Wall teichoic acid protects Staphylococcus aureus from inhibition by Congo red and other dyes

The use of nanoparticles in the treatment of infectious diseases and cancer, dental applications and tissue regeneration: a review

The emergence of nanotechnology as a field of study can be traced back to the 1980s, at which point the means to artificially produce, control, and observe matter on a nanometer level was made viable. Recent advancements in technology have enabled us to extend our reach to the nanoscale, which has presented an unparalleled opportunity to directly target biomolecular interactions. As a result of these developments, there is a drive to arise intelligent nanostructures capable of overcoming the obstacles that have impeded the progress of conventional pharmacological methodologies. After four decades, the gradual amalgamation of bio- and nanotechnologies is initiating a revolution in the realm of disease detection, treatment, and monitoring, as well as unsolved medical predicaments. Although a significant portion of research in the field is still confined to laboratories, the initial application of nanotechnology as treatments, vaccines, pharmaceuticals, and diagnostic equipment has now obtained endorsement for commercialization and clinical practice. The current issue presents an overview of the latest progress in nanomedical strategies towards alleviating antibiotic resistance, diagnosing and treating cancer, addressing neurodegenerative disorders, and an array of applications, encompassing dentistry and tuberculosis treatment. The current investigation also scrutinizes the deployment of sophisticated smart nanostructured materials in fields of application such as regenerative medicine, as well as the management of targeted and sustained release of pharmaceuticals and therapeutic interventions. The aforementioned concept exhibits the potential for revolutionary advancements within the field of immunotherapy, as it introduces the utilization of implanted vaccine technology to consistently regulate and augment immune functions. Concurrently with the endeavor to attain the advantages of nanomedical intervention, it is essential to enhance the unceasing emphasis on nanotoxicological research and the regulation of nanomedications' safety. This initiative is crucial in achieving the advancement in medicine that currently lies within our reach.

Impact of the pentose phosphate pathway on metabolism and pathogenesis of Staphylococcus aureus

Staphylococcus aureus is an important pathogen that leads to significant disease through multiple routes of infection. We recently published a transposon sequencing (Tn-seq) screen in a mouse acute pneumonia model and identified a hypothetical gene (SAUSA300_1902, pgl) with similarity to a lactonase of Escherichia coli involved in the pentose phosphate pathway (PPP) that was conditionally essential. Limited studies have investigated the role of the PPP in physiology and pathogenesis of S. aureus. We show here that mutation of pgl significantly impacts ATP levels and respiration. RNA-seq analysis of the pgl mutant and parent strains identified compensatory changes in gene expression for glucose and gluconate as well as reductions in the pyrimidine biosynthesis locus. These differences were also evident through unbiased metabolomics studies and 13C labeling experiments that showed mutation of pgl led to reductions in pyrimidine metabolism including decreases in ribose-5P, UMP and GMP. These nucleotide reductions impacted the amount of extracellular DNA in biofilms and reduced biofilm formation. Mutation also limited the capacity of the strain to resist oxidant damage induced by hydrogen peroxide and paraquat and subsequent intracellular survival inside macrophages. Changes in wall teichoic acid impacted susceptibility to hydrogen peroxide. We demonstrated the importance of these changes on virulence in three different models of infection, covering respiratory, skin and septicemia, demonstrating the need for proper PPP function in all models. This work demonstrates the multifaceted role metabolism can play in multiple aspects of S. aureus pathogenesis.

Novel antimicrobial strategies to treat multi-drug resistant Staphylococcus aureus infections

Antimicrobial resistance is a major obstacle for the treatment of infectious diseases and currently represents one of the most significant threats to global health. Staphylococcus aureus remains a formidable human pathogen with high mortality rates associated with severe systemic infections. S. aureus has become notorious as a multidrug resistant bacterium, which when combined with its extensive arsenal of virulence factors that exacerbate disease, culminates in an incredibly challenging pathogen to treat clinically. Compounding this major health issue is the lack of antibiotic discovery and development, with only two new classes of antibiotics approved for clinical use in the last 20 years. Combined efforts from the scientific community have reacted to the threat of dwindling treatment options to combat S. aureus disease in several innovative and exciting developments. This review describes current and future antimicrobial strategies aimed at treating staphylococcal colonization and/or disease, examining therapies that show significant promise at the preclinical development stage to approaches that are currently being investigated in clinical trials.

