Biochemical characterisation of the chlamydial MurF ligase, and possible sequence of the chlamydial peptidoglycan pentapeptide stem

The MraY Inhibitor Muraymycin D2 and Its Derivatives Induce Enlarged Cells in Obligate Intracellular Chlamydia and Wolbachia and Break the Persistence Phenotype in Chlamydia

Chlamydial infections and diseases caused by filarial nematodes are global health concerns. However, treatment presents challenges due to treatment failures potentially caused by persisting Chlamydia and long regimens against filarial infections accompanied by low compliance. A new treatment strategy could be the targeting of the reduced peptidoglycan structures involved in cell division in the obligate intracellular bacteria Chlamydia and Wolbachia, the latter being obligate endosymbionts supporting filarial development, growth, and survival. Here, cell culture experiments with C. trachomatis and Wolbachia showed that the nucleoside antibiotics muraymycin and carbacaprazamycin interfere with bacterial cell division and induce enlarged, aberrant cells resembling the penicillin-induced persistence phenotype in Chlamydia. Enzymatic inhibition experiments with purified C. pneumoniae MraY revealed that muraymycin derivatives abolish the synthesis of the peptidoglycan precursor lipid I. Comparative in silico analyses of chlamydial and wolbachial MraY with the corresponding well-characterized enzyme in Aquifex aeolicus revealed a high degree of conservation, providing evidence for a similar mode of inhibition. Muraymycin D2 treatment eradicated persisting non-dividing C. trachomatis cells from an established penicillin-induced persistent infection. This finding indicates that nucleoside antibiotics may have additional properties that can break bacterial persistence.

The Expandables: Cracking the Staphylococcal Cell Wall for Expansion Microscopy

Expansion Microscopy (ExM) is a novel tool improving the resolution of fluorescence microscopy by linking the sample into a hydrogel that gets physically expanded in water. Previously, we have used ExM to visualize the intracellular Gram-negative pathogens Chlamydia trachomatis, Simkania negevensis, and Neisseria gonorrhoeae. Gram-positive bacteria have a rigid and thick cell wall that impedes classic expansion strategies. Here we developed an approach, which included a series of enzymatic treatments resulting in isotropic 4× expansion of the Gram-positive pathogen Staphylococcus aureus. We further demonstrate the suitability of the technique for imaging of planktonic bacteria as well as endocytosed, intracellular bacteria at a spatial resolution of approximately 60 nm with conventional confocal laser scanning microscopy.

Division without Binary Fission: Cell Division in the FtsZ-Less Chlamydia

Chlamydia is an obligate intracellular bacterial pathogen that has significantly reduced its genome size in adapting to its intracellular niche. Among the genes that Chlamydia has eliminated is ftsZ, encoding the central organizer of cell division that directs cell wall synthesis in the division septum. These Gram-negative pathogens have cell envelopes that lack peptidoglycan (PG), yet they use PG for cell division purposes. Recent research into chlamydial PG synthesis, components of the chlamydial divisome, and the mechanism of chlamydial division have significantly advanced our understanding of these processes in a unique and important pathogen. For example, it has been definitively confirmed that chlamydiae synthesize a canonical PG structure during cell division. Various studies have suggested and provided evidence that Chlamydia uses MreB to substitute for FtsZ in organizing and coordinating the divisome during division, components of which have been identified and characterized. Finally, as opposed to using an FtsZ-dependent binary fission process, Chlamydia employs an MreB-dependent polarized budding process to divide. A brief historical context for these key advances is presented along with a discussion of the current state of knowledge of chlamydial cell division.

