Expression and characterization of a Mycoplasma genitalium glycosyltransferase in membrane glycolipid biosynthesis: potential target against mycoplasma infections

Engineering Mycoplasma pneumoniae to bypass the association with Guillain-Barré syndrome

A non-pathogenic Mycoplasma pneumoniae-based chassis is leading the development of live biotherapeutic products (LBPs) for respiratory diseases. However, reports connecting Guillain-Barré syndrome (GBS) cases to prior M. pneumoniae infections represent a concern for exploiting such a chassis. Galactolipids, especially galactocerebroside (GalCer), are considered the most likely M. pneumoniae antigens triggering autoimmune responses associated with GBS development. In this work, we generated different strains lacking genes involved in galactolipids biosynthesis. Glycolipid profiling of the strains demonstrated that some mutants show a complete lack of galactolipids. Cross-reactivity assays with sera from GBS patients with prior M. pneumoniae infection showed that certain engineered strains exhibit reduced antibody recognition. However, correlation analyses of these results with the glycolipid profile of the engineered strains suggest that other factors different from GalCer contribute to sera recognition, including total ceramide levels, dihexosylceramide (DHCer), and diglycosyldiacylglycerol (DGDAG). Finally, we discuss the best candidate strains as potential GBS-free Mycoplasma chassis.

The Cell Wall, Cell Membrane and Virulence Factors of Staphylococcus aureus and Their Role in Antibiotic Resistance

Antibiotic resistant strains of bacteria are a serious threat to human health. With increasing antibiotic resistance in common human pathogens, fewer antibiotics remain effective against infectious diseases. Staphylococcus aureus is a pathogenic bacterium of particular concern to human health as it has developed resistance to many of the currently used antibiotics leaving very few remaining as effective treatment. Alternatives to conventional antibiotics are needed for treating resistant bacterial infections. A deeper understanding of the cellular characteristics of resistant bacteria beyond well characterized resistance mechanisms can allow for increased ability to properly treat them and to potentially identify targetable changes. This review looks at antibiotic resistance in S aureus in relation to its cellular components, the cell wall, cell membrane and virulence factors. Methicillin resistant S aureus bacteria are resistant to most antibiotics and some strains have even developed resistance to the last resort antibiotics vancomycin and daptomycin. Modifications in cell wall peptidoglycan and teichoic acids are noted in antibiotic resistant bacteria. Alterations in cell membrane lipids affect susceptibility to antibiotics through surface charge, permeability, fluidity, and stability of the bacterial membrane. Virulence factors such as adhesins, toxins and immunomodulators serve versatile pathogenic functions in S aureus. New antimicrobial strategies can target cell membrane lipids and virulence factors including anti-virulence treatment as an adjuvant to traditional antibiotic therapy.

Pathogenicity and virulence of Mycoplasma genitalium: Unraveling Ariadne's Thread

Mycoplasma genitalium, a pathogen from class Mollicutes, has been linked to sexually transmitted diseases and sparked widespread concern. To adapt to its environment, M. genitalium has evolved specific adhesins and motility mechanisms that allow it to adhere to and invade various eukaryotic cells, thereby causing severe damage to the cells. Even though traditional exotoxins have not been identified, secreted nucleases or membrane lipoproteins have been shown to cause cell death and inflammatory injury in M. genitalium infection. However, as both innate and adaptive immune responses are important for controlling infection, the immune responses that develop upon infection do not necessarily eliminate the organism completely. Antigenic variation, detoxifying enzymes, immunoglobulins, neutrophil extracellular trap-degrading enzymes, cell invasion, and biofilm formation are important factors that help the pathogen overcome the host defence and cause chronic infections in susceptible individuals. Furthermore, M. genitalium can increase the susceptibility to several sexually transmitted pathogens, which significantly complicates the persistence and chronicity of M. genitalium infection. This review aimed to discuss the virulence factors of M. genitalium to shed light on its complex pathogenicity and pathogenesis of the infection.