Wall Teichoic Acids Facilitate the Release of Toxins from the Surface of Staphylococcus aureus

A major feature of the pathogenicity of Staphylococcus aureus is its ability to secrete cytolytic toxins. This process involves the translocation of the toxins from the cytoplasm through the bacterial membrane and the cell wall to the external environment. The process of their movement through the membrane is relatively well defined, involving both general and toxin-specific secretory systems. Movement of the toxins through the cell wall was considered to involve the passive diffusion of the proteins through the porous cell wall structures; however, recent work suggests that this is more complex, and here we demonstrate a role for the wall teichoic acids (WTA) in this process. Utilizing a genome-wide association approach, we identified a polymorphism in the locus encoding the WTA biosynthetic machinery as associated with the cytolytic activity of the bacteria. We verified this association using an isogenic mutant set and found that WTA are required for the release of several cytolytic toxins from the bacterial cells. We show that this effect is mediated by a change in the electrostatic charge across the cell envelope that results from the loss of WTA. As a major target for the development of novel therapeutics, it is important that we fully understand the entire process of cytolytic toxin production and release. These findings open up a new aspect to the process of toxin release by a major human pathogen while also demonstrating that clinical isolates can utilize WTA production to vary their cytotoxicity, thereby altering their pathogenic capabilities. IMPORTANCE The production and release of cytolytic toxins is a critical aspect for the pathogenicity of many bacterial pathogens. In this study, we demonstrate a role for wall teichoic acids, molecules that are anchored to the peptidoglycan of the bacterial cell wall, in the release of toxins from S. aureus cells into the extracellular environment. Our findings suggest that this effect is mediated by a gradient of electrostatic charge which the presence of the negatively charged WTA molecules create across the cell envelope. This work brings an entirely new aspect to our understanding of the cytotoxicity of S. aureus and demonstrates a further means by which this major human pathogen can adapt its pathogenic capabilities.

Electrostatic Interaction with the Bacterial Cell Envelope Tunes the Lytic Activity of Two Novel Peptidoglycan Hydrolases

Peptidoglycan (PG) hydrolases, due to their crucial role in the metabolism of the bacterial cell wall (CW), are increasingly being considered suitable targets for therapies, and a potent alternative to conventional antibiotics. In the light of contradictory data reported, detailed mechanism of regulation of enzymes activity based on electrostatic interactions between hydrolase molecule and bacterial CW surface remains unknown. Here, we report a comprehensive study on this phenomenon using as a model two novel PG hydrolases, SpM23_A, and SpM23_B, which although share the same bacterial host, similarities in sequence conservation, domain architecture, and structure, display surprisingly distinct net charges (in 2D electrophoresis, pI 6.8, and pI 9.7, respectively). We demonstrate a strong correlation between hydrolases surface net charge and the enzymes activity by modulating the charge of both, enzyme molecule and bacterial cell surface. Teichoic acids, anionic polymers present in the bacterial CW, are shown to be involved in the mechanism of enzymes activity regulation by the electrostatics-based interplay between charged bacterial envelope and PG hydrolases. These data serve as a hint for the future development of chimeric PG hydrolases of desired antimicrobial specificity. IMPORTANCE This study shows direct relationship between the surface charge of two recently described enzymes, SpM23_A and SpM23_B, and bacterial cell walls. We demonstrate that by (i) surface charge probing of bacterial strains collection, (ii) reduction of the net charge of the positively charged enzyme, and (iii) altering the net charge of the bacterial surface by modifying the content and composition of teichoic acids. In all cases, we observed that lytic activity and binding strength of SpM23 enzymes, are regulated by electrostatic interactions with the bacterial cell envelope and that this interaction contributes to the determination of the spectrum of susceptible bacterial species. Moreover, we revealed the regulatory role of charged cell wall components, namely, teichoic and lipoteichoic acids, over the SpM23 enzymes. We believe that our findings make an important contribution to understand the means of hydrolases activity regulation in the complex environment of the bacterial cell wall.