Chlamydia: what is on the outside does matter

This review summarises major highlights on the structural biology of the chlamydial envelope. Chlamydiae are obligate intracellular bacteria, characterised by a unique biphasic developmental cycle. Depending on the stage of their lifecycle, they appear in the form of elementary or reticulate bodies. Since these particles have distinctive functions, it is not surprising that their envelope differs in lipid as well as in protein content. Vice versa, by identifying surface proteins, specific characteristics of the particles such as rigidity or immunogenicity may be deduced. Detailed information on the bacterial membranes will increase our understanding on the host-pathogen interactions chlamydiae employ to survive and grow and might lead to new strategies to battle chlamydial infections.

d-Amino Acid Derivatives as in Situ Probes for Visualizing Bacterial Peptidoglycan Biosynthesis

The bacterial cell wall is composed of membrane layers and a rigid yet flexible scaffold called peptidoglycan (PG). PG provides mechanical strength to enable bacteria to resist damage from the environment and lysis due to high internal turgor. PG also has a critical role in dictating bacterial cell morphology. The essential nature of PG for bacterial propagation, as well as its value as an antibiotic target, has led to renewed interest in the study of peptidoglycan biosynthesis. However, significant knowledge gaps remain that must be addressed before a clear understanding of peptidoglycan synthesis and dynamics is realized. For example, the enzymes involved in the PG biosynthesis pathway have not been fully characterized. Our understanding of PG biosynthesis has been frequently revamped by the discovery of novel enzymes or newly characterized functions of known enzymes. In addition, we do not clearly know how the respective activities of these enzymes are coordinated with each other and how they control the spatial and temporal dynamics of PG synthesis. The emergence of molecular probes and imaging techniques has significantly advanced the study PG synthesis and modification. Prior efforts utilized the specificity of PG-targeting antibiotics and proteins to develop PG-specific probes, such as fluorescent vancomycin and fluorescent wheat germ agglutinin. However, these probes suffer from limitations due to toxic effects toward bacterial cells and poor membrane permeability. To address these issues, we designed and introduced a family of novel molecular probes, fluorescent d-amino acids (FDAAs), which are covalently incorporated into PG through the activities of endogenous bacterial transpeptidases. Their high biocompatibility and PG specificity have made them powerful tools for labeling peptidoglycan. In addition, their enzyme-mediated incorporation faithfully reflects the activity of PG synthases, providing a direct in situ method for studying PG formation during the bacterial life cycle. In this Account, we describe our efforts directed at the development of FDAAs and their derivatives. These probes have enabled for the first time the ability to visualize PG synthesis in live bacterial cells and in real time. We summarize experimental evidence for FDAA incorporation into PG and the enzyme-mediated incorporation pathway. We demonstrate various applications of FDAAs, including bacterial morphology analyses, PG growth model studies, investigation of PG-enzyme correlation, in vitro PG synthase activity assays, and antibiotic inhibition tests. Finally, we discuss the current limitations of the probes and our ongoing efforts to improve them. We are confident that these probes will prove to be valuable tools that will enable the discovery of new antibiotic targets and expand the available arsenal directed at the public health threat posed by antibiotic resistance.

Deconstructing the Chlamydial Cell Wall

The evolutionary separated Gram-negative Chlamydiales show a biphasic life cycle and replicate exclusively within eukaryotic host cells. Members of the genus Chlamydia are responsible for many acute and chronic diseases in humans, and Chlamydia-related bacteria are emerging pathogens. We revisit past efforts to detect cell wall material in Chlamydia and Chlamydia-related bacteria in the context of recent breakthroughs in elucidating the underlying cellular and molecular mechanisms of the chlamydial cell wall biosynthesis. In this review, we also discuss the role of cell wall biosynthesis in chlamydial FtsZ-independent cell division and immune modulation. In the past, penicillin susceptibility of an invisible wall was referred to as the "chlamydial anomaly." In light of new mechanistic insights, chlamydiae may now emerge as model systems to understand how a minimal and modified cell wall biosynthetic machine supports bacterial cell division and how cell wall-targeting beta-lactam antibiotics can also act bacteriostatically rather than bactericidal. On the heels of these discussions, we also delve into the effects of other cell wall antibiotics in individual chlamydial lineages.

