel approaches for the structure-based design of new drugs against M. tuberculosis./em>.

Total Synthesis of Tetrahydrolipstatin, Its Derivatives, and Evaluation of Their Ability to Potentiate Multiple Antibiotic Classes against Mycobacterium Species

Tetrahydrolipstatin (THL, 1a) has been shown to inhibit both mammalian and bacterial α/β hydrolases. In the case of bacterial systems, THL is a known inhibitor of several Mycobacterium tuberculosis hydrolases involved in mycomembrane biosynthesis. Herein we report a highly efficient eight-step asymmetric synthesis of THL using a route that allows modification of the THL α-chain substituent to afford compounds 1a through 1e. The key transformation in the synthesis was use of a (TPP)CrCl/Co2(CO)8-catalyzed regioselective and stereospecific carbonylation on an advanced epoxide intermediate to yield a trans-β-lactone. These compounds are modest inhibitors of Ag85A and Ag85C, two α/β hydrolases of M. tuberculosis involved in the biosynthesis of the mycomembrane. Among these compounds, 10d showed the highest inhibitory effect on Ag85A (34 ± 22 μM) and Ag85C (66 ± 8 μM), and its X-ray structure was solved in complex with Ag85C to 2.5 Å resolution. In contrast, compound 1e exhibited the best-in-class MICs of 50 μM (25 μg/mL) and 16 μM (8.4 μg/mL) against M. smegmatis and M. tuberculosis H37Ra, respectively, using a microtiter assay plate. Combination of 1e with 13 well-established antibiotics synergistically enhanced the potency of few of these antibiotics in M. smegmatis and M. tuberculosis H37Ra. Compound 1e applied at concentrations 4-fold lower than its MIC enhanced the MIC of the synergistic antibiotic by 2-256-fold. In addition to observing synergy with first-line drugs, rifamycin and isoniazid, the MIC of vancomycin against M. tuberculosis H37Ra was 65 μg/mL; however, the MIC was lowered to 0.25 μg/mL in the presence of 2.1 μg/mL 1e demonstrating the potential of targeting mycobacterial hydrolases involved in mycomembrane and peptidoglycan biosynthesis.

LD-transpeptidases: the great unknown among the peptidoglycan cross-linkers

The peptidoglycan (PG) cell wall is an essential polymer for the shape and viability of bacteria. Its protective role is in great part provided by its mesh-like character. Therefore, PG-cross-linking enzymes like the penicillin-binding proteins (PBPs) are among the best targets for antibiotics. However, while PBPs have been in the spotlight for more than 50 years, another class of PG-cross-linking enzymes called LD-transpeptidases (LDTs) seemed to contribute less to PG synthesis and, thus, has kept an aura of mystery. In the last years, a number of studies have associated LDTs with cell wall adaptation to stress including β-lactam antibiotics, outer membrane stability, and toxin delivery, which has shed light onto the biological meaning of these proteins. Furthermore, as some species display a great abundance of LD-cross-links in their cell wall, it has been hypothesized that LDTs could also be the main synthetic PG-transpeptidases in some bacteria. In this review, we introduce these enzymes and their role in PG biosynthesis and we highlight the most recent advances in understanding their biological role in diverse species.

N-Thio-β-lactams targeting L,D-transpeptidase-2, with activity against drug-resistant strains of Mycobacterium tuberculosis

Effective treatment of tuberculosis is frequently hindered by the emerging antimicrobial resistance of Mycobacterium tuberculosis. The present study evaluates monocyclic β-lactam compounds targeting the mycobacterial cell wall remodeling. Novel N-thio-β-lactams were designed, synthesized, and characterized on the L,D-transpeptidase-2, a validated target in M. tuberculosis. The candidates were evaluated in biochemical assays identifying five compounds presenting target-specific kinetic constants equal or superior to meropenem, a carbapenem currently considered for tuberculosis therapy. Mass spectrometry in line with the crystal structures of five target-ligand complexes revealed that the N-thio-β-lactams act via an unconventional mode of adduct formation, transferring the thio-residues from the lactam ring to the active-site cysteine of Ldt_Mt2. The resulting stable adducts lead to a long-term inactivation of the target protein. Finally, the candidates were evaluated in vitro against a drug-susceptible and multidrug-resistant clinical isolates of M. tuberculosis, confirming the antimycobacterial effect of these novel compounds.

