A Staphylococcus aureus autolysin that has an N-acetylmuramoyl-L-alanine amidase domain and an endo-beta-N-acetylglucosaminidase domain: cloning, sequence analysis, and characterization

Recombinant Amidases: Recent Insights and its Applications in the Production of Industrially Important Fine Chemicals

The current research for the synthesis of industrially important fine chemicals is more inclined towards developing enzyme-based processes. The biotransformation reactions wherein microbial cells/enzymes are used, have become essential in making the process efficient, green, and economical. Amongst industrially important enzymes, amidase is one of the most versatile tools in biocatalysis and biotransformation reactions. It shows broad substrate specificity and sturdy functional characteristics because of its promiscuous nature. Further, advancement in the area led to the development of amidase recombinant systems, which are developed using biotechnology and enzyme engineering tools. Additionally, recombinant amidases may be instrumental in commercializing the synthesis of fine chemicals such as hydroxamic acids that have a significant pharmaceutical market. Hence, the present review focuses on highlighting and assimilating the tools and techniques used in developing recombinant systems followed by their applications.

Characterization of the major autolysin (AtlC) of Staphylococcus carnosus

BACKGROUND

Autolysis by cellular peptidoglycan hydrolases (PGH) is a well-known phenomenon in bacteria. During food fermentation, autolysis of starter cultures can exert an accelerating effect, as described in many studies on cheese ripening. In contrast, very little is known about autolysis of starter cultures used in other fermentations. Staphylococcus (S.) carnosus is often used in raw sausage fermentations, contributing to nitrate reduction and flavor formation. In this study, we analyzed the influence of PGHs of the strains S. carnosus TMW 2.146 and S. carnosus TMW 2.2525 on their autolytic behavior. The staphylococcal major autolysin (Atl), a bifunctional enzyme with an N-acetylmuramoyl-L-alanine amidase and a glucosaminidase as an active site, is assumed to be the enzyme by which autolysis is mainly mediated.

RESULTS

AtlC mutant strains showed impaired growth and almost no autolysis compared to their respective wild-type strains. Light microscopy and scanning electron microscopy showed that the mutants could no longer appropriately separate from each other during cell division, resulting in the formation of cell clusters. The surface of the mutants appeared rough with an irregular morphology compared to the smooth cell surfaces of the wild-types. Moreover, zymograms showed that eight lytic bands of S. carnosus, with a molecular mass between 140 and 35 kDa, are processed intermediates of AtlC. It was noticed that additional bands were found that had not been described in detail before and that the banding pattern changes over time. Some bands disappear entirely, while others become stronger or are newly formed. This suggests that AtlC is degraded into smaller fragments over time. A second knockout was generated for the gene encoding a N-acetylmuramoyl-L-alanine amidase domain-containing protein. Still, no phenotypic differences could be detected in this mutant compared to the wild-type, implying that the autolytic activity of S. carnosus is mediated by AtlC.

CONCLUSIONS

In this study, two knockout mutants of S. carnosus were generated. The atlC mutant showed a significantly altered phenotype compared to the wild-type, revealing AtlC as a key factor in staphylococcal autolysis. Furthermore, we show that Atl is degraded into smaller fragments, which are still cell wall lytic active.

FacZ is a GpsB-interacting protein that prevents aberrant division-site placement in Staphylococcus aureus

Staphylococcus aureus is a Gram-positive pathogen responsible for antibiotic-resistant infections. To identify vulnerabilities in cell envelope biogenesis that may overcome resistance, we enriched for S. aureus transposon mutants with defects in cell surface integrity or cell division by sorting for cells that stain with propidium iodide or have increased light-scattering properties, respectively. Transposon sequencing of the sorted populations identified more than 20 previously uncharacterized factors impacting these processes. Cells inactivated for one of these proteins, factor preventing extra Z-rings (FacZ, SAOUHSC_01855), showed aberrant membrane invaginations and multiple FtsZ cytokinetic rings. These phenotypes were suppressed in mutants lacking the conserved cell-division protein GpsB, which forms an interaction hub bridging envelope biogenesis factors with the cytokinetic ring in S. aureus. FacZ was found to interact directly with GpsB in vitro and in vivo. We therefore propose that FacZ is an envelope biogenesis factor that antagonizes GpsB function to prevent aberrant division events in S. aureus.

Gausemycin A-Resistant Staphylococcus aureus Demonstrates Affected Cell Membrane and Cell Wall Homeostasis

Antibiotic resistance is a significant and pressing issue in the medical field, as numerous strains of infectious bacteria have become resistant to commonly prescribed antibiotics. Staphylococcus aureus is a bacterium that poses a grave threat, as it is responsible for a large number of nosocomial infections and has high mortality rates worldwide. Gausemycin A is a new lipoglycopeptide antibiotic that has considerable efficacy against multidrug-resistant S. aureus strains. Although the cellular targets of gausemycin A have been previously identified, detailing the molecular processes of action is still needed. We performed gene expression analysis to identify molecular mechanisms that may be involved in bacterial resistance to gausemycin A. In the present study, we observed that gausemycin A-resistant S. aureus in the late-exponential phase showed an increased expression of genes involved in cell wall turnover (sceD), membrane charge (dltA), phospholipid metabolism (pgsA), the two-component stress-response system (vraS), and the Clp proteolytic system (clpX). The increased expression of these genes implies that changes in the cell wall and cell membrane are essential for the bacterial resistance to gausemycin A. In the stationary phase, we observed a decrease in the expression of genes involved in the phospholipid metabolism (mprF) and Clp proteolytic system (clpX).

Autolysin-mediated peptidoglycan hydrolysis is required for the surface display of Staphylococcus aureus cell wall-anchored proteins

Peptidoglycan hydrolases, or autolysins, play a critical role in cell wall remodeling and degradation, facilitating bacterial growth, cell division, and cell separation. In Staphylococcus aureus, the so-called "major" autolysin, Atl, has long been associated with host adhesion; however, the molecular basis underlying this phenomenon remains understudied. To investigate, we used the type V glycopeptide antibiotic complestatin, which binds to peptidoglycan and blocks the activity of autolysins, as a chemical probe of autolysin function. We also generated a chromosomally encoded, catalytically inactive variant of the Atl enzyme. Autolysin-mediated peptidoglycan hydrolysis, in particular Atl-mediated daughter cell separation, was shown to be critical for maintaining optimal surface levels of S. aureus cell wall-anchored proteins, including the fibronectin-binding proteins (FnBPs) and protein A (Spa). As such, disrupting autolysin function reduced the affinity of S. aureus for host cell ligands, and negatively impacted early stages of bacterial colonization in a systemic model of S. aureus infection. Phenotypic studies revealed that Spa was sequestered at the septum of complestatin-treated cells, highlighting that autolysins are required to liberate Spa during cell division. In summary, we reveal the hydrolytic activities of autolysins are associated with the surface display of S. aureus cell wall-anchored proteins. We demonstrate that by blocking autolysin function, type V glycopeptide antibiotics are promising antivirulence agents for the development of strategies to control S. aureus infections.