SmdA is a Novel Cell Morphology Determinant in Staphylococcus aureus

Cell division and cell wall synthesis in staphylococci need to be precisely coordinated and controlled to allow the cell to multiply while maintaining its nearly spherical shape. The mechanisms ensuring correct placement of the division plane and synthesis of new cell wall have been studied intensively. However, hitherto unknown factors and proteins are likely to play key roles in this complex interplay. Here, we identified and investigated a protein with a major influence on cell morphology in Staphylococcus aureus. The protein, named SmdA (for staphylococcal morphology determinant A), is a membrane protein with septum-enriched localization. By CRISPRi knockdown and overexpression combined with different microscopy techniques, we demonstrated that proper levels of SmdA were necessary for cell division, including septum formation and cell splitting. We also identified conserved residues in SmdA that were critical for its functionality. Pulldown and bacterial two-hybrid interaction experiments showed that SmdA interacted with several known cell division and cell wall synthesis proteins, including penicillin-binding proteins (PBPs) and EzrA. Notably, SmdA also affected susceptibility to cell wall targeting antibiotics, particularly in methicillin-resistant S. aureus (MRSA). Together, our results showed that S. aureus was dependent on balanced amounts of membrane attached SmdA to carry out proper cell division.

Systematic Assessment of Accessibility to the Surface of Staphylococcus aureus

Proteins from bacterial foes, antimicrobial peptides, and host immune proteins must navigate past a dense layer of bacterial surface biomacromolecules to reach the peptidoglycan (PG) layer of Gram-positive bacteria. A subclass of molecules (e.g., antibiotics with intracellular targets) also must permeate through the PG (in a molecular sieving manner) to reach the cytoplasmic membrane. Despite the biological and therapeutic importance of surface accessibility, systematic analyses in live bacterial cells have been lacking. We describe a live cell fluorescence assay that is robust, shows a high level of reproducibility, and reports on the permeability of molecules to and within the PG scaffold. Moreover, our study shows that teichoic acids impede the permeability of molecules of a wide range of sizes and chemical composition.

Streptococcus mutans Lacking sufCDSUB Is Viable, but Displays Major Defects in Growth, Stress Tolerance Responses and Biofilm Formation

Streptococcus mutans appears to possess a sole iron-sulfur (Fe-S) cluster biosynthesis system encoded by the sufCDSUB cluster. This study was designed to examine the role of sufCDSUB in S. mutans physiology. Allelic exchange mutants deficient of the whole sufCDSUB cluster and in individual genes were constructed. Compared to the wild-type, UA159, the sufCDSUB-deficient mutant, Δsuf::kan^r, had a significantly reduced growth rate, especially in medium with the absence of isoleucine, leucine or glutamate/glutamine, amino acids that require Fe-S clusters for biosynthesis and when grown with medium adjusted to pH 6.0 and under oxidative and nitrosative stress conditions. Relative to UA159, Δsuf::kan^r had major defects in stress tolerance responses with reduced survival rate of > 2-logs following incubation at low pH environment or after hydrogen peroxide challenge. When compared to UA159, Δsuf::kan^r tended to form aggregates in broth medium and accumulated significantly less biofilm. As shown by luciferase reporter fusion assays, the expression of sufCDSUB was elevated by > 5.4-fold when the reporter strain was transferred from iron sufficient medium to iron-limiting medium. Oxidative stress induced by methyl viologen increased sufCDSUB expression by > 2-fold, and incubation in a low pH environment led to reduction of sufCDSUB expression by > 7-fold. These results suggest that lacking of SufCDSUB in S. mutans causes major defects in various cellular processes of the deficient mutant, including growth, stress tolerance responses and biofilm formation. In addition, the viability of the deficient mutant also suggests that SUF, the sole Fe-S cluster machinery identified is non-essential in S. mutans, which is not known in any other bacterium lacking the NIF and/or ISC system. However, how the bacterium compensates the Fe-S deficiency and if any novel Fe-S assembly systems exist in this bacterium await further investigation.

Apt (Adenine Phosphoribosyltransferase) Mutation in Laboratory-Selected Vancomycin-Intermediate Staphylococcus aureus