Facile Synthesis and Metabolic Incorporation of m-DAP Bioisosteres Into Cell Walls of Live Bacteria

Bacterial cell walls contain peptidoglycan (PG), a scaffold that provides proper rigidity to resist lysis from internal osmotic pressure and a barrier to protect cells against external stressors. It consists of repeating sugar units with a linkage to a stem peptide that becomes cross-linked by cell wall transpeptidases (TP). While synthetic PG fragments containing l-lysine in the third position on the stem peptide are easier to access, those with meso-diaminopimelic acid (m-DAP) pose a severe synthetic challenge. Herein, we describe a solid phase synthetic scheme based on widely available building blocks to assemble meso-cystine (m-CYT), which mimics key structural features of m-DAP. To demonstrate proper mimicry of m-DAP, cell wall probes were synthesized with m-CYT in place of m-DAP and evaluated for their metabolic processing in live bacterial cells. We found that m-CYT-based cell wall probes were properly processed by TPs in various bacterial species that endogenously contain m-DAP in their PG. Additionally, we have used hybrid quantum mechanical/molecular mechanical (QM/MM) and molecular dynamics (MD) simulations to explore the influence of m-DAP analogs on the PG cross-linking. The results showed that the cross-linking mechanism of transpeptidases occurred through a concerted process. We anticipate that this strategy, which is based on the use of inexpensive and commercially available building blocks, can be widely adopted to provide greater accessibility of PG mimics for m-DAP containing organisms.

Structure and Function of L,D- and D,D-Transpeptidase Family Enzymes from Mycobacterium tuberculosis

Peptidoglycan, the exoskeleton of bacterial cell and an essential barrier that protects the cell, is synthesized by a pathway where the final steps are catalysed by transpeptidases. Knowledge of the structure and function of these vital enzymes that generate this macromolecule in M. tuberculosis could facilitate the development of potent lead compounds against tuberculosis. This review summarizes the experimental and computational studies to date on these aspects of transpeptidases in M. tuberculosis that have been identified and validated. The reported structures of L,D- and D,D-transpeptidases, as well as their functionalities, are reviewed and the proposed enzymatic mechanisms for L,D-transpeptidases are summarized. In addition, we provide bioactivities of known tuberculosis drugs against these enzymes based on both experimental and computational approaches. Advancing knowledge about these prominent targets supports the development of new drugs with novel inhibition mechanisms overcoming the current need for new drugs against tuberculosis.

MSMEG_2432 of Mycobacterium smegmatis mc2155 is a dual function enzyme that exhibits DD-carboxypeptidase and β-lactamase activities

Mycobacterial peptidoglycan (PG) is an unsolved puzzle due to its complex structure and involvement of multiple enzymes in the process of its remodelling. dd-Carboxypeptidases are low molecular mass penicillin-binding proteins (LMM-PBPs) that catalyzes the cleavage of terminal d-Ala of muramyl pentapeptide branches and thereby helps in the PG remodelling process. Here, we have assigned the function of a putative LMM-PBP, MSMEG_2432 of Mycobacterium smegmatis, by showing that it exhibits both dd-CPase and β-lactamase activities. Like conventional dd-CPase (PBP5 from E. coli), upon ectopic complementation in a deformed seven PBP deletion mutant of E. coli, MSMEG_2432 has manifested its ability to restore ~75 % of the cell population to their normal rod shape. Further, in vitrodd-CPase assay has confirmed its ability to release terminal d-Ala from the synthetic tripeptide and the peptidoglycan mimetic pentapeptide substrates ending with d-Ala-d-Ala. Also, elevated resistance against penicillins and cephalosporins upon ectopic expression of MSMEG_2432 suggests the presence of β-lactamase activity, which is further confirmed in vitro through nitrocefin hydrolysis assay. Moreover, it is found apparent that D169A substitution in MSMEG_2432 influences both of its in vivo and in vitrodd-CPase and β-lactamase activities. Thus, we infer that MSMEG_2432 is a dual function enzyme that possesses both dd-CPase and β-lactamase activities.