Characterization of a Glycolipid Synthase Producing α-Galactosylceramide in Bacteroides fragilis

Glycolipids are complex molecules involved in important cellular processes. Among them, the glycosphingolipid α-galactosylceramide has proven to be of interest in biomedicine for its immunostimulatory capabilities. Given its structural requirements, the use of ceramide glycosyltransferase enzymes capable of synthesizing this molecule under in vivo or in vitro conditions is a potential production strategy. Several GT4 enzymes from Bacteroides fragilis were considered as potential candidates in addition to the known BF9343_3149, but only this one showed glycolipid synthase activity. The enzyme was expressed as a SUMO fusion protein to produce soluble protein. It is a non-processive glycosyltransferase that prefers UDP-Gal over UDP-Glc as a donor substrate, and maximum activity was found at pH 7.3 and around 30-35 °C. It does not require metal cations for activity as other GT4 enzymes, but Zn²⁺ inactivates the enzyme. The reaction occurs when the ceramide lipid acceptor is solubilized with BSA (100% conversion) but not when it is presented in mixed micelles, and anionic lipids do not increase activity, as in other membrane-associated glycolipid synthases. Further protein engineering to increase stability and activity can make feasible the enzymatic synthesis of α-GalCer for biomedical applications.

From the banal to the bizarre: unravelling immune recognition and response to microbial lipids

Microbes produce a rich array of lipidic species that through their location in the cell wall and ability to mingle with host lipids represent a privileged class of immune-active molecules. Lipid-sensing immunity recognizes microbial lipids from pathogens and commensals causing immune responses. Yet microbial lipids are often heterogeneous, in limited supply and in some cases their structures are incompletely defined. Total synthesis can assist in structural determination, overcome supply issues, and provide access to high-purity, homogeneous samples and analogues. This account highlights synthetic approaches to lipidic species from pathogenic and commensal bacteria and fungi that have supported immunological studies involving lipid sensing through the pattern recognition receptor Mincle and cell-mediated immunity through the CD1-T cell axis.

Chemical Synthesis of Cell Wall Constituents of Mycobacterium tuberculosis

The pathogen Mycobacterium tuberculosis (Mtb), causing tuberculosis disease, features an extraordinary thick cell envelope, rich in Mtb-specific lipids, glycolipids, and glycans. These cell wall components are often directly involved in host–pathogen interaction and recognition, intracellular survival, and virulence. For decades, these mycobacterial natural products have been of great interest for immunology and synthetic chemistry alike, due to their complex molecular structure and the biological functions arising from it. The synthesis of many of these constituents has been achieved and aided the elucidation of their function by utilizing the synthetic material to study Mtb immunology. This review summarizes the synthetic efforts of a quarter century of total synthesis and highlights how the synthesis layed the foundation for immunological studies as well as drove the field of organic synthesis and catalysis to efficiently access these complex natural products.

A Glycolipid Glycosyltransferase with Broad Substrate Specificity from the Marine Bacterium "Candidatus Pelagibacter sp." Strain HTCC7211

In the marine environment, phosphorus availability significantly affects the lipid composition in many cosmopolitan marine heterotrophic bacteria, including members of the SAR11 clade and the Roseobacter clade. Under phosphorus stress conditions, nonphosphorus sugar-containing glycoglycerolipids are substitutes for phospholipids in these bacteria. Although these glycoglycerolipids play an important role as surrogates for phospholipids under phosphate deprivation, glycoglycerolipid synthases in marine microbes are poorly studied. In the present study, we biochemically characterized a glycolipid glycosyltransferase (GT_cp) from the marine bacterium “Candidatus Pelagibacter sp.” strain HTCC7211, a member of the SAR11 clade. Our results showed that GT_cp is able to act as a multifunctional enzyme by synthesizing different glycoglycerolipids with UDP-glucose, UDP-galactose, or UDP-glucuronic acid as sugar donors and diacylglycerol (DAG) as the acceptor. Analyses of enzyme kinetic parameters demonstrated that Mg²⁺ notably changes the enzyme’s affinity for UDP-glucose, which improves its catalytic efficiency. Homology modeling and mutational analyses revealed binding sites for the sugar donor and the diacylglycerol lipid acceptor, which provided insights into the retaining mechanism of GT_cp with its GT-B fold. A phylogenetic analysis showed that GT_cp and its homologs form a group in the GT4 glycosyltransferase family. These results not only provide new insights into the glycoglycerolipid synthesis mechanism in lipid remodeling but also describe an efficient enzymatic tool for the future synthesis of bioactive molecules.