Comparative genomic sequencing of laboratory-derived vancomycin-intermediate Staphylococcusaureus (VISA) (MM66-3 and MM66-4) revealed unique mutations in both MM66-3 (in apt and ssaA6), and MM66-4 (in apt and walK), compared to hetero-VISA parent strain MM66. Transcriptional profiling revealed that both MM66 VISA shared 79 upregulated genes and eight downregulated genes. Of these, 30.4% of the upregulated genes were associated with the cell envelope, whereas 75% of the downregulated genes were associated with virulence. In concordance with mutations and transcriptome alterations, both VISA strains demonstrated reduced autolysis, reduced growth in the presence of salt and reduced virulence factor activity. In addition to mutations in genes linked to cell wall metabolism (ssaA6 and walK), the same mutation in apt which encodes adenine phosphoribosyltransferase, was confirmed in both MM66 VISA. Apt plays a role in both adenine metabolism and accumulation and both MM66 VISA grew better than MM66 in the presence of adenine or 2-fluoroadenine indicating a reduction in the accumulation of these growth inhibiting compounds in the VISA strains. MM66 apt mutants isolated via 2-fluoroadenine selection also demonstrated reduced susceptibility to the cell wall lytic dye Congo red and vancomycin. Finding that apt mutations contribute to reduced vancomycin susceptibility once again suggests a role for altered purine metabolism in a VISA mechanism.

β-Lactams against the Fortress of the Gram-Positive Staphylococcus aureus Bacterium

The biological diversity of the unicellular bacteria-whether assessed by shape, food, metabolism, or ecological niche-surely rivals (if not exceeds) that of the multicellular eukaryotes. The relationship between bacteria whose ecological niche is the eukaryote, and the eukaryote, is often symbiosis or stasis. Some bacteria, however, seek advantage in this relationship. One of the most successful-to the disadvantage of the eukaryote-is the small (less than 1 μm diameter) and nearly spherical Staphylococcus aureus bacterium. For decades, successful clinical control of its infection has been accomplished using β-lactam antibiotics such as the penicillins and the cephalosporins. Over these same decades S. aureus has perfected resistance mechanisms against these antibiotics, which are then countered by new generations of β-lactam structure. This review addresses the current breadth of biochemical and microbiological efforts to preserve the future of the β-lactam antibiotics through a better understanding of how S. aureus protects the enzyme targets of the β-lactams, the penicillin-binding proteins. The penicillin-binding proteins are essential enzyme catalysts for the biosynthesis of the cell wall, and understanding how this cell wall is integrated into the protective cell envelope of the bacterium may identify new antibacterials and new adjuvants that preserve the efficacy of the β-lactams.

Phylogenomics reveals the basis of adaptation of Pseudorhizobium species to extreme environments and supports a taxonomic revision of the genus

The family Rhizobiaceae includes many genera of soil bacteria, often isolated for their association with plants. Herein, we investigate the genomic diversity of a group of Rhizobium species and unclassified strains isolated from atypical environments, including seawater, rock matrix or polluted soil. Based on whole-genome similarity and core genome phylogeny, we show that this group corresponds to the genus Pseudorhizobium. We thus reclassify Rhizobium halotolerans, R. marinum, R. flavum and R. endolithicum as P. halotolerans sp. nov., P. marinum comb. nov., P. flavum comb. nov. and P. endolithicum comb. nov., respectively, and show that P. pelagicum is a synonym of P. marinum. We also delineate a new chemolithoautotroph species, P. banfieldiae sp. nov., whose type strain is NT-26^T (=DSM 106348^T=CFBP 8663^T). This genome-based classification was supported by a chemotaxonomic comparison, with increasing taxonomic resolution provided by fatty acid, protein and metabolic profiles. In addition, we used a phylogenetic approach to infer scenarios of duplication, horizontal transfer and loss for all genes in the Pseudorhizobium pangenome. We thus identify the key functions associated with the diversification of each species and higher clades, shedding light on the mechanisms of adaptation to their respective ecological niches. Respiratory proteins acquired at the origin of Pseudorhizobium were combined with clade-specific genes to enable different strategies for detoxification and nutrition in harsh, nutrient-poor environments.

Inactivation of the pgmA Gene in Streptococcus mutans Significantly Decreases Biofilm-Associated Antimicrobial Tolerance

Screening of a Streptococcus mutans mutant library indicated that pgmA mutants displayed a reduced biofilm-associated tolerance toward gentamicin. The biofilms formed by the S. mutanspgmA mutant also displayed decreased tolerance towards linezolid and vancomycin compared to wild-type biofilms. On the contrary, the resistance of planktonic S. mutanspgmA cells to gentamycin, linezolid, and vancomycin was more similar to wild-type levels. Investigations of biofilms grown in microtiter trays and on submerged glass slides showed that pgmA mutants formed roughly the same amount of biofilm as the wild type, indicating that the reduced antimicrobial tolerance of these mutants is not due to diminished biofilm formation. The pgmA gene product is known to be involved in the synthesis of precursors for cell wall components such as teichoic acids and membrane glycolipids. Accordingly, the S. mutanspgmA mutant showed increased sensitivity to Congo Red, indicating that it has impaired cell wall integrity. A changed cell wall composition of the S. mutanspgmA mutant may play a role in the increased sensitivity of S. mutanspgmA biofilms toward antibiotics.