Glucomannan as a polysaccharide adjuvant improved immune responses against Staphylococcus aureus: Potency and efficacy studies

Staphylococcus aureus is a gram-positive bacterium, representing one of the most important nosocomial pathogens. The treatment of infections, caused by S. aureus, has become increasingly intricate due to the emergence of highly resistant strains. Therefore, it is obvious that an effective prevention strategy against this bacterium could significantly decrease such infections. In the present study, the protective efficacy and immunological properties of recombinant autolysin, formulated in Montanide ISA266 and Alum adjuvants with Glucomannan as a polysaccharide, were assessed in the systemic mouse model of infection. Mice were immunized with the purified recombinant protein in various formulations in different groups and, subsequently, mice were challenged with 5 × 10⁸ CFU of bacteria for the evaluation of their survival and bacterial clearances in the internal organs. ELISA was performed to determine the type of induced immunity, cytokine secretion (IFN-γ, IL-4, IL-2, and IL-17), and isotyping (IgG1 and IgG2a). In addition, we measured the opsonophagocytic activities of the antibodies. Results showed that immunization with r-autolysin + Alum + Glucomannan and r-autolysin + MontanideISA266+Glucomannan formulations significantly increased total IgG and isotypes (IgG1 and IgG2a), as compared with other vaccinated and control groups. Furthermore, the formulation of r-autolysin in Alum and MontanideISA266 adjuvants with Glucomannan enhanced IFN-γ, IL-4, and IL-17 cytokine secretion as well as protectivity, following experimental challenge. We concluded that Glucomannan has the potential to induce immune responses and would be used as an adjuvant factor in vaccine formulation.

Mechanistic Insights into the Activities of Major Families of Enzymes in Bacterial Peptidoglycan Assembly and Breakdown

Serving as an exoskeletal scaffold, peptidoglycan is a polymeric macromolecule that is essential and conserved across all bacteria, yet is absent in mammalian cells; this has made bacterial peptidoglycan a well-established excellent antibiotic target. In addition, soluble peptidoglycan fragments derived from bacteria are increasingly recognised as key signalling molecules in mediating diverse intra- and inter-species communication in nature, including in gut microbiota-host crosstalk. Each bacterial species encodes multiple redundant enzymes for key enzymatic activities involved in peptidoglycan assembly and breakdown. In this review, we discuss recent findings on the biochemical activities of major peptidoglycan enzymes, including peptidoglycan glycosyltransferases (PGT) and transpeptidases (TPs) in the final stage of peptidoglycan assembly, as well as peptidoglycan glycosidases, lytic transglycosylase (LTs), amidases, endopeptidases (EPs) and carboxypeptidases (CPs) in peptidoglycan turnover and metabolism. Biochemical characterisation of these enzymes provides valuable insights into their substrate specificity, regulation mechanisms and potential modes of inhibition.

The cell cycle of Staphylococcus aureus: An updated review

As bacteria proliferate, DNA replication, chromosome segregation, cell wall synthesis, and cytokinesis occur concomitantly and need to be tightly regulated and coordinated. Although these cell cycle processes have been studied for decades, several mechanisms remain elusive, specifically in coccus-shaped cells such as Staphylococcus aureus. In recent years, major progress has been made in our understanding of how staphylococci divide, including new, fundamental insights into the mechanisms of cell wall synthesis and division site selection. Furthermore, several novel proteins and mechanisms involved in the regulation of replication initiation or progression of the cell cycle have been identified and partially characterized. In this review, we will summarize our current understanding of the cell cycle processes in the spheroid model bacterium S. aureus, with a focus on recent advances in the understanding of how these processes are regulated.

The Discovery of the Role of Outer Membrane Vesicles against Bacteria

Gram-negative bacteria are intrinsically resistant to many commercialized antibiotics. The outer membrane (OM) of Gram-negative bacteria prevents the entry of such antibiotics. Outer membrane vesicles (OMV) are naturally released from the OM of Gram-negative bacteria for a range of purposes, including competition with other bacteria. OMV may carry, as part of the membrane or lumen, molecules with antibacterial activity. Such OMV can be exposed to and can fuse with the cell surface of different bacterial species. In this review we consider how OMV can be used as tools to deliver antimicrobial agents. This includes the characteristics of OMV production and how this process can be used to create the desired antibacterial activity of OMV.

Comparative Study of Bacterial SPOR Domains Identifies Functionally Important Differences in Glycan Binding Affinity

Bacterial SPOR domains target proteins to the divisome by binding septal peptidoglycan (PG) at sites where cell wall amidases have removed stem peptides. These PG structures are referred to as denuded glycans. Although all characterized SPOR domains bind denuded glycans, whether there are differences in affinity is not known. Here, we use isothermal titration calorimetry (ITC) to determine the relative PG glycan binding affinity (<i>K</i><sub>d</sub>) of four Escherichia coli SPOR domains and one Cytophaga hutchinsonii SPOR domain. We found that the <i>K</i><sub>d</sub> values ranged from approximately 1 μM for E. coli DamX<sup>SPOR</sup> and <i>C. hutchinsonii</i> CHU2221<sup>SPOR</sup> to about 10 μM for E. coli FtsN<sup>SPOR</sup>. To investigate whether these differences in PG binding affinity are important for SPOR domain protein function, we constructed and characterized a set of DamX and FtsN "swap" proteins. As expected, all SPOR domain swap proteins localized to the division site, and, in the case of FtsN, all of the heterologous SPOR domains supported cell division. However, for DamX, only the high-affinity SPOR domain from CHU2221 supported normal function in cell division. In summary, different SPOR domains bind denuded PG glycans with different affinities, which appears to be important for the functions of some SPOR domain proteins (e.g., DamX) but not for the functions of others (e.g., FtsN). <b>IMPORTANCE</b> SPOR domain proteins are prominent components of the cell division apparatus in a wide variety of bacteria. The primary function of SPOR domains is targeting proteins to the division site, which they accomplish by binding to septal peptidoglycan. However, whether SPOR domains have any functions beyond septal targeting is unknown. Here, we show that SPOR domains vary in their PG binding affinities and that, at least in the case of the E. coli cell division protein DamX, having a high-affinity SPOR domain contributes to proper function.