Phylogenetic and Biochemical Analyses of Mycobacterial l,d-Transpeptidases Reveal a Distinct Enzyme Class That Is Preferentially Acylated by Meropenem

The genomes of diverse mycobacterial species encode multiple proteins with the canonical l,d-transpeptidase (Ldt) sequence motif. The reason for this apparent redundancy is not well understood, but evidence suggests paralogous Ldts may serve niche roles in maintaining and/or remodeling mycobacterial peptidoglycan. We examined 323 mycobacterial Ldts and determined these enzymes cluster into six clades. We identified a variably represented yet distinct Ldt class (class 6) containing Mycobacterium smegmatis (Msm) LdtF and built a homology model of Msm LdtF toward elucidating class 6 structural and functional differences. We report class 6 Ldts have structurally divergent catalytic domains containing a 10-residue insertion near the active site and additionally determined that meropenem preferentially acylates LdtF. Our data demonstrate an evolutionary basis for mycobacterial Ldt multiplicity that lends support to the idea that paralogous Ldts serve nonredundant roles in vivo and suggests class 6 Ldts can be selectively targeted by specific carbapenem antibiotics.

L,D-Transpeptidase Specific Probe Reveals Spatial Activity of Peptidoglycan Cross-Linking

Peptidoglycan (PG) is a cross-linked, meshlike scaffold endowed with the strength to withstand the internal pressure of bacteria. Bacteria are known to heavily remodel their peptidoglycan stem peptides, yet little is known about the physiological impact of these chemical variations on peptidoglycan cross-linking. Furthermore, there are limited tools to study these structural variations, which can also have important implications on cell wall integrity and host immunity. Cross-linking of peptide chains within PG is an essential process, and its disruption thereof underpins the potency of several classes of antibiotics. Two primary cross-linking modes have been identified that are carried out by D,D-transpeptidases and L,D-transpeptidases (Ldts). The nascent PG from each enzymatic class is structurally unique, which results in different cross-linking configurations. Recent advances in PG cellular probes have been powerful in advancing the understanding of D,D-transpeptidation by Penicillin Binding Proteins (PBPs). In contrast, no cellular probes have been previously described to directly interrogate Ldt function in live cells. Herein, we describe a new class of Ldt-specific probes composed of structural analogs of nascent PG, which are metabolically incorporated into the PG scaffold by Ldts. With a panel of tetrapeptide PG stem mimics, we demonstrated that subtle modifications such as amidation of iso-Glu can control PG cross-linking. Ldt probes were applied to quantify and track the localization of Ldt activity in Enterococcus faecium, Mycobacterium smegmatis, and Mycobacterium tuberculosis. These results confirm that our Ldt probes are specific and suggest that the primary sequence of the stem peptide can control Ldt cross-linking levels. We anticipate that unraveling the interplay between Ldts and other cross-linking modalities may reveal the organization of the PG structure in relation to the spatial localization of cross-linking machineries.

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.

Coxiella burnetii RpoS Regulates Genes Involved in Morphological Differentiation and Intracellular Growth

Coxiella burnetii, the etiological agent of Q fever, undergoes a unique biphasic developmental cycle where bacteria transition from a replicating (exponential-phase) large cell variant (LCV) form to a nonreplicating (stationary-phase) small cell variant (SCV) form. The alternative sigma factor RpoS is an essential regulator of stress responses and stationary-phase physiology in several bacterial species, including Legionella pneumophila, which has a developmental cycle superficially similar to that of C. burnetii Here, we used a C. burnetii ΔrpoS mutant to define the role of RpoS in intracellular growth and SCV development. Growth yields following infection of Vero epithelial cells or THP-1 macrophage-like cells with the rpoS mutant in the SCV form, but not the LCV form, were significantly lower than that of wild-type bacteria. RNA sequencing and whole-cell mass spectrometry of the C. burnetii ΔrpoS mutant revealed that a substantial portion of the C. burnetii genome is regulated by RpoS during SCV development. Regulated genes include those involved in stress responses, arginine transport, peptidoglycan remodeling, and synthesis of the SCV-specific protein ScvA. Genes comprising the dot/icm locus, responsible for production of the Dot/Icm type 4B secretion system, were also dysregulated in the rpoS mutant. These data were corroborated with independent assays demonstrating that the C. burnetii ΔrpoS strain has increased sensitivity to hydrogen peroxide and carbenicillin and a thinner cell wall/outer membrane complex. Collectively, these results demonstrate that RpoS is an important regulator of genes involved in C. burnetii SCV development and intracellular growth.IMPORTANCE The Q fever bacterium Coxiella burnetii has spore-like environmental stability, a characteristic that contributes to its designation as a potential bioweapon. Stability is likely conferred by a highly resistant, small cell variant (SCV) stationary-phase form that arises during a biphasic developmental cycle. Here, we define the role of the alternative sigma factor RpoS in regulating genes associated with SCV development. Genes involved in stress responses, amino acid transport, cell wall remodeling, and type 4B effector secretion were dysregulated in the rpoS mutant. Cellular impairments included defects in intracellular growth, cell wall structure, and resistance to oxidants. These results support RpoS as a central regulator of the Coxiella developmental cycle and identify developmentally regulated genes involved in morphological differentiation.