Tet38 of Staphylococcus aureus Binds to Host Cell Receptor Complex CD36-Toll-Like Receptor 2 and Protects from Teichoic Acid Synthesis Inhibitors Tunicamycin and Congo Red

Using an affinity column retention assay, we showed that the purified Tet38 membrane transporter of Staphylococcus aureus bound specifically to host cell CD36 and to the complex CD36-Toll-like receptor 2 (TLR-2), but not to TLR-2 alone or TLR-2 and S. aureus lipoteichoic acid (LTA). We tested the effect of LTA on the internalization of S. aureus tet38 mutant QT7 versus RN6390 by A549 epithelial cells. Addition of anti-LTA antibody to the bacteria prior to adding to A549 cells reduced internalization of QT7 2-fold compared to that with nonspecific antibody treatment. QT7 internalized 4- to 6-fold less than RN6390 with or without anti-LTA antibody. These data suggested that Tet38 and LTA were independently involved in the invasion process. The wall teichoic acid (WTA) inhibitor tunicamycin had an 8-fold decrease in activity with overexpression of tet38 and a 2-fold increase in activity in QT7 (tet38). Reserpine (an inhibitor of efflux pumps) reduced the effect of tet38 overexpression on tunicamycin resistance 4-fold. In addition, tet38 affected growth in the presence of LTA inhibitor Congo red, with overexpression increasing growth and deletion of tet38 reducing growth. In conclusion, Tet38 contributes to S. aureus invasion of A549 via direct binding to CD36 of the complex CD36-TLR-2, and LTA independently bound to TLR-2. The reduction of tunicamycin resistance in the presence of reserpine and the survival ability of the tet38 overexpressor in the presence of Congo red suggest that Tet38 can also protect the synthesis of LTA and WTA in S. aureus against their inhibitors, possibly functioning as an efflux pump.

Unprotonated Short-Chain Alkylamines Inhibit Staphylolytic Activity of Lysostaphin in a Wall Teichoic Acid-Dependent Manner

Lysostaphin (Lst) is a potent bacteriolytic enzyme that kills Staphylococcus aureus, a common bacterial pathogen of humans and animals. With high activity against both planktonic cells and biofilms, Lst has the potential to be used in industrial products, such as commercial cleansers, for decontamination. However, Lst is inhibited in the presence of monoethanolamine (MEA), a chemical widely used in cleaning solutions and pharmaceuticals, and the underlying mechanism of inhibition remains unknown. In this study, we examined the cell binding and killing capabilities of Lst against S. aureus ATCC 6538 in buffered salt solution with MEA at different pH values (7.5 to 10.5) and discovered that only the unprotonated form of MEA inhibited Lst binding to the cell surface, leading to low Lst activity, despite retention of its secondary structure. This reduced enzyme activity could be largely recovered via a reduction in wall teichoic acid (WTA) biosynthesis through tunicamycin treatment, indicating that the suppression of Lst activity was dependent on the presence and amount of WTA. We propose that the decreased cell binding and killing capabilities of Lst are associated with the influence of uncharged MEA on the conformation of WTA. A similar effect was confirmed with other short-chain alkylamines. This study offers new insight into the impact of short-chain alkylamines on both Lst and WTA structure and function and provides guidance for the application of Lst in harsh environments.IMPORTANCE Lysostaphin (Lst) effectively and selectively kills Staphylococcus aureus, the bacterial culprit of many hospital- and community-acquired skin and respiratory infections and food poisoning. Lst has been investigated in animal models and clinical trials, industrial formulations, and environmental settings. Here, we studied the mechanistic basis of the inhibitory effect of alkylamines, such as monoethanolamine (MEA), a widely used chemical in commercial detergents, on Lst activity, for the potential incorporation of Lst in disinfectant solutions. We have found that protonated MEA has little influence on Lst activity, while unprotonated MEA prevents Lst from binding to S. aureus cells and hence dramatically decreases the enzyme's bacteriolytic efficacy. Following partial removal of the wall teichoic acid, an important component of the bacterial cell envelope, the inhibitory effect of unprotonated MEA on Lst is reduced. This phenomenon can be extended to other short-chain alkylamines. This mechanistic report of the impact of alkylamines on Lst functionality will help guide future applications of Lst in disinfection and decontamination of health-related commercial products.