Evidence of Neutralizing and Non-Neutralizing Anti-Glucosaminidase Antibodies in Patients With S. Aureus Osteomyelitis and Their Association With Clinical Outcome Following Surgery in a Clinical Pilot

Staphylococcus aureus osteomyelitis remains a very challenging condition; recent clinical studies have shown infection control rates following surgery/antibiotics to be ~60%. Additionally, prior efforts to produce an effective S. aureus vaccine have failed, in part due to lack of knowledge of protective immunity. Previously, we demonstrated that anti-glucosaminidase (Gmd) antibodies are protective in animal models but found that only 6.7% of culture-confirmed S. aureus osteomyelitis patients in the AO Clinical Priority Program (AO-CPP) Registry had basal serum levels (>10 ng/ml) of anti-Gmd at the time of surgery (baseline). We identified a small subset of patients with high levels of anti-Gmd antibodies and adverse outcomes following surgery, not explained by Ig class switching to non-functional isotypes. Here, we aimed to test the hypothesis that clinical cure following surgery is associated with anti-Gmd neutralizing antibodies in serum. Therefore, we first optimized an in vitro assay that quantifies recombinant Gmd lysis of the M. luteus cell wall and used it to demonstrate the 50% neutralizing concentration (NC50) of a humanized anti-Gmd mAb (TPH-101) to be ~15.6 μg/ml. We also demonstrated that human serum deficient in anti-Gmd antibodies can be complemented by TPH-101 to achieve the same dose-dependent Gmd neutralizing activity as purified TPH-101. Finally, we assessed the anti-Gmd physical titer and neutralizing activity in sera from 11 patients in the AO-CPP Registry, who were characterized into four groups post-hoc. Group 1 patients (n=3) had high anti-Gmd physical and neutralizing titers at baseline that decreased with clinical cure of the infection over time. Group 2 patients (n=3) had undetectable anti-Gmd antibodies throughout the study and adverse outcomes. Group 3 (n=3) had high titers +/- neutralizing anti-Gmd at baseline with adverse outcomes. Group 4 (n=2) had low titers of non-neutralizing anti-Gmd at baseline with delayed high titers and adverse outcomes. Collectively, these findings demonstrate that both neutralizing and non-neutralizing anti-Gmd antibodies exist in S. aureus osteomyelitis patients and that screening for these antibodies could have a value for identifying patients in need of passive immunization prior to surgery. Future prospective studies to test the prognostic value of anti-Gmd antibodies to assess the potential of passive immunization with TPH-101 are warranted.

Staphylococcus aureus adhesion to the host

Staphylococcus aureus is a pathobiont capable of colonizing and infecting most tissues within the human body, resulting in a multitude of different clinical outcomes. Adhesion of S. aureus to the host is crucial for both host colonization and the establishment of infections. Underlying the pathogen's success is a complex and diverse arsenal of adhesins. In this review, we discuss the different classes of adhesins, including a consideration of the various adhesion sites throughout the body and the clinical outcomes of each infection type. The development of therapeutics targeting the S. aureus host-pathogen interaction is a relatively understudied area. Due to the increasing global threat of antimicrobial resistance, it is crucial that innovative and alternative approaches are considered. Neutralizing virulence factors, through the development of antivirulence agents, could reduce bacterial pathogenicity and the ever-increasing burden of S. aureus infections. This review provides insight into potentially efficacious adhesion-associated targets for the development of novel decolonizing and antivirulence strategies.

Low Concentration of the Neutrophil Proteases Cathepsin G, Cathepsin B, Proteinase-3 and Metalloproteinase-9 Induce Biofilm Formation in Non-Biofilm-Forming Staphylococcus epidermidis Isolates

Neutrophils play a crucial role in eliminating bacteria that invade the human body; however, cathepsin G can induce biofilm formation in a non-biofilm-forming Staphylococcus epidermidis 1457 strain, suggesting that neutrophil proteases may be involved in biofilm formation. Cathepsin G, cathepsin B, proteinase-3, and metalloproteinase-9 (MMP-9) from neutrophils were tested on the biofilm induction in commensal (skin isolated) and clinical non-biofilm-forming S. epidermidis isolates. From 81 isolates, 53 (74%) were aap+, icaA−, icaD− genotype, and without the capacity of biofilm formation under conditions of 1% glucose, 4% ethanol or 4% NaCl, but these 53 non-biofilm-forming isolates induced biofilm by the use of different neutrophil proteases. Of these, 62.3% induced biofilm with proteinase-3, 15% with cathepsin G, 10% with cathepsin B and 5% with MMP -9, where most of the protease-induced biofilm isolates were commensal strains (skin). In the biofilm formation kinetics analysis, the addition of phenylmethylsulfonyl fluoride (PMSF; a proteinase-3 inhibitor) showed that proteinase-3 participates in the cell aggregation stage of biofilm formation. A biofilm induced with proteinase-3 and DNAse-treated significantly reduced biofilm formation at an early time (initial adhesion stage of biofilm formation) compared to untreated proteinase-3-induced biofilm (p < 0.05). A catheter inoculated with a commensal (skin) non-biofilm-forming S. epidermidis isolate treated with proteinase-3 and another one without the enzyme were inserted into the back of a mouse. After 7 days of incubation period, the catheters were recovered and the number of grown bacteria was quantified, finding a higher amount of adhered proteinase-3-treated bacteria in the catheter than non-proteinase-3-treated bacteria (p < 0.05). Commensal non-biofilm-forming S. epidermidis in the presence of neutrophil cells significantly induced the biofilm formation when multiplicity of infection (MOI) 1:0.01 (neutrophil:bacteria) was used, but the addition of a cocktail of protease inhibitors impeded biofilm formation. A neutrophil:bacteria assay did not induce neutrophil extracellular traps (NETs). Our results suggest that neutrophils, in the presence of commensal non-biofilm-forming S. epidermidis, do not generate NETs formation. The effect of neutrophils is the production of proteases, and proteinase-3 releases bacterial DNA at the initial adhesion, favoring cell aggregation and subsequently leading to biofilm formation.