Imaging Bacterial Cell Wall Biosynthesis

Peptidoglycan is an essential component of the cell wall that protects bacteria from environmental stress. A carefully coordinated biosynthesis of peptidoglycan during cell elongation and division is required for cell viability. This biosynthesis involves sophisticated enzyme machineries that dynamically synthesize, remodel, and degrade peptidoglycan. However, when and where bacteria build peptidoglycan, and how this is coordinated with cell growth, have been long-standing questions in the field. The improvement of microscopy techniques has provided powerful approaches to study peptidoglycan biosynthesis with high spatiotemporal resolution. Recent development of molecular probes further accelerated the growth of the field, which has advanced our knowledge of peptidoglycan biosynthesis dynamics and mechanisms. Here, we review the technologies for imaging the bacterial cell wall and its biosynthesis activity. We focus on the applications of fluorescent d-amino acids, a newly developed type of probe, to visualize and study peptidoglycan synthesis and dynamics, and we provide direction for prospective research.

The Paradigms They Are a-Changin': past, present and future of PVC bacteria research

These are exciting times for PVC researchers! The PVC superphylum is composed of the bacterial phyla Planctomycetes, Verrucomicrobia, Chlamydiae (those three founders giving it its name), Lentisphaerae and Kirimatiellaeota as well as some uncultured candidate phyla, such as the Candidatus Omnitrophica (previously known as OP3). Despite early debates, most of the disagreements that surround this group of bacteria have been recently resolved. In this article, we review the history of the study of PVC bacteria, with a particular focus on the misinterpretations that emerged early in the field and their resolution. We begin with a historical perspective that describes the relevant facts of PVC research from the early times when they were not yet termed PVC. Those were controversial times and we refer to them as the “discovery age” of the field. We continue by describing new discoveries due to novel techniques and data that combined with the reinterpretations of old ones have contributed to solve most of the discordances and we refer to these times as the “illumination age” of PVC research. We follow by arguing that we are just entering the “golden age” of PVC research and that the future of this growing community is looking bright. We finish by suggesting a few of the directions that PVC researches might take in the future.

Evolutionary Cell Biology of Division Mode in the Bacterial Planctomycetes-Verrucomicrobia- Chlamydiae Superphylum

Bacteria from the Planctomycetes, Verrucomicrobia, and Chlamydiae (PVC) superphylum are exceptions to the otherwise dominant mode of division by binary fission, which is based on the interaction between the FtsZ protein and the peptidoglycan (PG) biosynthesis machinery. Some PVC bacteria are deprived of the FtsZ protein and were also thought to lack PG. How these bacteria divide is still one of the major mysteries of microbiology. The presence of PG has recently been revealed in Planctomycetes and Chlamydiae, and proteins related to PG synthesis have been shown to be implicated in the division process in Chlamydiae, providing important insights into PVC mechanisms of division. Here, we review the historical lack of observation of PG in PVC bacteria, its recent detection in two phyla and its involvement in chlamydial cell division. Based on the detection of PG-related proteins in PVC proteomes, we consider the possible evolution of the diverse division mechanisms in these bacteria. We conclude by summarizing what is known and what remains to be understood about the evolutionary cell biology of PVC division modes.

Pathogenic Chlamydia Lack a Classical Sacculus but Synthesize a Narrow, Mid-cell Peptidoglycan Ring, Regulated by MreB, for Cell Division

The peptidoglycan (PG) cell wall is a peptide cross-linked glycan polymer essential for bacterial division and maintenance of cell shape and hydrostatic pressure. Bacteria in the Chlamydiales were long thought to lack PG until recent advances in PG labeling technologies revealed the presence of this critical cell wall component in Chlamydia trachomatis. In this study, we utilize bio-orthogonal D-amino acid dipeptide probes combined with super-resolution microscopy to demonstrate that four pathogenic Chlamydiae species each possess a ≤ 140 nm wide PG ring limited to the division plane during the replicative phase of their developmental cycles. Assembly of this PG ring is rapid, processive, and linked to the bacterial actin-like protein, MreB. Both MreB polymerization and PG biosynthesis occur only in the intracellular form of pathogenic Chlamydia and are required for cell enlargement, division, and transition between the microbe’s developmental forms. Our kinetic, molecular, and biochemical analyses suggest that the development of this limited, transient, PG ring structure is the result of pathoadaptation by Chlamydia to an intracellular niche within its vertebrate host.