IMPORTANCE The bilayer formed by membrane lipids serves as the containment unit for living microbial cells. In the marine environment, it has been firmly established that phytoplankton and heterotrophic bacteria can replace phospholipids with nonphosphorus sugar-containing glycoglycerolipids in response to phosphorus limitation. However, little is known about how these glycoglycerolipids are synthesized. Here, we determined the biochemical characteristics of a glycolipid glycosyltransferase (GT_cp) from the marine bacterium “Candidatus Pelagibacter sp.” strain HTCC7211. GT_cp and its homologs form a group in the GT4 glycosyltransferase family and can synthesize neutral glycolipids (monoglucosyl-1,2-diacyl-sn-glycerol [MGlc-DAG] and monogalactosyl [MGal]-DAG) and monoglucuronic acid diacylglycerol (MGlcA-DAG). We also uncovered the key residues for DAG binding through molecular docking, site-direct mutagenesis, and subsequent enzyme activity assays. Our data provide new insights into the glycoglycerolipid synthesis mechanism in lipid remodeling.

Galactocerebroside biosynthesis pathways of Mycoplasma species: an antigen triggering Guillain-Barré-Stohl syndrome

Infection by Mycoplasma pneumoniae has been identified as a preceding factor of Guillain-Barré-Stohl syndrome. The Guillain-Barré-Stohl syndrome is triggered by an immune reaction against the major glycolipids and it has been postulated that M. pneumoniae infection triggers this syndrome due to bacterial production of galactocerebroside. Here, we present an extensive comparison of 224 genome sequences from 104 Mycoplasma species to characterize the genetic determinants of galactocerebroside biosynthesis. Hidden Markov models were used to analyse glycosil transferases, leading to identification of a functional protein domain, termed M2000535 that appears in about a third of the studied genomes. This domain appears to be associated with a potential UDP-glucose epimerase, which converts UDP-glucose into UDP-galactose, a main substrate for the biosynthesis of galactocerebroside. These findings clarify the pathogenic mechanisms underlining the triggering of Guillain-Barré-Stohl syndrome by M. pneumoniae infections.

Stereoselective Synthesis of Diglycosyl Diacylglycerols with Glycosyl Donors Bearing a β-Stereodirecting 2,3-Naphthalenedimethyl Protecting Group

Diglycosyl diacylglycerols (DGDGs) are major components of Gram-positive bacterial plasma membranes and are involved in the immune response systems. The chemical synthesis of DGDGs has been highly demanded, as it will allow the elucidation of their biological functions at the molecular level. In this study, we have developed a novel β-stereodirecting 2,3-naphthalenedimethyl (NapDM) protecting group that is orthogonal to protecting groups commonly used in oligosaccharide synthesis. The NapDM group can be easily cleaved under TFA-mediated acidic conditions. Futhermore, we demonstrated the application of this protecting group to an acyl protecting-group-free strategy by utilizing the NapDM group for the synthesis of DGDGs. This strategy features the use of the β-stereodirecting NapDM group as an acid-cleavable permanent protecting group and late-stage glycosylation of monoglycosyl diacylglycerol acceptors, enabling the stereoselective synthesis of three different bacterial DGDGs with unsaturated fatty acid chain(s).