Exploiting transposons in the study of Staphylococcus aureus pathogenesis and virulence

The opportunistic pathogen Staphylococcus aureus has an extremely complex relationship with humans. While the bacteria can exist as a commensal in many, it can cause a wide range of diseases and infections when turned pathogenic. Its presence is a determinant of chronicity and poor prognosis in numerous diseases, and its genomic plasticity causes S. aureus antimicrobial resistance to be one of the most dire contemporary medical problems to solve. Genetic manipulation of S. aureus has led to numerous findings that are vital in the fight against its pathogenesis. The utilisation of transposon mutant libraries for the systematic inspection of the S. aureus genome led to many landmark discoveries pertaining to the bacteria's pathogenicity, antimicrobial resistance acquisition, and virulence regulation. In this review, we describe mutant libraries, and their significant contributions, from various S. aureus strains created with commonly used transposons. The general workflow for the construction of libraries will be presented, along with a discussion of the challenges of undertaking the task of large-scale library construction. As the accessibility of transposon mutant library construction, screening, and analysis increases, this genetic tool could be further exploited in the study of the S. aureus genome.

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.

The Major Autolysin Atl Regulates the Virulence of Staphylococcus aureus by Controlling the Sorting of LukAB

Infections caused by the Gram-positive bacterium Staphylococcus aureus remain a significant health threat globally. The production of bicomponent pore-forming leukocidins plays an important role in S. aureus pathogenesis. Transcriptionally, these toxins are primarily regulated by the Sae and Agr regulatory systems. However, the posttranslational regulation of these toxins is largely unexplored. In particular, one of the leukocidins, LukAB, has been shown to be both secreted into the extracellular milieu and associated with the bacterial cell envelope. Here, we report that a major cell wall hydrolase, autolysin (Atl), controls the sorting of LukAB from the cell envelope to the extracellular milieu, an effect independent of transcriptional regulation. By influencing the sorting of LukAB, Atl modulates S. aureus cytotoxicity toward primary human neutrophils. Mechanistically, we found that the reduction in peptidoglycan cleavage and increased LukAB secretion in the atl mutant can be reversed through the supplementation of exogenous mutanolysin. Altogether, our study revealed that the cell wall hydrolase activity of Atl and the cleavage of peptidoglycan play an important role in controlling the sorting of S. aureus toxins during secretion.

NamZ1 and NamZ2 from the oral pathogen Tannerella forsythia are peptidoglycan processing exo-β-N-acetylmuramidases with distinct substrate specificity

The Gram-negative periodontal pathogen Tannerella forsythia is inherently auxotrophic for N-acetylmuramic acid (MurNAc), which is an essential carbohydrate constituent of the peptidoglycan (PGN) of the bacterial cell wall. Thus, to build up its cell wall, T. forsythia strictly depends on the salvage of exogenous MurNAc or sources of MurNAc, such as polymeric or fragmentary PGN, derived from cohabiting bacteria within the oral microbiome. In our effort to elucidate how T. forsythia satisfies its demand for MurNAc, we recognized that the organism possesses three putative orthologs of the exo-β-N-acetylmuramidase BsNamZ from Bacillus subtilis, which cleaves non-reducing end, terminal MurNAc entities from the artificial substrate pNP-MurNAc and the naturally-occurring disaccharide substrate MurNAc-N-acetylglucosamine (GlcNAc). TfNamZ1 and TfNamZ2 were successfully purified as soluble, pure recombinant His₆-fusions and characterized as exo-lytic β-N-acetylmuramidases with distinct substrate specificities. The activity of TfNamZ1 was considerably lower compared to TfNamZ2 and BsNamZ, in the cleavage of MurNAc-GlcNAc. When peptide-free PGN glycans were used as substrates, we revealed striking differences in the specificity and mode of action of these enzymes, as analyzed by mass spectrometry. TfNamZ1, but not TfNamZ2 or BsNamZ, released GlcNAc-MurNAc disaccharides from these glycans. In addition, glucosamine (GlcN)-MurNAc disaccharides were generated when partially N-deacetylated PGN glycans from B. subtilis 168 were applied. This characterizes TfNamZ1 as a unique disaccharide-forming exo-lytic β-N-acetylmuramidase (exo-disaccharidase), and, TfNamZ2 and BsNamZ as sole MurNAc monosaccharide-lytic exo-β-N-acetylmuramidases. IMPORTANCE Two exo-N-acetylmuramidases from T. forsythia belonging to glycosidase family GH171 (www.cazy.org) were shown to differ in their activities, thus revealing a functional diversity within this family: NamZ1 releases disaccharides (GlcNAc-MurNAc/GlcN-MurNAc) from the non-reducing ends of PGN glycans, whereas NamZ2 releases terminal MurNAc monosaccharides. This work provides a better understanding of how T. forsythia may acquire the essential growth factor MurNAc by the salvage of PGN from cohabiting bacteria in the oral microbiome, which may pave avenues for the development of anti-periodontal drugs. On a broad scale, our study indicates that the utilization of PGN as a nutrient source, involving exo-lytic N-acetylmuramidases with different modes of action, appears to be a general feature of bacteria, particularly among the phylum Bacteroidetes.

Demonstration of the role of cell wall homeostasis in Staphylococcus aureus growth and the action of bactericidal antibiotics

Bacterial cell wall peptidoglycan is essential, maintaining both cellular integrity and morphology, in the face of internal turgor pressure. Peptidoglycan synthesis is important, as it is targeted by cell wall antibiotics, including methicillin and vancomycin. Here, we have used the major human pathogen Staphylococcus aureus to elucidate both the cell wall dynamic processes essential for growth (life) and the bactericidal effects of cell wall antibiotics (death) based on the principle of coordinated peptidoglycan synthesis and hydrolysis. The death of S. aureus due to depletion of the essential, two-component and positive regulatory system for peptidoglycan hydrolase activity (WalKR) is prevented by addition of otherwise bactericidal cell wall antibiotics, resulting in stasis. In contrast, cell wall antibiotics kill via the activity of peptidoglycan hydrolases in the absence of concomitant synthesis. Both methicillin and vancomycin treatment lead to the appearance of perforating holes throughout the cell wall due to peptidoglycan hydrolases. Methicillin alone also results in plasmolysis and misshapen septa with the involvement of the major peptidoglycan hydrolase Atl, a process that is inhibited by vancomycin. The bactericidal effect of vancomycin involves the peptidoglycan hydrolase SagB. In the presence of cell wall antibiotics, the inhibition of peptidoglycan hydrolase activity using the inhibitor complestatin results in reduced killing, while, conversely, the deregulation of hydrolase activity via loss of wall teichoic acids increases the death rate. For S. aureus, the independent regulation of cell wall synthesis and hydrolysis can lead to cell growth, death, or stasis, with implications for the development of new control regimes for this important pathogen.