Pathogenic Chlamydia do not assemble their peptidoglycan (PG) cell wall in a classical, mesh-like sacculus, but instead apparently confine it to the mid-cell in the actively dividing, non-infectious form. We characterize the assembly and aging of this PG-ring and link its synthesis to MreB, an actin-like protein associated with lateral cell wall synthesis in bacteria. As PG is recognized by the host innate immune system, we hypothesize that the limited amount of PG synthesized by Chlamydia is an adaptation to the microbe’s intracellular lifestyle.

Identification and Partial Characterization of a Novel UDP-N-Acetylenolpyruvoylglucosamine Reductase/UDP-N-Acetylmuramate:l-Alanine Ligase Fusion Enzyme from Verrucomicrobium spinosum DSM 4136(T)

The enzymes involved in synthesizing the bacterial cell wall are attractive targets for the design of antibacterial compounds, since this pathway is essential for bacteria and is absent in animals, particularly humans. A survey of the genome of a bacterium that belongs to the phylum Verrucomicrobia, the closest free-living relative to bacteria from the Chlamydiales phylum, shows genetic evidence that Verrucomicrobium spinosum possesses a novel fusion open reading frame (ORF) annotated by the locus tag (VspiD_010100018130). The ORF, which is predicted to encode the enzymes UDP-N-acetylenolpyruvoylglucosamine reductase (MurB) and UDP-N-acetylmuramate:l-alanine ligase (MurC) that are involved in the cytoplasmic steps of peptidoglycan biosynthesis, was cloned. In vivo analyses using functional complementation showed that the fusion gene was able to complement Escherichia coli murB and murC temperature sensitive mutants. The purified recombinant fusion enzyme (MurB/C_Vs) was shown to be endowed with UDP-N-acetylmuramate:l-alanine ligase activity. In vitro analyses demonstrated that the latter enzyme had a pH optimum of 9.0, a magnesium optimum of 10 mM and a temperature optimum of 44–46°C. Its apparent K_m values for ATP, UDP-MurNAc, and l-alanine were 470, 90, and 25 μM, respectively. However, all attempts to demonstrate an in vitro UDP-N-acetylenolpyruvoylglucosamine reductase (MurB) activity were unsuccessful. Lastly, Hidden Markov Model-based similarity search and phylogenetic analysis revealed that this fusion enzyme could only be identified in specific lineages within the Verrucomicrobia phylum.

Structural characterization of muropeptides from Chlamydia trachomatis peptidoglycan by mass spectrometry resolves "chlamydial anomaly"

The "chlamydial anomaly," first coined by James Moulder, describes the inability of researchers to detect or purify peptidoglycan (PG) from pathogenic Chlamydiae despite genetic and biochemical evidence and antibiotic susceptibility data that suggest its existence. We recently detected PG in Chlamydia trachomatis by a new metabolic cell wall labeling method, however efforts to purify PG from pathogenic Chlamydiae have remained unsuccessful. Pathogenic chlamydial species are known to activate nucleotide-binding oligomerization domain-containing protein 2 (NOD2) innate immune receptors by as yet uncharacterized ligands, which are presumed to be PG fragments (muramyl di- and tripeptides). We used the NOD2-dependent activation of NF-κB by C. trachomatis-infected cell lysates as a biomarker for the presence of PG fragments within specific lysate fractions. We designed a new method of muropeptide isolation consisting of a double filtration step coupled with reverse-phase HPLC fractionation of Chlamydia-infected HeLa cell lysates. Fractions that displayed NOD2 activity were analyzed by electrospray ionization mass spectrometry, confirming the presence of muramyl di- and tripeptides in Chlamydia-infected cell lysate fractions. Moreover, the mass spectrometry data of large muropeptide fragments provided evidence that transpeptidation and transglycosylation reactions occur in pathogenic Chlamydiae. These results reveal the composition of chlamydial PG and disprove the "glycanless peptidoglycan" hypothesis.