Reconstitution of Staphylococcus aureus Lipoteichoic Acid Synthase Activity Identifies Congo Red as a Selective Inhibitor

Lipoteichoic acid (LTA) is an anionic surface polymer that is essential for normal growth of Staphylococcus aureus, making the LTA polymerase, LTA synthase (LtaS), a proposed drug target for combating Staphylococcal infections. LtaS is a polytopic membrane protein with five membrane-spanning helices and an extracellular domain, and it uses phosphatidylglycerol to assemble a glycerol phosphate chain on a glycosylated diacylglycerol membrane anchor. We report here the first reconstitution of LtaS polymerization activity and show that the azo dye Congo red inhibits this enzyme both in vitro and in cells. Related azo dyes and the previously reported LtaS inhibitor 1771 have weak or no in vitro inhibitory activity. Synthetic lethality with mutant strains known to be nonviable in the absence of LTA confirms selective inhibition by Congo red. As the only validated LtaS inhibitor, Congo red can serve as a probe to understand how inhibiting lipoteichoic acid biosynthesis affects cell physiology and may also guide the discovery of more potent inhibitors for use in treating S. aureus infections.

Genome-wide screen for genes involved in eDNA release during biofilm formation by Staphylococcus aureus

Staphylococcus aureus is a leading cause of both nosocomial and community-acquired infection. Biofilm formation at the site of infection reduces antimicrobial susceptibility and can lead to chronic infection. During biofilm formation, a subset of cells liberate cytoplasmic proteins and DNA, which are repurposed to form the extracellular matrix that binds the remaining cells together in large clusters. Using a strain that forms robust biofilms in vitro during growth under glucose supplementation, we carried out a genome-wide screen for genes involved in the release of extracellular DNA (eDNA). A high-density transposon insertion library was grown under biofilm-inducing conditions, and the relative frequency of insertions was compared between genomic DNA (gDNA) collected from cells in the biofilm and eDNA from the matrix. Transposon insertions into genes encoding functions necessary for eDNA release were identified by reduced representation in the eDNA. On direct testing, mutants of some of these genes exhibited markedly reduced levels of eDNA and a concomitant reduction in cell clustering. Among the genes with robust mutant phenotypes were gdpP, which encodes a phosphodiesterase that degrades the second messenger cyclic-di-AMP, and xdrA, the gene for a transcription factor that, as revealed by RNA-sequencing analysis, influences the expression of multiple genes, including many involved in cell wall homeostasis. Finally, we report that growth in biofilm-inducing medium lowers cyclic-di-AMP levels and does so in a manner that depends on the gdpP phosphodiesterase gene.

In vitro reconstitution demonstrates the cell wall ligase activity of LCP proteins

Sacculus is a peptidoglycan (PG) matrix that protects bacteria from osmotic lysis. In Gram-positive organisms, the sacculus is densely functionalized with glycopolymers important for survival, but the way in which assembly occurs is not known. In Staphylococcus aureus, three LCP (LytR-CpsA-Psr) family members have been implicated in attaching the major glycopolymer wall teichoic acid (WTA) to PG, but ligase activity has not been demonstrated for these or any other LCP proteins. Using WTA and PG substrates produced chemoenzymatically, we show that all three proteins can transfer WTA precursors to nascent PGs, establishing that LCP proteins are PG-glycopolymer ligases. Although all S. aureus LCP proteins have the capacity to attach WTA to PG, we show that their cellular functions are not redundant. Strains lacking lcpA have phenotypes similar to those of WTA-null strains, indicating that this is the most important WTA ligase. This work provides a foundation for studying how LCP enzymes participate in cell wall assembly.