Unraveling the complex enzymatic machinery making a key galactolipid in chloroplast membrane: a multiscale computer simulation

Chloroplast membranes have a high content of the uncharged galactolipids monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG). These galactolipids are essential for the biogenesis of plastids and functioning of the photosynthetic machinery. A monotopic glycosyltransferase, monogalactosyldiacylglycerol synthase synthesizes the bulk of MGDG. It is embedded in the outer leaflet of the inner envelope membrane of chloroplasts. The protein transfers a galactose residue from UDP-galactose to diacylglycerol (DAG); it needs anionic lipids such as phosphatidylglycerol (PG) to be active. The intricacy of the organization and the process of active complex assembly and synthesis have been investigated at the Coarse-Grained and All-Atom of computer simulation levels to cover large spatial and temporal scales. The following self-assembly process and catalytic events can be drawn; (1) in the membrane, in the absence of protein, there is a spontaneous formation of PG clusters to which DAG molecules associate, (2) a reorganization of the clusters occurs in the vicinity of the protein once inserted in the membrane, (3) an accompanying motion of the catalytic domain of the protein brings DAG in the proper position for the formation of the active complex MGD1/UDP-Gal/DAG/PG for which an atomistic model of interaction is proposed.

Metabolic engineering for glycoglycerolipids production in E. coli: Tuning phosphatidic acid and UDP-glucose pathways

Glycolipids are target molecules in biotechnology and biomedicine as biosurfactants, biomaterials and bioactive molecules. An engineered E. coli strain for the production of glycoglycerolipids (GGL) used the MG517 glycolipid synthase from M. genitalium for glucosyl transfer from UDPGlc to diacylglycerol acceptor (Mora-Buyé et al., 2012). The intracellular diacylglycerol pool proved to be the limiting factor for GGL production. Here we designed different metabolic engineering strategies to enhance the availability of precursor substrates for the glycolipid synthase by modulating fatty acids, acyl donor and phosphatidic acid biosynthesis. Knockouts of tesA, fadE and fabR genes involved in fatty acids degradation, overexpression of the transcriptional regulator FadR, the acyltransferases PlsB and C, and the pyrophosphatase Cdh for phosphatidic acid biosynthesis, as well as the phosphatase PgpB for conversion to diacylglycerol were explored with the aim of improving GGL titers. Among the different engineered strains, the ΔtesA strain co-expressing MG517 and a fusion PlsCxPgpB protein was the best producer, with a 350% increase of GGL titer compared to the parental strain expressing MG517 alone. Attempts to boost UDPGlc availability by overexpressing the uridyltransferase GalU or knocking out the UDP-sugar diphosphatase encoding gene ushA did not further improve GGL titers. Most of the strains produced GGL containing a variable number of glucosyl units from mono-to tetra-saccharides. Interestingly, the strains co-expressing Cdh showed a shift in the GGL profile towards the diglucosylated lipid (up to 80% of total GGLs) whereas the strains with a fadR knockout presented a higher amount of unsaturated acyl chains. In all cases, GGL production altered the lipidic composition of the E. coli membrane, observing that GGL replace phosphatidylethanolamine to maintain the overall membrane charge balance.

Essential Mycoplasma Glycolipid Synthase Adheres to the Cell Membrane by Means of an Amphipathic Helix

Because of the lack of cell wall, Micoplasma species require a fine control of membrane fluidity and integrity. mg517 is an essential gene of Mycoplasma genitalium responsible for the biosynthesis of membrane glycoglycerolipids. It encodes for a unique glycosyltransferase (MG517) with processive activity, transferring activated glycosyl donors to either nude diacylglycerol or already glycosylated diacylglycerol. This dual activity, asserted to different enzymes in other species, is sensitive to and regulated by the presence of anionic lipid vesicles in vitro. We present here a computational model of the C-terminus domain of MG517 that complements a previous structural model of the N-terminus domain. By means of sequence analysis, molecular dynamics and metadynamics simulations, we have identified a short α-helix at the apical C-terminus of MG517 with clear amphipathic character. Binding to a membrane model is thermodynamically favored which suggests that this structural element guides the adhesion of MG517 to the cell membrane. We have experimentally verified that truncation of part of this helix causes a substantial reduction of glycoglycerolipids synthesis. The model proposes that MG517 recognizes and binds the diacylglycerol substrate embedded in the membrane by means of this α-helix at the C-terminus together with a previously identified binding pocket at the N-terminus.