Structural Basis for the Interaction and Processing of β-Lactam Antibiotics by l,d-Transpeptidase 3 (LdtMt3) from Mycobacterium tuberculosis

Targeting Mycobacterium tuberculosis peptidoglycans with β-lactam antibiotics represents a strategy to address increasing resistance to antitubercular drugs. β-Lactams inhibit peptidoglycan synthases such as l,d-transpeptidases, a group of carbapenem-sensitive enzymes that stabilize peptidoglycans through 3 → 3 cross-links. M. tuberculosis encodes five l,d-transpeptidases (Ldt_Mt1-5), of which Ldt_Mt3 is one of the less understood. Herein, we structurally characterized the apo and faropenem-acylated forms of Ldt_Mt3 at 1.3 and 1.8 Å resolution, respectively. These structures revealed a fold and catalytic diad similar to those of other Ldts_Mt enzymes, supporting its involvement in transpeptidation reactions despite divergences in active site size and charges. The Ldt_Mt3-faropenem structure indicated that faropenem is degraded after Cys-246 acylation, and possibly only a β-OH-butyrate or an acetyl group (C₂H₃O) covalently attached to the enzyme remains, an observation that strongly supports the notion that Ldt_Mt3 is inactivated by β-lactams. Docking simulations with intact β-lactams predicted key Ldt_Mt3 residues that interact with these antibiotics. We also characterized the heat of acylation involved in the binding and reaction of Ldt_Mt3 for ten β-lactams belonging to four different classes, and imipenem had the highest inactivation constant. This work provides key insights into the structure, binding mechanisms, and degradation of β-lactams by Ldt_Mt3, which may be useful for the development of additional β-lactams with potential antitubercular activity.

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.

High-Resolution Analysis of the Peptidoglycan Composition in Streptomyces coelicolor

The bacterial cell wall maintains cell shape and protects against bursting by turgor. A major constituent of the cell wall is peptidoglycan (PG), which is continuously modified to enable cell growth and differentiation through the concerted activity of biosynthetic and hydrolytic enzymes. Streptomycetes are Gram-positive bacteria with a complex multicellular life style alternating between mycelial growth and the formation of reproductive spores. This involves cell wall remodeling at apical sites of the hyphae during cell elongation and autolytic degradation of the vegetative mycelium during the onset of development and antibiotic production. Here, we show that there are distinct differences in the cross-linking and maturation of the PGs between exponentially growing vegetative hyphae and the aerial hyphae that undergo sporulation. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis identified over 80 different muropeptides, revealing that major PG hydrolysis takes place over the course of mycelial growth. Half of the dimers lacked one of the disaccharide units in transition-phase cells, most likely due to autolytic activity. The deacetylation of MurNAc to MurN was particularly pronounced in spores and strongly reduced in sporulation mutants with a deletion of bldD or whiG, suggesting that MurN is developmentally regulated. Altogether, our work highlights the dynamic and growth phase-dependent changes in the composition of the PG in StreptomycesIMPORTANCE Streptomycetes are bacteria with a complex lifestyle and are model organisms for bacterial multicellularity. From a single spore, a large multigenomic multicellular mycelium is formed, which differentiates to form spores. Programmed cell death is an important event during the onset of morphological differentiation. In this work, we provide new insights into the changes in the peptidoglycan composition and over time, highlighting changes over the course of development and between growing mycelia and spores. This revealed dynamic changes in the peptidoglycan when the mycelia aged, with extensive peptidoglycan hydrolysis and, in particular, an increase in the proportion of 3-3 cross-links. Additionally, we identified a muropeptide that accumulates predominantly in the spores and may provide clues toward spore development.

Mycobacterium abscessus and β-Lactams: Emerging Insights and Potential Opportunities

β-lactams, the most widely used class of antibiotics, are well-tolerated, and their molecular mechanisms of action against many bacteria are well-documented. Mycobacterium abscessus (Mab) is a highly drug-resistant rapidly-growing nontuberculous mycobacteria (NTM). Only in recent years have we started to gain insight into the unique relationship between β-lactams and their targets in Mab. In this mini-review, we summarize recent findings that have begun to unravel the molecular basis for overall efficacy of β-lactams against Mab and discuss emerging evidence that indicates that we have yet to harness the full potential of this antibiotic class to treat Mab infections.