TarO-specific inhibitors of wall teichoic acid biosynthesis restore β-lactam efficacy against methicillin-resistant staphylococci

The widespread emergence of methicillin-resistant Staphylococcus aureus (MRSA) has dramatically eroded the efficacy of current β-lactam antibiotics and created an urgent need for new treatment options. We report an S. aureus phenotypic screening strategy involving chemical suppression of the growth inhibitory consequences of depleting late-stage wall teichoic acid biosynthesis. This enabled us to identify early-stage pathway-specific inhibitors of wall teichoic acid biosynthesis predicted to be chemically synergistic with β-lactams. We demonstrated by genetic and biochemical means that each of the new chemical series discovered, herein named tarocin A and tarocin B, inhibited the first step in wall teichoic acid biosynthesis (TarO). Tarocins do not have intrinsic bioactivity but rather demonstrated potent bactericidal synergy in combination with broad-spectrum β-lactam antibiotics against diverse clinical isolates of methicillin-resistant staphylococci as well as robust efficacy in a murine infection model of MRSA. Tarocins and other inhibitors of wall teichoic acid biosynthesis may provide a rational strategy to develop Gram-positive bactericidal β-lactam combination agents active against methicillin-resistant staphylococci.

Wall Teichoic Acids Are Involved in the Medium-Induced Loss of Function of the Autolysin CD11 against Clostridium difficile

Bacterial lysins are potent antibacterial enzymes with potential applications in the treatment of bacterial infections. Some lysins lose activity in the growth media of target bacteria, and the underlying mechanism remains unclear. Here we use CD11, an autolysin of Clostridium difficile, as a model lysin to demonstrate that the inability of this enzyme to kill C. difficile in growth medium is not associated with inhibition of the enzyme activity by medium, or the modification of the cell wall peptidoglycan. Rather, wall teichoic acids (WTAs) appear to prevent the enzyme from binding to the cells and cleaving the cell wall peptidoglycan. By partially blocking the biosynthetic pathway of WTAs with tunicamycin, cell binding improved and the lytic efficacy of CD11 was significantly enhanced. This is the first report of the mechanism of lysin inactivation in growth medium, and provides insights into understanding the behavior of lysins in complex environments, including the gastrointestinal tract.

Chemical Genetic Analysis and Functional Characterization of Staphylococcal Wall Teichoic Acid 2-Epimerases Reveals Unconventional Antibiotic Drug Targets

Here we describe a chemical biology strategy performed in Staphylococcus aureus and Staphylococcus epidermidis to identify MnaA, a 2-epimerase that we demonstrate interconverts UDP-GlcNAc and UDP-ManNAc to modulate substrate levels of TarO and TarA wall teichoic acid (WTA) biosynthesis enzymes. Genetic inactivation of mnaA results in complete loss of WTA and dramatic in vitro β-lactam hypersensitivity in methicillin-resistant S. aureus (MRSA) and S. epidermidis (MRSE). Likewise, the β-lactam antibiotic imipenem exhibits restored bactericidal activity against mnaA mutants in vitro and concomitant efficacy against 2-epimerase defective strains in a mouse thigh model of MRSA and MRSE infection. Interestingly, whereas MnaA serves as the sole 2-epimerase required for WTA biosynthesis in S. epidermidis, MnaA and Cap5P provide compensatory WTA functional roles in S. aureus. We also demonstrate that MnaA and other enzymes of WTA biosynthesis are required for biofilm formation in MRSA and MRSE. We further determine the 1.9Å crystal structure of S. aureus MnaA and identify critical residues for enzymatic dimerization, stability, and substrate binding. Finally, the natural product antibiotic tunicamycin is shown to physically bind MnaA and Cap5P and inhibit 2-epimerase activity, demonstrating that it inhibits a previously unanticipated step in WTA biosynthesis. In summary, MnaA serves as a new Staphylococcal antibiotic target with cognate inhibitors predicted to possess dual therapeutic benefit: as combination agents to restore β-lactam efficacy against MRSA and MRSE and as non-bioactive prophylactic agents to prevent Staphylococcal biofilm formation.