Essential metabolism for a minimal cell

JCVI-syn3A, a robust minimal cell with a 543 kbp genome and 493 genes, provides a versatile platform to study the basics of life. Using the vast amount of experimental information available on its precursor, Mycoplasma mycoides capri, we assembled a near-complete metabolic network with 98% of enzymatic reactions supported by annotation or experiment. The model agrees well with genome-scale in vivo transposon mutagenesis experiments, showing a Matthews correlation coefficient of 0.59. The genes in the reconstruction have a high in vivo essentiality or quasi-essentiality of 92% (68% essential), compared to 79% in silico essentiality. This coherent model of the minimal metabolism in JCVI-syn3A at the same time also points toward specific open questions regarding the minimal genome of JCVI-syn3A, which still contains many genes of generic or completely unclear function. In particular, the model, its comparison to in vivo essentiality and proteomics data yield specific hypotheses on gene functions and metabolic capabilities; and provide suggestions for several further gene removals. In this way, the model and its accompanying data guide future investigations of the minimal cell. Finally, the identification of 30 essential genes with unclear function will motivate the search for new biological mechanisms beyond metabolism.

One way that researchers can test whether they understand a biological system is to see if they can accurately recreate it as a computer model. The more they learn about living things, the more the researchers can improve their models and the closer the models become to simulating the original. In this approach, it is best to start by trying to model a simple system.

Biologists have previously succeeded in creating ‘minimal bacterial cells’. These synthetic cells contain fewer genes than almost all other living things and they are believed to be among the simplest possible forms of life that can grow on their own. The minimal cells can produce all the chemicals that they need to survive – in other words, they have a metabolism. Accurately recreating one of these cells in a computer is a key first step towards simulating a complete living system.

Breuer et al. have developed a computer model to simulate the network of the biochemical reactions going on inside a minimal cell with just 493 genes. By altering the parameters of their model and comparing the results to experimental data, Breuer et al. explored the accuracy of their model. Overall, the model reproduces experimental results, but it is not yet perfect. The differences between the model and the experiments suggest new questions and tests that could advance our understanding of biology. In particular, Breuer et al. identified 30 genes that are essential for life in these cells but that currently have no known purpose.

Continuing to develop and expand models like these to reproduce more complex living systems provides a tool to test current knowledge of biology. These models may become so advanced that they could predict how living things will respond to changing situations. This would allow scientists to test ideas sooner and make much faster progress in understanding life on Earth. Ultimately, these models could one day help to accelerate medical and industrial processes to save lives and enhance productivity.

Mycobacterium tuberculosis β-gentiobiosyl diacylglycerides signal through the pattern recognition receptor Mincle: total synthesis and structure activity relationships

Mycobacterium tuberculosis H37Ra produces a range of immunogenic β-gentiobiosyl diacylglycerides. We report the total synthesis of several candidate structures and show that these compounds signal weakly through mouse, but not human, Mincle. Structure-activity relationships reveal a striking dependence upon acyl chain length for gentiobiosyl diacylglyceride signalling through Mincle. Significantly, a truncated β-glucosyl diglyceride was shown to provide potent signalling through both human and mouse Mincle and could activate murine bone marrow derived dendritic cells.

Free exopolysaccharide from Mycoplasma mycoides subsp. mycoides possesses anti-inflammatory properties

In this study we explored the immunomodulatory properties of highly purified free galactan, the soluble exopolysaccharide secreted by Mycoplasma mycoides subsp. mycoides (Mmm). Galactan was shown to bind to TLR2 but not TLR4 using HEK293 reporter cells and to induce the production of the anti-inflammatory cytokine IL-10 in bovine macrophages, whereas low IL-12p40 and no TNF-α, both pro-inflammatory cytokines, were induced in these cells. In addition, pre-treatment of macrophages with galactan substantially reduced lipopolysaccharide (LPS)-induced production of pro-inflammatory cytokines TNF- and IL-12p40 while increasing LPS-induced secretion of immunosuppressive IL-10. Also, galactan did not activate naïve lymphocytes and induced only low production of the Th1 cytokine IFN-γ in Mmm-experienced lymphocytes. Finally, galactan triggered weak recall proliferation of CD4+ T lymphocytes from contagious bovine pleuropneumonia-infected animals despite having a positive effect on the expression of co-stimulatory molecules on macrophages. All together, these results suggest that galactan possesses anti-inflammatory properties and potentially provides Mmm with a mechanism to evade host innate and adaptive cell-mediated immune responses.