The role of peptidoglycan in chlamydial cell division: towards resolving the chlamydial anomaly

Chlamydiales are obligate intracellular bacteria including some important pathogens causing trachoma, genital tract infections and pneumonia, among others. They share an atypical division mechanism, which is independent of an FtsZ homologue. However, they divide by binary fission, in a process inhibited by penicillin derivatives, causing the formation of an aberrant form of the bacteria, which is able to survive in the presence of the antibiotic. The paradox of penicillin sensitivity of chlamydial cells in the absence of detectable peptidoglycan (PG) was dubbed the chlamydial anomaly, since no PG modified by enzymes (Pbps) that are the usual target of penicillin could be detected in Chlamydiales. We review here the recent advances in this field with the first direct and indirect evidences of PG-like material in both Chlamydiaceae and Chlamydia-related bacteria. Moreover, PG biosynthesis is required for proper localization of the newly described septal proteins RodZ and NlpD. Taken together, these new results set the stage for a better understanding of the role of PG and septal proteins in the division mechanism of Chlamydiales and illuminate the long-standing chlamydial anomaly. Moreover, understanding the chlamydial division mechanism is critical for the development of new antibiotics for the treatment of chlamydial chronic infections.

Discovery of chlamydial peptidoglycan reveals bacteria with murein sacculi but without FtsZ

Chlamydiae are important pathogens and symbionts, with unique cell biology features. They lack the cell-division protein FtsZ, which functions in maintaining cell shape and orchestrating cell division in almost all other bacteria. In addition, the existence of peptidoglycan (PG) in chlamydial cell envelopes has been highly controversial. Using electron cryotomography, mass spectrometry and fluorescent labeling dyes, here we show that some environmental chlamydiae have cell-wall sacculi consisting of an unusual PG type. Treatment with fosfomycin (a PG synthesis inhibitor) leads to lower infection rates and aberrant cell shapes, suggesting that PG synthesis is crucial for the chlamydial life cycle. Our findings demonstrate for the first time the presence of PG in a member of the Chlamydiae. They also present a unique example of a bacterium with a PG sacculus but without FtsZ, challenging the current hypothesis that it is the absence of a cell wall that renders FtsZ non-essential.

AmiA is a penicillin target enzyme with dual activity in the intracellular pathogen Chlamydia pneumoniae

Intracellular Chlamydiaceae do not need to resist osmotic challenges and a functional cell wall was not detected in these pathogens. Nevertheless, a recent study revealed evidence for circular peptidoglycan-like structures in Chlamydiaceae and penicillin inhibits cytokinesis, a phenomenon known as the chlamydial anomaly. Here, by characterizing a cell wall precursor-processing enzyme, we provide insights into the mechanisms underlying this mystery. We show that AmiA from Chlamydia pneumoniae separates daughter cells in an Escherichia coli amidase mutant. Contrary to homologues from free-living bacteria, chlamydial AmiA uses lipid II as a substrate and has dual activity, acting as an amidase and a carboxypeptidase. The latter function is penicillin sensitive and assigned to a penicillin-binding protein motif. Consistent with the lack of a regulatory domain in AmiA, chlamydial CPn0902, annotated as NlpD, is a carboxypeptidase, rather than an amidase activator, which is the case for E. coli NlpD. Functional conservation of AmiA implicates a role in cytokinesis and host response modulation.

Penicillin inhibits growth of chlamydial pathogens despite their lack of a conventional peptidoglycan cell wall. Here the authors report that the chlamydial amidase, AmiA, which can rescue cell division defects of an E. coli amiA mutant, has dual activity as a penicillin sensitive, lipid II-targetting carboxypeptidase.