Accumulation of Succinyl Coenzyme A Perturbs the Methicillin-Resistant Staphylococcus aureus (MRSA) Succinylome and Is Associated with Increased Susceptibility to Beta-Lactam Antibiotics

Penicillin binding protein 2a (PBP2a)-dependent resistance to β-lactam antibiotics in methicillin-resistant Staphylococcus aureus (MRSA) is regulated by the activity of the tricarboxylic acid (TCA) cycle via a poorly understood mechanism. We report that mutations in sucC and sucD, but not other TCA cycle enzymes, negatively impact β-lactam resistance without changing PBP2a expression. Increased intracellular levels of succinyl coenzyme A (succinyl-CoA) in the sucC mutant significantly perturbed lysine succinylation in the MRSA proteome. Suppressor mutations in sucA or sucB, responsible for succinyl-CoA biosynthesis, reversed sucC mutant phenotypes. The major autolysin (Atl) was the most succinylated protein in the proteome, and increased Atl succinylation in the sucC mutant was associated with loss of autolytic activity. Although PBP2a and PBP2 were also among the most succinylated proteins in the MRSA proteome, peptidoglycan architecture and cross-linking were unchanged in the sucC mutant. These data reveal that perturbation of the MRSA succinylome impacts two interconnected cell wall phenotypes, leading to repression of autolytic activity and increased susceptibility to β-lactam antibiotics.

Tracking the Subcellular Localization of Surface Proteins in Staphylococcus aureus by Immunofluorescence Microscopy

Surface proteins of Staphylococcus aureus and other Gram-positive bacteria play essential roles in bacterial colonization and host-microbe interactions. Surface protein precursors containing a YSIRK/GXXS signal peptide are translocated across the septal membrane at mid-cell, anchored to the cell wall peptidoglycan at the cross-wall compartment, and presented on the new hemispheres of the daughter cells following cell division. After several generations of cell division, these surface proteins will eventually cover the entire cell surface. To understand how these proteins travel from the bacterial cytoplasm to the cell surface, we describe a series of immunofluorescence microscopy protocols designed to detect the stepwise subcellular localization of the surface protein precursors: surface display (protocol A), cross-wall localization (protocol B), and cytoplasmic/septal membrane localization (protocol C). Staphylococcal protein A (SpA) is the model protein used in this work. The protocols described here are readily adapted to study the localization of other surface proteins as well as other cytoplasmic or membrane proteins in S. aureus in general. Furthermore, the protocols can be modified and adapted for use in other Gram-positive bacteria. Graphic abstract: Tracking the subcellular localization of surface proteins in S. aureus.

Peptidoglycan Contribution to the B Cell Superantigen Activity of Staphylococcal Protein A

Staphylococcus aureus causes reiterative and chronic persistent infections. This can be explained by the formidable ability of this pathogen to escape immune surveillance mechanisms. Cells of S. aureus display the abundant staphylococcal protein A (SpA). SpA binds to immunoglobulin (Ig) molecules and coats the bacterial surface to prevent phagocytic uptake. SpA also binds and cross-links variable heavy 3 (VH3) idiotype (IgM) B cell receptors, promoting B cell expansion and the secretion of nonspecific VH3-IgM via a mechanism requiring CD4+ T cell help. SpA binding to antibodies is mediated by the N-terminal Ig-binding domains (IgBDs). The so-called region X, uncharacterized LysM domain, and C-terminal LPXTG sorting signal for peptidoglycan attachment complete the linear structure of the protein. Here, we report that both the LysM domain and the LPXTG motif sorting signal are required for the B cell superantigen activity of SpA in a mouse model of infection. SpA molecules purified from staphylococcal cultures are sufficient to exert B cell superantigen activity and promote immunoglobulin secretion as long as they carry intact LysM and LPXTG motif domains with bound peptidoglycan fragments. The LysM domain binds the glycan chains of peptidoglycan fragments, whereas the LPXTG motif is covalently linked to wall peptides lacking glycan. These findings emphasize the complexity of SpA interactions with B cell receptors.IMPORTANCE The LysM domain is found in all kingdoms of life. While their function in mammals is not known, LysM domains of bacteria and their phage parasites are associated with enzymes that cleave or remodel peptidoglycan. Plants recognize microbe-associated molecular patterns such as chitin via receptors endowed with LysM-containing ectodomains. In plants, such receptors play equally important roles in defense and symbiosis signaling. SpA of S. aureus carries a LysM domain that binds glycan strands of peptidoglycan to influence defined B cell responses that divert pathogen-specific adaptive immune responses.

Staphylococcus aureus cell wall structure and dynamics during host-pathogen interaction

Peptidoglycan is the major structural component of the Staphylococcus aureus cell wall, in which it maintains cellular integrity, is the interface with the host, and its synthesis is targeted by some of the most crucial antibiotics developed. Despite this importance, and the wealth of data from in vitro studies, we do not understand the structure and dynamics of peptidoglycan during infection. In this study we have developed methods to harvest bacteria from an active infection in order to purify cell walls for biochemical analysis ex vivo. Isolated ex vivo bacterial cells are smaller than those actively growing in vitro, with thickened cell walls and reduced peptidoglycan crosslinking, similar to that of stationary phase cells. These features suggested a role for specific peptidoglycan homeostatic mechanisms in disease. As S. aureus missing penicillin binding protein 4 (PBP4) has reduced peptidoglycan crosslinking in vitro its role during infection was established. Loss of PBP4 resulted in an increased recovery of S. aureus from the livers of infected mice, which coincided with enhanced fitness within murine and human macrophages. Thicker cell walls correlate with reduced activity of peptidoglycan hydrolases. S. aureus has a family of 4 putative glucosaminidases, that are collectively crucial for growth. Loss of the major enzyme SagB, led to attenuation during murine infection and reduced survival in human macrophages. However, loss of the other three enzymes Atl, SagA and ScaH resulted in clustering dependent attenuation, in a zebrafish embryo, but not a murine, model of infection. A combination of pbp4 and sagB deficiencies resulted in a restoration of parental virulence. Our results, demonstrate the importance of appropriate cell wall structure and dynamics during pathogenesis, providing new insight to the mechanisms of disease.