Have we realized the full potential of β-lactams for treating drug-resistant TB?

β-lactams are the most widely used antibiotics and are effective against a spectrum of pathogenic bacteria. Here, we focus on the state-of-the-art understanding of the molecular underpinnings that determine the overall efficacy of β-lactams against TB and include historical perspectives of this antibiotic class against this ancient disease. We summarize literature that describes why earlier generations of β-lactams are ineffective and the potential promise of newer β-lactams that exhibit improved efficacy against TB. Emerging evidence warrants renewed consideration of newer β-lactams in regimens for treatment of drug-resistant TB. © 2018 IUBMB Life, 70(9):881-888, 2018.

Two dd-Carboxypeptidases from Mycobacterium smegmatis Affect Cell Surface Properties through Regulation of Peptidoglycan Cross-Linking and Glycopeptidolipids

During the peptidoglycan (PG) maturation of mycobacteria, the glycan strands are interlinked by both 3-3 (between two meso-diaminopimelic acids [meso-DAPs]) and 4-3 cross-links (between d-Ala and meso-DAP), though there is a predominance (60 to 80%) of 3-3 cross-links. The dd-carboxypeptidases (dd-CPases) act on pentapeptides to generate tetrapeptides that are used by ld-transpeptidases as substrates to form 3-3 cross-links. Therefore, dd-CPases play a crucial role in mycobacterial PG cross-link formation. However, the physiology of dd-CPases in mycobacteria is relatively unexplored. In this study, we deleted two dd-CPase genes, msmeg_2433 and msmeg_2432, both individually and in combination, from Mycobacterium smegmatis mc²155. Though the single dd-CPase gene deletions had no significant impact on the mycobacterial physiology, many interesting functional alterations were observed in the double-deletion mutant, viz, a predominance in PG cross-link formation was shifted from 3-3 cross-links to 4-3, cell surface glycopeptidolipid (GPL) expression was reduced, and susceptibility to β-lactams and antitubercular agents was enhanced. Moreover, the survival rate of the double mutant within murine macrophages was higher than that of the parent. Interestingly, the complementation with any one of the dd-CPase genes could restore the wild-type phenotype. In a nutshell, we infer that the altered ratio of 4-3 to 3-3 PG cross-links might have influenced the expression of surface GPLs, colony morphology, biofilm formation, drug susceptibility, and subsistence of the cells within macrophages.IMPORTANCE The glycan strands in mycobacterial peptidoglycan (PG) are interlinked by both 3-3 and 4-3 cross-links. The dd-CPases generate tetrapeptides by acting on the pentapeptides, and ld-transpeptidases use tetrapeptides as substrates to form 3-3 cross-links. In this study, we showed that simultaneous deletions of two dd-CPases alter the nature of PG cross-linking from 3-3 cross-links to 4-3 cross-links. The deletions subsequently decrease the expression of glycopeptidolipids (significant surface lipid present in many nontuberculous mycobacteria, including Mycobacterium smegmatis) and affect other physiological parameters, like cell morphology, growth rate, biofilm formation, antibiotic susceptibility, and survival within murine macrophages. Thus, unraveling the physiology of dd-CPases might help us design antimycobacterial therapeutics in the future.

β-lactam resistance: The role of low molecular weight penicillin binding proteins, β-lactamases and ld-transpeptidases in bacteria associated with respiratory tract infections