Characterization of serine hydroxymethyltransferase GlyA as a potential source of D-alanine in Chlamydia pneumoniae

For intracellular Chlamydiaceae, there is no need to withstand osmotic challenges, and a functional cell wall has not been detected in these pathogens so far. Nevertheless, penicillin inhibits cell division in Chlamydiaceae resulting in enlarged aberrant bodies, a phenomenon known as chlamydial anomaly. D-alanine is a unique and essential component in the biosynthesis of bacterial cell walls. In free-living bacteria like Escherichia coli, penicillin-binding proteins such as monofunctional transpeptidases PBP2 and PBP3, the putative targets of penicillin in Chlamydiaceae, cross-link adjacent peptidoglycan strands via meso-diaminopimelic acid and D-Ala-D-Ala moieties of pentapeptide side chains. In the absence of genes coding for alanine racemase Alr and DadX homologs, the source of D-Ala and thus the presence of substrates for PBP2 and PBP3 activity in Chlamydiaceae has puzzled researchers for years. Interestingly, Chlamydiaceae genomes encode GlyA, a serine hydroxymethyltransferase that has been shown to exhibit slow racemization of D- and L-alanine as a side reaction in E. coli. We show that GlyA from Chlamydia pneumoniae can serve as a source of D-Ala. GlyA partially reversed the D-Ala auxotrophic phenotype of an E. coli racemase double mutant. Moreover, purified chlamydial GlyA had racemase activity on L-Ala in vitro and was inhibited by D-cycloserine, identifying GlyA, besides D-Ala ligase MurC/Ddl, as an additional target of this competitive inhibitor in Chlamydiaceae. Proof of D-Ala biosynthesis in Chlamydiaceae helps to clarify the structure of cell wall precursor lipid II and the role of chlamydial penicillin-binding proteins in the development of non-dividing aberrant chlamydial bodies and persistence in the presence of penicillin.

A new metabolic cell-wall labelling method reveals peptidoglycan in Chlamydia trachomatis

Peptidoglycan (PG), an essential structure in the cell walls of the vast majority of bacteria, is critical for division and maintaining cell shape and hydrostatic pressure. Bacteria comprising the Chlamydiales were thought to be one of the few exceptions. Chlamydia harbour genes for PG biosynthesis and exhibit susceptibility to 'anti-PG' antibiotics, yet attempts to detect PG in any chlamydial species have proven unsuccessful (the 'chlamydial anomaly'). We used a novel approach to metabolically label chlamydial PG using d-amino acid dipeptide probes and click chemistry. Replicating Chlamydia trachomatis were labelled with these probes throughout their biphasic developmental life cycle, and the results of differential probe incorporation experiments conducted in the presence of ampicillin are consistent with the presence of chlamydial PG-modifying enzymes. These findings culminate 50 years of speculation and debate concerning the chlamydial anomaly and are the strongest evidence so far that chlamydial species possess functional PG.

Biochemical characterization of UDP-N-acetylmuramoyl-L-alanyl-D-glutamate: meso-2,6-diaminopimelate ligase (MurE) from Verrucomicrobium spinosum DSM 4136(T.)

Verrucomicrobium spinosum is a Gram-negative bacterium that is related to bacteria from the genus Chlamydia. The bacterium is pathogenic towards Drosophila melanogaster and Caenorhabditis elegans, using a type III secretion system to facilitate pathogenicity. V. spinosum employs the recently discovered l,l-diaminopimelate aminotransferase biosynthetic pathway to generate the bacterial cell wall and protein precursors diaminopimelate and lysine. A survey of the V. spinosum genome provides evidence that the bacterium should be able to synthesize peptidoglycan de novo, since all of the necessary genes are present. The enzyme UDP-N-acetylmuramoyl-l-alanyl-d-glutamate: meso-2,6-diaminopimelate ligase (MurE) (E.C. 6.3.2.15) catalyzes a reaction in the cytoplasmic step of peptidoglycan biosynthesis by adding the third amino acid residue to the peptide stem. The murE ortholog from V. spinosum (murE_Vs) was cloned and was shown to possess UDP-MurNAc-l-Ala-d-Glu:meso-2,6-diaminopimelate ligase activity in vivo using functional complementation. In vitro analysis using the purified recombinant enzyme demonstrated that MurE_Vs has a pH optimum of 9.6 and a magnesium optimum of 30 mM. meso-Diaminopimelate was the preferred substrate with a K_m of 17 µM, when compared to other substrates that are structurally related. Sequence alignment and structural analysis using homology modeling suggest that key residues that make up the active site of the enzyme are conserved in MurE_Vs. Our kinetic analysis and structural model of MurE_Vs is consistent with other MurE enzymes from Gram-negative bacteria that have been characterized. To verify that V. spinosum incorporates diaminopimelate into its cell wall, we purified peptidoglycan from a V. spinosum culture; analysis revealed the presence of diaminopimelate, consistent with that of a bona fide peptidoglycan from Gram-negative bacteria.