The prevalence of methicillin resistant Staphylococcus aureus (MRSA) in both hospitals and the wider community places a huge weight on healthcare providers. To discover new control regimes, it is therefore important to understand how the pathogen behaves within the relevant environment of the host. This is often hampered by the ability to obtain sufficient ex vivo pathogen samples for study. We have developed a method to isolate S. aureus from the infected host to be able to analyse cellular morphology and structure. S. aureus, isolated from an infected kidney abscess are smaller in size, with thicker cell walls than exponentially growing cells in vitro. Their cell wall peptidoglycan also is less crosslinked. These features suggested the role of components controlling cell wall homeostasis as being important for infections. We tested the role of PBP4, known to increase cell wall crosslinking and found a pbp4 mutant to have increased survival in macrophages and fitness within the murine host. Conversely the peptidoglycan hydrolase SagB, whose loss results in thinner cell walls was attenuated in the murine systemic model of infection, with concomitant loss of fitness within macrophages. Our study reveals an important adaptation to the host environment and the role of those components involved in cell wall homeostasis in vivo.

Staphylococcal Biofilms: Challenges and Novel Therapeutic Perspectives

Staphylococci, like Staphylococcus aureus and S. epidermidis, are common colonizers of the human microbiota. While being harmless in many cases, many virulence factors result in them being opportunistic pathogens and one of the major causes of hospital-acquired infections worldwide. One of these virulence factors is the ability to form biofilms-three-dimensional communities of microorganisms embedded in an extracellular polymeric matrix (EPS). The EPS is composed of polysaccharides, proteins and extracellular DNA, and is finely regulated in response to environmental conditions. This structured environment protects the embedded bacteria from the human immune system and decreases their susceptibility to antimicrobials, making infections caused by staphylococci particularly difficult to treat. With the rise of antibiotic-resistant staphylococci, together with difficulty in removing biofilms, there is a great need for new treatment strategies. The purpose of this review is to provide an overview of our current knowledge of the stages of biofilm development and what difficulties may arise when trying to eradicate staphylococcal biofilms. Furthermore, we look into promising targets and therapeutic methods, including bacteriocins and phage-derived antibiofilm approaches.

Role of SrtA in Pathogenicity of Staphylococcus lugdunensis

Among coagulase-negative staphylococci (CoNS), Staphylococcus lugdunensis has a special position as causative agent of aggressive courses of infectious endocarditis (IE) more reminiscent of IEs caused by Staphylococcus aureus than those by CoNS. To initiate colonization and invasion, bacterial cell surface proteins are required; however, only little is known about adhesion of S. lugdunensis to biotic surfaces. Cell surface proteins containing the LPXTG anchor motif are covalently attached to the cell wall by sortases. Here, we report the functionality of Staphylococcus lugdunensis sortase A (SrtA) to link LPXTG substrates to the cell wall. To determine the role of SrtA dependent surface proteins in biofilm formation and binding eukaryotic cells, we generated SrtA-deficient mutants (ΔsrtA). These mutants formed a smaller amount of biofilm and bound less to immobilized fibronectin, fibrinogen, and vitronectin. Furthermore, SrtA absence affected the gene expression of two different adhesins on transcription level. Surprisingly, we found no decreased adherence and invasion in human cell lines, probably caused by the upregulation of further adhesins in ΔsrtA mutant strains. In conclusion, the functionality of S. lugdunensis SrtA in anchoring LPXTG substrates to the cell wall let us define it as the pathogen’s housekeeping sortase.

New insights in the coordinated amidase and glucosaminidase activity of the major autolysin (Atl) in Staphylococcus aureus

After bacterial cell division, the daughter cells are still covalently interlinked by the peptidoglycan network which is resolved by specific hydrolases (autolysins) to release the daughter cells. In staphylococci, the major autolysin (Atl) with its two domain enzymes, N-acetylmuramyl-L-alanine amidase (AmiA) and β-N-acetylglucosaminidase (GlcA), resolves the peptidoglycan to release the daughter cells. Internal deletions in each of the enzyme domains revealed defined morphological alterations such as cell cluster formation in ΔamiA, ΔglcA and Δatl, and asymmetric cell division in the ΔglcA. A most important finding was that GlcA activity requires the prior removal of the stem peptide by AmiA for its activity thus the naked glycan strand is its substrate. Furthermore, GlcA is not an endo-β-N-acetylglucosaminidase but an exo-enzyme that cuts the glycan backbone to disaccharides independent of its O-acetylation modification. Our results shed new light into the sequential peptidoglycan hydrolysis by AmiA and GlcA during cell division in staphylococci.

Nega et al. shed light on the interplay of the two domain enzymes of the major autolysin, AmiA and GlcA, in S. aureus for peptidoglycan hydrolysis during bacterial cell division. They show that GlcA requires the prior removal of the stem peptide by AmiA for its activity and that GlcA is not an endo-enzyme as previously thought, but an exo-enzyme that chops down the glycan backbone to disaccharides independent of its O-acetylation modification.

Lack of Humoral Immunity Against Glucosaminidase Is Associated with Postoperative Complications in Staphylococcus aureus Osteomyelitis

BACKGROUND

Glucosaminidase (Gmd) is known to be a protective antigen in animal models of Staphylococcus aureus osteomyelitis. We compared the endogenous anti-Gmd antibody levels in sera of patients with culture-confirmed S. aureus bone infections to their sera at 1 year after operative treatment of the infection.

METHODS

A novel global biospecimen registry of 297 patients with deep-wound culture-confirmed S. aureus osteomyelitis was analyzed to assess relationships between baseline anti-Gmd serum titers (via custom Luminex assay), known host risk factors for infection, and 1-year postoperative clinical outcomes (e.g., infection control, inconclusive, refracture, persistent infection, septic nonunion, amputation, and septic death).

RESULTS

All patients had measurable humoral immunity against some S. aureus antigens, but only 20 patients (6.7%; p < 0.0001) had high levels of anti-Gmd antibodies (>10 ng/mL) in serum at baseline. A subset of 194 patients (65.3%) who completed 1 year of follow-up was divided into groups based on anti-Gmd level: low (<1 ng/mL, 54 patients; 27.8%), intermediate (<10 ng/mL, 122 patients; 62.9%), and high (>10 ng/mL, 18 patients; 9.3%), and infection control rates were 40.7%, 50.0%, and 66.7%, respectively. The incidence of adverse outcomes in these groups was 33.3%, 16.4%, and 11.1%, respectively. Assessing anti-Gmd level as a continuous variable showed a 60% reduction in adverse-event odds (p = 0.04) for every tenfold increase in concentration. No differences in patient demographics, body mass index of >40 kg/m, diabetes status, age of ≥70 years, male sex, Charlson Comorbidity Index of >1, or Cierny-Mader host type were observed between groups, and these risk factors were not associated with adverse events. Patients with low anti-Gmd titer demonstrated a significant 2.68-fold increased odds of adverse outcomes (p = 0.008).