Galactofuranose in Mycoplasma mycoides is important for membrane integrity and conceals adhesins but does not contribute to serum resistance

Mycoplasma mycoides subsp. capri (Mmc) and subsp. mycoides (Mmm) are important ruminant pathogens worldwide causing diseases such as pleuropneumonia, mastitis and septicaemia. They express galactofuranose residues on their surface, but their role in pathogenesis has not yet been determined. The M. mycoides genomes contain up to several copies of the glf gene, which encodes an enzyme catalysing the last step in the synthesis of galactofuranose. We generated a deletion of the glf gene in a strain of Mmc using genome transplantation and tandem repeat endonuclease coupled cleavage (TREC) with yeast as an intermediary host for the genome editing. As expected, the resulting YCp1.1-Δglf strain did not produce the galactofuranose-containing glycans as shown by immunoblots and immuno-electronmicroscopy employing a galactofuranose specific monoclonal antibody. The mutant lacking galactofuranose exhibited a decreased growth rate and a significantly enhanced adhesion to small ruminant cells. The mutant was also 'leaking' as revealed by a β-galactosidase-based assay employing a membrane impermeable substrate. These findings indicate that galactofuranose-containing polysaccharides conceal adhesins and are important for membrane integrity. Unexpectedly, the mutant strain showed increased serum resistance.

Highly dynamic genomic loci drive the synthesis of two types of capsular or secreted polysaccharides within the Mycoplasma mycoides cluster

Mycoplasmas of the Mycoplasma mycoides cluster are all ruminant pathogens. Mycoplasma mycoides subsp. mycoides is responsible for contagious bovine pleuropneumonia and is known to produce capsular polysaccharide (CPS) and exopolysaccharide (EPS). Previous studies have strongly suggested a role for Mycoplasma mycoides subsp. mycoides polysaccharides in pathogenicity. Mycoplasma mycoides subsp. mycoides-secreted EPS was recently characterized as a β(1→6)-galactofuranose homopolymer (galactan) identical to the capsular product. Here, we extended the characterization of secreted polysaccharides to all other members of the M. mycoides cluster: M. capricolum subsp. capripneumoniae, M. capricolum subsp. capricolum, M. leachii, and M. mycoides subsp. capri (including the LC and Capri serovars). Extracted EPS was characterized by nuclear magnetic resonance, resulting in the identification of a homopolymer of β(1→2)-glucopyranose (glucan) in M. capricolum subsp. capripneumoniae and M. leachii. Monoclonal antibodies specific for this glucan and for the Mycoplasma mycoides subsp. mycoides-secreted galactan were used to detect the two polysaccharides. While M. mycoides subsp. capri strains of serovar LC produced only capsular galactan, no polysaccharide could be detected in strains of serovar Capri. All strains of M. capricolum subsp. capripneumoniae and M. leachii produced glucan CPS and EPS, whereas glucan production and localization varied among M. capricolum subsp. capricolum strains. Genes associated with polysaccharide synthesis and forming a biosynthetic pathway were predicted in all cluster members. These genes were organized in clusters within two loci representing genetic variability hot spots. Phylogenetic analysis showed that some of these genes, notably galE and glf, were acquired via horizontal gene transfer. These findings call for a reassessment of the specificity of the serological tests based on mycoplasma polysaccharides.