Staphylococcus aureus and Staphylococcus epidermidis cause life-threatening infections that are commonly acquired in hospitals as well as the community and remain difficult to treat with current antibiotics. In part, this is due to the emergence of methicillin-resistant S. aureus and S. epidermidis (MRSA and MRSE), which exhibit broad resistance to β-lactams such as penicillin and other members of this important founding class of antibiotics. Compounding this problem, Staphylococci commonly colonize the surface of catheters and other medical devices, forming bacterial communities that are intrinsically resistant to antibiotics. Here we functionally characterize a family of 2-epimerases, named MnaA and Cap5P, that we demonstrate by genetic, biochemical, and X-ray crystallography means are essential for wall teichoic acid biosynthesis and that upon their genetic inactivation render methicillin-resistant Staphylococci unable to form biofilms as well as broadly hypersusceptible to β-lactam antibiotics both in vitro and in a host infection setting. WTA 2-epimerases therefore constitute a novel class of methicillin-resistant Staphylococcal drug targets.

In vitro and in vivo antimicrobial efficacy of natural plant-derived compounds against Vibrio cholerae of O1 El Tor Inaba serotype

In this study, we investigated antibacterial activities of 20 plant-derived natural compounds against Gram-negative enteric pathogens. We found that both flavonoids and non-flavonoids, including honokiol and magnolol, possess specific antibacterial activities against V. cholerae, but not against other species of Gram-negative bacterium which we tested. Using various antibacterial assays, we determined that there was a dose-dependent bactericidal and biofilm inhibitory activity of honokiol and magnolol against Vibrio cholerae. In addition to antibacterial activities, these molecules also induced an attenuating effect on reactive oxygen species (ROS) production and pro-inflammatory responses generated by macrophages in response to lipopolysaccharides (LPS). Additionally, Caenorhabditis elegans lethality assay revealed that honokiol and magnolol have an ability to extend a lifespan of V. cholerae-infected worms, contributing to prolonged survival of worms after lethal infection. Altogether, our data show for the first time that honokiol and magnolol may be considered as attractive protective or preventive food adjuncts for cholera.

Compound-gene interaction mapping reveals distinct roles for Staphylococcus aureus teichoic acids

Staphylococcus aureus contains two distinct teichoic acid (TA) polymers, lipoteichoic acid (LTA) and wall teichoic acid (WTA), which are proposed to play redundant roles in regulating cell division. To gain insight into the underlying biology of S. aureus TAs, we used a small molecule inhibitor to screen a highly saturated transposon library for cellular factors that become essential when WTA is depleted. We constructed an interaction network connecting WTAs with genes involved in LTA synthesis, peptidoglycan synthesis, surface protein display, and D-alanine cell envelope modifications. Although LTAs and WTAs are synthetically lethal, we report that they do not have the same synthetic interactions with other cell envelope genes. For example, D-alanylation, a tailoring modification of both WTAs and LTAs, becomes essential when the former, but not the latter, are removed. Therefore, D-alanine-tailored LTAs are required for survival when WTAs are absent. Examination of terminal phenotoypes led to the unexpected discovery that cells lacking both LTAs and WTAs lose their ability to form Z rings and can no longer divide. We have concluded that the presence of either LTAs or WTAs on the cell surface is required for initiation of S. aureus cell division, but these polymers act as part of distinct cellular networks.

Whole animal automated platform for drug discovery against multi-drug resistant Staphylococcus aureus

Staphylococcus aureus, the leading cause of hospital-acquired infections in the United States, is also pathogenic to the model nematode Caenorhabditis elegans. The C. elegans-S. aureus infection model was previously carried out on solid agar plates where the bacteriovorous C. elegans feeds on a lawn of S. aureus. However, agar-based assays are not amenable to large scale screens for antibacterial compounds. We have developed a high throughput liquid screening assay that uses robotic instrumentation to dispense a precise amount of methicillin resistant S. aureus (MRSA) and worms in 384-well assay plates, followed by automated microscopy and image analysis. In validation of the liquid assay, an MRSA cell wall defective mutant, MW2ΔtarO, which is attenuated for killing in the agar-based assay, was found to be less virulent in the liquid assay. This robust assay with a Z'-factor consistently greater than 0.5 was utilized to screen the Biomol 4 compound library consisting of 640 small molecules with well characterized bioactivities. As proof of principle, 27 of the 30 clinically used antibiotics present in the library conferred increased C. elegans survival and were identified as hits in the screen. Surprisingly, the antihelminthic drug closantel was also identified as a hit in the screen. In further studies, we confirmed the anti-staphylococcal activity of closantel against vancomycin-resistant S. aureus isolates and other Gram-positive bacteria. The liquid C. elegans-S. aureus assay described here allows screening for anti-staphylococcal compounds that are not toxic to the host.