CONCLUSIONS

Deficiency in circulating anti-Gmd antibodies was associated serious adverse outcomes following operative treatment of S. aureus osteomyelitis. At 1 year, high levels of anti-Gmd antibodies were associated with a nearly 3-fold increase in infection-control odds. Additional prospective studies clarifying Gmd immunization for osteomyelitis are needed.

LEVEL OF EVIDENCE

Prognostic Level IV. See Instructions for Authors for a complete description of levels of evidence.

The Role of Bacterial Membrane Vesicles in the Dissemination of Antibiotic Resistance and as Promising Carriers for Therapeutic Agent Delivery

The rapid emergence and spread of antibiotic-resistant bacteria continues to be an issue difficult to deal with, especially in the clinical, animal husbandry, and food fields. The occurrence of multidrug-resistant bacteria renders treatment with antibiotics ineffective. Therefore, the development of new therapeutic methods is a worthwhile research endeavor in treating infections caused by antibiotic-resistant bacteria. Recently, bacterial membrane vesicles (BMVs) have been investigated as a possible approach to drug delivery and vaccine development. The BMVs are released by both pathogenic and non-pathogenic Gram-positive and Gram-negative bacteria, containing various components originating from the cytoplasm and the cell envelope. The BMVs are able to transform bacteria with genes that encode enzymes such as proteases, glycosidases, and peptidases, resulting in the enhanced antibiotic resistance in bacteria. The BMVs can increase the resistance of bacteria to antibiotics. However, the biogenesis and functions of BMVs are not fully understood in association with the bacterial pathogenesis. Therefore, this review aims to discuss BMV-associated antibiotic resistance and BMV-based therapeutic interventions.

Domain sliding of two Staphylococcus aureus N-acetylglucosaminidases enables their substrate-binding prior to its catalysis

To achieve productive binding, enzymes and substrates must align their geometries to complement each other along an entire substrate binding site, which may require enzyme flexibility. In pursuit of novel drug targets for the human pathogen S. aureus, we studied peptidoglycan N-acetylglucosaminidases, whose structures are composed of two domains forming a V-shaped active site cleft. Combined insights from crystal structures supported by site-directed mutagenesis, modeling, and molecular dynamics enabled us to elucidate the substrate binding mechanism of SagB and AtlA-gl. This mechanism requires domain sliding from the open form observed in their crystal structures, leading to polysaccharide substrate binding in the closed form, which can enzymatically process the bound substrate. We suggest that these two hydrolases must exhibit unusual extents of flexibility to cleave the rigid structure of a bacterial cell wall.

Deriving a dose and regimen for anti-glucosaminidase antibody passive-immunisation for patients with Staphylococcus aureus osteomyelitis

Staphylococcus aureus (S. aureus) osteomyelitis remains a major clinical problem. Anti-glucosaminidase (Gmd) antibodies (1C11) are efficacious in prophylactic and therapeutic murine models. Feasibility, safety and pharmacokinetics of 1C11 passive immunisation in sheep and endogenous anti-Gmd levels were quantified in osteomyelitis patients. 3 sheep received a 500 mg intravenous (i.v.) bolus of 1C11 and its levels in sera were determined by enzyme-linked immunosorbent assay (ELISA) over 52 d. A humanised anti-Gmd monoclonal antibody, made by grafting the antigen-binding fragment (Fab) portion of 1C11 onto the fragment crystallisable region (Fc) of human IgG1, was used to make a standard curve of mean fluorescent intensity versus concentration of anti-Gmd. Anti-Gmd serum levels were determined in 297 patients with culture-confirmed S. aureus osteomyelitis and 40 healthy controls. No complications or adverse events were associated with the sheep 1C11 i.v. infusion and the estimated circulating half-life of 1C11 was 23.7 d. Endogenous anti-Gmd antibody levels in sera of osteomyelitis patients ranged from < 1 ng/mL to 300 µg/mL, with a mean concentration of 21.7 µg/mL. The estimated circulating half-life of endogenous anti-Gmd antibodies in sera of 12 patients with cured osteomyelitis was 120.4 d. A clinically relevant administration of anti-Gmd (500 mg i.v. = 7 mg/kg/70 kg human) was safe in sheep. This dose was 8 times more than the endogenous anti-Gmd levels observed in osteomyelitis patients and was predicted to have a half-life of > 3 weeks. Anti-Gmd passive immunisation has potential to prevent and treat S. aureus osteomyelitis. Further clinical development is warranted.

Uncovering the activities, biological roles, and regulation of bacterial cell wall hydrolases and tailoring enzymes

Bacteria account for 1000-fold more biomass than humans. They vary widely in shape and size. The morphological diversity of bacteria is due largely to the different peptidoglycan-based cell wall structures that encase bacterial cells. Although the basic structure of peptidoglycan is highly conserved, consisting of long glycan strands that are cross-linked by short peptide chains, the mature cell wall is chemically diverse. Peptidoglycan hydrolases and cell wall-tailoring enzymes that regulate glycan strand length, the degree of cross-linking, and the addition of other modifications to peptidoglycan are central in determining the final architecture of the bacterial cell wall. Historically, it has been difficult to biochemically characterize these enzymes that act on peptidoglycan because suitable peptidoglycan substrates were inaccessible. In this review, we discuss fundamental aspects of bacterial cell wall synthesis, describe the regulation and diverse biochemical and functional activities of peptidoglycan hydrolases, and highlight recently developed methods to make and label defined peptidoglycan substrates. We also review how access to these substrates has now enabled biochemical studies that deepen our understanding of how bacterial cell wall enzymes cooperate to build a mature cell wall. Such improved understanding is critical to the development of new antibiotics that disrupt cell wall biogenesis, a process essential to the survival of bacteria.