A bifunctional glycosyltransferase from Agrobacterium tumefaciens synthesizes monoglucosyl and glucuronosyl diacylglycerol under phosphate deprivation

Glycolipids are mainly found in phototrophic organisms (like plants and cyanobacteria), in Gram-positive bacteria, and a few other bacterial phyla. Besides the function as bulk membrane lipids, they often play a role under phosphate deprivation as surrogates for phospholipids. The Gram-negative Agrobacterium tumefaciens accumulates four different glycolipids under phosphate deficiency, including digalactosyl diacylglycerol and glucosylgalactosyl diacylglycerol synthesized by a processive glycosyltransferase. The other two glycolipids have now been identified by mass spectrometry and nuclear magnetic resonance spectroscopy as monoglucosyl diacylglycerol and glucuronosyl diacylglycerol. These two lipids are synthesized by a single promiscuous glycosyltransferase encoded by the ORF atu2297, with UDP-glucose or UDP-glucuronic acid as sugar donors. The transfer of sugars differing in their chemistry is a novel feature not observed before for lipid glycosyltransferases. Furthermore, this enzyme is the first glucuronosyl diacylglycerol synthase isolated. Deletion mutants of Agrobacterium lacking monoglucosyl diacylglycerol and glucuronosyl diacylglycerol or all glycolipids are not impaired in growth or virulence during infection of tobacco leaf discs. Our data suggest that the four glycolipids and the nonphospholipid diacylglyceryl trimethylhomoserine can mutually replace each other during phosphate deprivation. This redundancy of different nonphospholipids may represent an adaptation mechanism to enhance the competitiveness in nature.

Mycoplasma genitalium: a review

Mycoplasma genitalium (M. genitalium) was first isolated from the urethral swabs of two symptomatic men with urethritis in 1980. Published prevalence rates vary greatly between populations studied. A number of urogenital conditions have been ascribed to M. genitalium, which is recognised to cause a sexually transmitted infection. The association of M. genitalium with non-specific urethritis is now well established, but the evidence supporting its role in both male and female infertility remains inconclusive. Laboratory methods are challenging and there is a lack of test standardisation. The recommended treatment of the infection is azithromycin as a single 1 gm dose. However, in recent years macrolide resistance has been observed. More studies are required to establish the clinical importance of M. genitalium in urogenital conditions, particularly infertility, and to establish the role for screening and treatment in high-risk populations.

Subcellular localization of monoglucosyldiacylglycerol synthase in Synechocystis sp. PCC6803 and its unique regulation by lipid environment

Synthesis of monogalactosyldiacylglycerol (GalDAG) and digalactosyldiacylglycerol (GalGalDAG), the major membrane lipids in cyanobacteria, begins with production of the intermediate precursor monoglucosyldiacylglycerol (GlcDAG), by monoglucosyldiacylglycerol synthase (MGS). In Synechocystis sp. PCC6803 (Synechocystis) this activity is catalyzed by an integral membrane protein, Sll1377 or MgdA. In silico sequence analysis revealed that cyanobacterial homologues of MgdA are highly conserved and comprise a distinct group of lipid glycosyltransferases. Global regulation of lipid synthesis in Synechocystis and, more specifically, the influence of the lipid environment on MgdA activity have not yet been fully elucidated. Therefore, we purified membrane subfractions from this organism and assayed MGS activity in vitro, with and without different lipids and other potential effectors. Sulfoquinovosyldiacylglycerol (SQDAG) potently stimulates MgdA activity, in contrast to other enzymes of a similar nature, which are activated by phosphatidylglycerol instead. Moreover, the final products of galactolipid synthesis, GalDAG and GalGalDAG, inhibited this activity. Western blotting revealed the presence of MgdA both in plasma and thylakoid membranes, with a high specific level of the MgdA protein in the plasma membrane but highest MGS activity in the thylakoid membrane. This discrepancy in the subcellular localization of enzyme activity and protein may indicate the presence of either an unknown regulator and/or an as yet unidentified MGS-type enzyme. Furthermore, the stimulation of MgdA activity by SQDAG observed here provides a new insight into regulation of the biogenesis of both sulfolipids and galactolipids in cyanobacteria.