Crystallization and preliminary X-ray analysis of a UDP-MurNAc-tripeptide D-alanyl-D-alanine-adding enzyme (PaMurF) from Pseudomonas aeruginosa

The ATP-dependent UDP-MurNAc-tripeptide:D-Ala-D-Ala ligase MurF catalyses the last step in the cytoplasmic phase of peptidoglycan biosynthesis, which is critical in the formation of the bacterial cell wall and in the recycling of peptidoglycan intermediates. In this study, the crystallization of MurF from the Gram-negative pathogen Pseudomonas aeruginosa in the presence of its UDP-MurNAc-tripeptide substrate is reported. The crystals belonged to space group P212121, with unit-cell parameters a = 57.81, b = 87.29, c = 92.61 Å, and data were collected to 1.92 Å resolution, allowing study of the enzyme in the substrate-liganded form for the first time.

Toll-like receptor 2-dependent activity of native major outer membrane protein proteosomes of Chlamydia trachomatis

Chlamydia trachomatis is the most common sexually transmitted bacterial pathogen and the etiologic agent of blinding trachoma. Intracellular signaling pathways leading to host cell inflammation and innate immunity to Chlamydia include those mediated by Toll-like receptors (TLRs) and nucleotide binding oligomerization domain 1 (Nod1) protein. In epithelial cells, TLR-dependent signaling contributes to local immune responses via induction of inflammatory mediators. There is evidence that TLR3, TLR4, and, particularly, TLR2 are critical for Chlamydia-mediated host cell activation and pathology. Despite the importance of TLR2, major chlamydial TLR2 antigens have not been identified so far. Numerous bacterial porins are known TLR2 agonists, i.e., porins from Neisseriae, Shigella, Salmonella, Haemophilus influenzae, and Fusobacterium nucleatum, which share structural and functional similarities with the chlamydial major outer membrane protein (MOMP), a strong antigen candidate for a potential vaccine against C. trachomatis. We describe the ability of purified, detergent-free MOMP to signal via TLR2 in vitro in TLR-overexpressing cells and TLR2-competent human reproductive tract epithelial cell lines. Using MOMP formed in pure protein micelles (proteosomes), we show the induction of TLR2-dependent interleukin-8 (IL-8) and IL-6 secretion in vitro, the involvement of TLR1 as a TLR2 coreceptor, and the activation of both NF-κB and mitogen-activated protein (MAP) kinase intracellular pathways. Interestingly, MOMP proteosomes induce cytokine secretion in endocervical epithelial cells (End/E6E7) but not in urethral epithelial cells (THUECs). A detailed understanding of the TLR2-dependent molecular mechanisms that characterize the effect of MOMP proteosomes on host cells may provide new insights for its successful development as an immunotherapeutic target against Chlamydia.

MurD enzymes from different bacteria: evaluation of inhibitors

D-Glutamic acid-adding enzyme (MurD ligase) catalyses the addition of D-glutamic acid to UDP-N-acetylmuramoyl-L-alanine, an essential cytoplasmic step in the pathway for bacterial cell-wall peptidoglycan synthesis. As such, it represents an important antibacterial drug-discovery target enzyme. Recently, several series of compounds have been synthesised and found to inhibit MurD from Escherichia coli, the best one having an IC(50) value of 8 μM. In the present work, we have tested 20 of these compounds against the MurD enzymes from Staphylococcus aureus, Streptococcus pneumoniae, Borrelia burgdorferi and Mycobacterium tuberculosis. Most of the E. coli MurD inhibitors appeared less efficient against the four other orthologues. This divergent result can be explained by the differences in amino acid sequences and topologies of the active sites of the MurD ligases studied.