Disruption of peptidoglycan (PG) biosynthesis in the bacterial cell wall by β-lactam antibiotics has transformed therapeutic options for bacterial infections. These antibiotics target the transpeptidase domains in penicillin binding proteins (PBPs), which can be classified into high and low molecular weight (LMW) counterparts. While the essentiality of the former has been extensively demonstrated, the physiological roles of LMW PBPs remain poorly understood. Herein, we review the function of LMW PBPs, β-lactamases and ld-transpeptidases (Ldts) in pathogens associated with respiratory tract infections. More specifically, we explore their roles in mediating β-lactam resistance. Using a comparative genomics approach, we identified a high degree of genetic redundancy for LMW PBPs which retain the motifs, SxxN, SxN and KTG required for catalytic activity. Differences in domain architecture suggest distinct physiological roles, possibly related to bacterial cell cycle and/or adaptation to various environmental conditions. Many of the LMW PBPs play an important role in β-lactam resistance either through mutation or variation in abundance. In all of the bacterial genomes assessed, at least one β-lactamase homologue is present, suggesting that enzymatic degradation of β-lactams is a highly conserved resistance mechanism. Furthermore, the presence of Ldt homologues in the majority of species surveyed suggests that alternative PG crosslinking may further mediate β-lactam drug resistance. A deeper understanding of the interplay between these different mechanisms of β-lactam resistance will provide a framework for new therapeutics, which are urgently required given the rapid emergence of antimicrobial resistance. © 2018 IUBMB Life, 70(9):855-868, 2018.

Cell Wall Biogenesis During Elongation and Division in the Plant Pathogen Agrobacterium tumefaciens

A great diversity of bacterial cell shapes can be found in nature, suggesting that cell wall biogenesis is regulated both spatially and temporally. Although Agrobacterium tumefaciens has a rod-shaped morphology, the mechanisms underlying cell growth are strikingly different than other well-studied rod-shaped bacteria including Escherichia coli. Technological advances, such as the ability to deplete essential genes and the development of fluorescent D-amino acids, have enabled recent advances in our understanding of cell wall biogenesis during cell elongation and division of A. tumefaciens. In this review, we address how the field has evolved over the years by providing a historical overview of cell elongation and division in rod-shaped bacteria. Next, we summarize the current understanding of cell growth and cell division processes in A. tumefaciens. Finally, we highlight the need for further research to answer key questions related to the regulation of cell wall biogenesis in A. tumefaciens.

The RipA and RipB Peptidoglycan Endopeptidases Are Individually Nonessential to Mycobacterium smegmatis

Mycobacteria possess a series of Rip peptidoglycan endopeptidases that have been characterized in various levels of detail. The RipA and RipB proteins have been extensively studied and are DL-endopeptidases, and RipA has been considered essential to Mycobacterium smegmatis and Mycobacterium tuberculosis We show here that the ripA and ripB genes are individually dispensable in M. smegmatis and that at least one of the genes must be expressed for viability. We characterized strains carrying in-frame deletion mutations of ripA and ripB and found that both mutant strains exhibited increased susceptibility to a limited number of antibiotics and to detergent but that only the ΔripA mutant displayed hypersusceptibility to lysozyme. We also constructed and characterized ΔripD and ΔripAΔripD mutants and found that the single mutant had only an intermediate lysozyme hypersusceptibility phenotype compared to that of wild-type cells while loss of ripD in the ΔripA background partially rescued the antibiotic and lysozyme phenotypes of the ΔripA mutant.

IMPORTANCE

We show that the RipA endopeptidase, which has been considered essential for cell division in certain mycobacteria, is not essential but that at least it or a similar protein, RipB, must be expressed by the bacteria for viability. This work is the first description of strains carrying single deletion mutations of RipA, RipB, and a novel endopeptidase-like protein, RipD.

Transcriptional Profiling of Coxiella burnetii Reveals Extensive Cell Wall Remodeling in the Small Cell Variant Developmental Form

A hallmark of Coxiella burnetii, the bacterial cause of human Q fever, is a biphasic developmental cycle that generates biologically, ultrastructurally, and compositionally distinct large cell variant (LCV) and small cell variant (SCV) forms. LCVs are replicating, exponential phase forms while SCVs are non-replicating, stationary phase forms. The SCV has several properties, such as a condensed nucleoid and an unusual cell envelope, suspected of conferring enhanced environmental stability. To identify genetic determinants of the LCV to SCV transition, we profiled the C. burnetii transcriptome at 3 (early LCV), 5 (late LCV), 7 (intermediate forms), 14 (early SCV), and 21 days (late SCV) post-infection of Vero epithelial cells. Relative to early LCV, genes downregulated in the SCV were primarily involved in intermediary metabolism. Upregulated SCV genes included those involved in oxidative stress responses, arginine acquisition, and cell wall remodeling. A striking transcriptional signature of the SCV was induction (>7-fold) of five genes encoding predicted L,D transpeptidases that catalyze nonclassical 3-3 peptide cross-links in peptidoglycan (PG), a modification that can influence several biological traits in bacteria. Accordingly, of cross-links identified, muropeptide analysis showed PG of SCV with 46% 3-3 cross-links as opposed to 16% 3-3 cross-links for LCV. Moreover, electron microscopy revealed SCV with an unusually dense cell wall/outer membrane complex as compared to LCV with its clearly distinguishable periplasm and inner and outer membranes. Collectively, these results indicate the SCV produces a unique transcriptome with a major component directed towards remodeling a PG layer that likely contributes to Coxiella's environmental resistance.