Structure-function features of a Mycoplasma glycolipid synthase derived from structural data integration, molecular simulations, and mutational analysis

Glycoglycerolipids are structural components of mycoplasma membranes with a fundamental role in membrane properties and stability. Their biosynthesis is mediated by glycosyltransferases (GT) that catalyze the transfer of glycosyl units from a sugar nucleotide donor to diacylglycerol. The essential function of glycolipid synthases in mycoplasma viability, and the absence of glycoglycerolipids in animal host cells make these GT enzymes a target for drug discovery by designing specific inhibitors. However, rational drug design has been hampered by the lack of structural information for any mycoplasma GT. Most of the annotated GTs in pathogenic mycoplasmas belong to family GT2. We had previously shown that MG517 in Mycoplasma genitalium is a GT-A family GT2 membrane-associated glycolipid synthase. We present here a series of structural models of MG517 obtained by homology modeling following a multiple-template approach. The models have been validated by mutational analysis and refined by long scale molecular dynamics simulations. Based on the models, key structure-function relationships have been identified: The N-terminal GT domain has a GT-A topology that includes a non-conserved variable region involved in acceptor substrate binding. Glu193 is proposed as the catalytic base in the GT mechanism, and Asp40, Tyr126, Tyr169, Ile170 and Tyr218 define the substrates binding site. Mutation Y169F increases the enzyme activity and significantly alters the processivity (or sequential transferase activity) of the enzyme. This is the first structural model of a GT-A glycoglycerolipid synthase and provides preliminary insights into structure and function relationships in this family of enzymes.

Characterization of free exopolysaccharides secreted by Mycoplasma mycoides subsp. mycoides

Contagious bovine pleuropneumonia is a severe respiratory disease of cattle that is caused by a bacterium of the Mycoplasma genus, namely Mycoplasma mycoides subsp. mycoides (Mmm). In the absence of classical virulence determinants, the pathogenicity of Mmm is thought to rely on intrinsic metabolic functions and specific components of the outer cell surface. One of these latter, the capsular polysaccharide galactan has been notably demonstrated to play a role in Mmm persistence and dissemination. The free exopolysaccharides (EPS), also produced by Mmm and shown to circulate in the blood stream of infected cattle, have received little attention so far. Indeed, their characterization has been hindered by the presence of polysaccharide contaminants in the complex mycoplasma culture medium. In this study, we developed a method to produce large quantities of EPS by transfer of mycoplasma cells from their complex broth to a chemically defined medium and subsequent purification. NMR analyses revealed that the purified, free EPS had an identical β(1−>6)-galactofuranosyl structure to that of capsular galactan. We then analyzed intraclonal Mmm variants that produce opaque/translucent colonies on agar. First, we demonstrated that colony opacity was related to the production of a capsule, as observed by electron microscopy. We then compared the EPS extracts and showed that the non-capsulated, translucent colony variants produced higher amounts of free EPS than the capsulated, opaque colony variants. This phenotypic variation was associated with an antigenic variation of a specific glucose phosphotransferase permease. Finally, we conducted in silico analyses of candidate polysaccharide biosynthetic pathways in order to decipher the potential link between glucose phosphotransferase permease activity and attachment/release of galactan. The co-existence of variants producing alternative forms of galactan (capsular versus free extracellular galactan) and associated with an antigenic switch constitutes a finely tuned mechanism that may be involved in virulence.

Recent perspectives in the diagnosis and evidence-based treatment of Mycoplasma genitalium

Mycoplasma genitalium is a globally important sexually transmitted pathogen. Men infected with M. genitalium frequently present with dysuria, while women may present with or without urogenital symptoms. In some populations, M. genitalium is significantly associated with HIV-1 infection, and is also an etiological agent in pelvic inflammatory disease. However, there is insufficient evidence to establish a causative role of the organism in obstetric complications, including tubal factor infertility. Although several nucleic acid amplification tests offer rapid, sensitive methods for detecting M. genitalium, there is no standardized assay. Available evidence supports treatment of M. genitalium infections with an extended regimen of azithromycin and resistant strains respond to moxifloxacin. Accumulating evidence indicates growing fluoroquinolone resistance, including against moxifloxacin, emphasizing the need for new therapeutic strategies to treat M. genitalium infections.