Peptidoglycan synthesis in Mycobacterium tuberculosis is organized into networks with varying drug susceptibility

Peptidoglycan (PG), a complex polymer composed of saccharide chains cross-linked by short peptides, is a critical component of the bacterial cell wall. PG synthesis has been extensively studied in model organisms but remains poorly understood in mycobacteria, a genus that includes the important human pathogen Mycobacterium tuberculosis (Mtb). The principle PG synthetic enzymes have similar and, at times, overlapping functions. To determine how these are functionally organized, we carried out whole-genome transposon mutagenesis screens in Mtb strains deleted for ponA1, ponA2, and ldtB, major PG synthetic enzymes. We identified distinct factors required to sustain bacterial growth in the absence of each of these enzymes. We find that even the homologs PonA1 and PonA2 have unique sets of genetic interactions, suggesting there are distinct PG synthesis pathways in Mtb. Either PonA1 or PonA2 is required for growth of Mtb, but both genetically interact with LdtB, which has its own distinct genetic network. We further provide evidence that each interaction network is differentially susceptible to antibiotics. Thus, Mtb uses alternative pathways to produce PG, each with its own biochemical characteristics and vulnerabilities.

Loss of a Functionally and Structurally Distinct ld-Transpeptidase, LdtMt5, Compromises Cell Wall Integrity in Mycobacterium tuberculosis

The final step of peptidoglycan (PG) biosynthesis in bacteria involves cross-linking of peptide side chains. This step in Mycobacterium tuberculosis is catalyzed by ld- and dd-transpeptidases that generate 3→3 and 4→3 transpeptide linkages, respectively. M. tuberculosis PG is predominantly 3→3 cross-linked, and LdtMt2 is the dominant ld-transpeptidase. There are four additional sequence paralogs of LdtMt2 encoded by the genome of this pathogen, and the reason for this apparent redundancy is unknown. Here, we studied one of the paralogs, LdtMt5, and found it to be structurally and functionally distinct. The structures of apo-LdtMt5 and its meropenem adduct presented here demonstrate that, despite overall architectural similarity to LdtMt2, the LdtMt5 active site has marked differences. The presence of a structurally divergent catalytic site and a proline-rich C-terminal subdomain suggest that this protein may have a distinct role in PG metabolism, perhaps involving other cell wall-anchored proteins. Furthermore, M. tuberculosis lacking a functional copy of LdtMt5 displayed aberrant growth and was more susceptible to killing by crystal violet, osmotic shock, and select carbapenem antibiotics. Therefore, we conclude that LdtMt5 is not a functionally redundant ld-transpeptidase, but rather it serves a unique and important role in maintaining the integrity of the M. tuberculosis cell wall.

Targeting the cell wall of Mycobacterium tuberculosis: a molecular modeling investigation of the interaction of imipenem and meropenem with L,D-transpeptidase 2

The single crystal X-ray structure of the extracellular portion of the L,D-transpeptidase (ex-LdtMt2 - residues 120-408) enzyme was recently reported. It was observed that imipenem and meropenem inhibit activity of this enzyme, responsible for generating L,D-transpeptide linkages in the peptidoglycan layer of Mycobacterium tuberculosis. Imipenem is more active and isothermal titration calorimetry experiments revealed that meropenem is subjected to an entropy penalty upon binding to the enzyme. Herein, we report a molecular modeling approach to obtain a molecular view of the inhibitor/enzyme interactions. The average binding free energies for nine commercially available inhibitors were calculated using MM/GBSA and Solvation Interaction Energy (SIE) approaches and the calculated energies corresponded well with the available experimentally observed results. The method reproduces the same order of binding energies as experimentally observed for imipenem and meropenem. We have also demonstrated that SIE is a reasonably accurate and cost-effective free energy method, which can be used to predict carbapenem affinities for this enzyme. A theoretical explanation was offered for the experimental entropy penalty observed for meropenem, creating optimism that this computational model can serve as a potential computational model for other researchers in the field.