Staphylococcus aureus cell growth and division are regulated by an amidase that trims peptides from uncrosslinked peptidoglycan

Bacteria are protected by a polymer of peptidoglycan that serves as an exoskeleton¹. In Staphylococcus aureus, the peptidoglycan assembly enzymes relocate during the cell cycle from the periphery, where they are active during growth, to the division site where they build the partition between daughter cells^2-4. But how peptidoglycan synthesis is regulated throughout the cell cycle is poorly understood^5,6. Here, we used a transposon screen to identify a membrane protein complex that spatially regulates S. aureus peptidoglycan synthesis. This complex consists of an amidase that removes stem peptides from uncrosslinked peptidoglycan and a partner protein that controls its activity. Amidases typically hydrolyse crosslinked peptidoglycan between daughter cells so that they can separate⁷. However, this amidase controls cell growth. In its absence, peptidoglycan synthesis becomes spatially dysregulated, which causes cells to grow so large that cell division is defective. We show that the cell growth and division defects due to loss of this amidase can be mitigated by attenuating the polymerase activity of the major S. aureus peptidoglycan synthase. Our findings lead to a model wherein the amidase complex regulates the density of peptidoglycan assembly sites to control peptidoglycan synthase activity at a given subcellular location. Removal of stem peptides from peptidoglycan at the cell periphery promotes peptidoglycan synthase relocation to midcell during cell division. This mechanism ensures that cell expansion is properly coordinated with cell division.

The gastrointestinal pathogen Campylobacter jejuni metabolizes sugars with potential help from commensal Bacteroides vulgatus

Although the gastrointestinal pathogen Campylobacter jejuni was considered asaccharolytic, >50% of sequenced isolates possess an operon for L-fucose utilization. In C. jejuni NCTC11168, this pathway confers L-fucose chemotaxis and competitive colonization advantages in the piglet diarrhea model, but the catabolic steps remain unknown. Here we solved the putative dehydrogenase structure, resembling FabG of Burkholderia multivorans. The C. jejuni enzyme, FucX, reduces L-fucose and D-arabinose in vitro and both sugars are catabolized by fuc-operon encoded enzymes. This enzyme alone confers chemotaxis to both sugars in a non-carbohydrate-utilizing C. jejuni strain. Although C. jejuni lacks fucosidases, the organism exhibits enhanced growth in vitro when co-cultured with Bacteroides vulgatus, suggesting scavenging may occur. Yet, when excess amino acids are available, C. jejuni prefers them to carbohydrates, indicating a metabolic hierarchy exists. Overall this study increases understanding of nutrient metabolism by this pathogen, and identifies interactions with other gut microbes.

The Sle1 Cell Wall Amidase Is Essential for β-Lactam Resistance in Community-Acquired Methicillin-Resistant Staphylococcus aureus USA300

Most clinically relevant methicillin-resistant Staphylococcus aureus (MRSA) strains have become resistant to β-lactams antibiotics through horizontal acquisition of the mecA gene encoding PBP2a, a peptidoglycan transpeptidase with low affinity for β-lactams. The level of resistance conferred by mecA is, however, strain dependent, and the mechanisms underlying this phenomenon remain poorly understood. We show here that β-lactam resistance correlates to expression of the Sle1 cell wall amidase in the fast-spreading and highly virulent community-acquired MRSA USA300 clone. Sle1 is a substrate of the ClpXP protease, and while the high Sle1 levels in cells lacking ClpXP activity confer β-lactam hyper-resistance, USA300 cells lacking Sle1 are as susceptible to β-lactams as cells lacking mecA This finding prompted us to assess the cellular roles of Sle1 in more detail, and we demonstrate that high Sle1 levels accelerate the onset of daughter cells splitting and decrease cell size. Vice versa, oxacillin decreases the Sle1 level and imposes a cell separation defect that is antagonized by high Sle1 levels, suggesting that high Sle1 levels increase tolerance to oxacillin by promoting cell separation. In contrast, increased oxacillin sensitivity of sle1 cells appears linked to a synthetic lethal effect on septum synthesis. In conclusion, this study demonstrates that Sle1 is a key factor in resistance to β-lactam antibiotics in the JE2 USA300 model strain and that PBP2a is required for the expression of Sle1 in JE2 cells exposed to oxacillin.

High-throughput transposon sequencing highlights the cell wall as an important barrier for osmotic stress in methicillin resistant Staphylococcus aureus and underlines a tailored response to different osmotic stressors

Staphylococcus aureus is an opportunistic pathogen that can cause soft tissue infections but is also a frequent cause of foodborne illnesses. One contributing factor for this food association is its high salt tolerance allowing this organism to survive commonly used food preservation methods. How this resistance is mediated is poorly understood, particularly during long-term exposure. In this study, we used transposon sequencing (TN-seq) to understand how the responses to osmotic stressors differ. Our results revealed distinctly different long-term responses to NaCl, KCl and sucrose stresses. In addition, we identified the DUF2538 domain containing gene SAUSA300_0957 (gene 957) as essential under salt stress. Interestingly, a 957 mutant was less susceptible to oxacillin and showed increased peptidoglycan crosslinking. The salt sensitivity phenotype could be suppressed by amino acid substitutions in the transglycosylase domain of the penicillin-binding protein Pbp2, and these changes restored the peptidoglycan crosslinking to WT levels. These results indicate that increased crosslinking of the peptidoglycan polymer can be detrimental and highlight a critical role of the bacterial cell wall for osmotic stress resistance. This study will serve as a starting point for future research on osmotic stress response and help develop better strategies to tackle foodborne staphylococcal infections.

Design, synthesis, and discovery of novel oxindoles bearing 3-heterocycles as species-specific and combinatorial agents in eradicating Staphylococcus species

A series of new functionalized 3-indolylindolin-2-ones, 3-(1-methylpyrrol-2-yl)indolin-2-ones, and 3-(thiophen-2-yl)indolin-2-ones were synthesized by using novel indium (III)-catalysed reaction of various 3-diazoindolin-2-ones with indoles, 1-methylpyrrole, or thiophene via one-pot procedure. The newly synthesized compounds were characterized and screened for their in vitro antibacterial activity against various Staphylococcus species, including methicillin-resistant Staphylococcus aureus. results revealed that five compounds KS15, KS16, KS17, KS19, and KS20 exhibited potent and specific antibacterial activity against Staphylococcus species albeit inactive against Gram-negative bacteria. Especially, compounds exhibited superior antibacterial potency against Staphylococcus epidermidis compared to the reference drug streptomycin. The most potential compound KS16 also increased the susceptibility of Staphylococcus aureus to ciprofloxacin, gentamicin, kanamycin, and streptomycin. Among them, KS16 was found to be a synergistic compound with gentamicin and kanamycin. Furthermore, the cellular level of autolysin protein was increased from the KS16-treated Staphylococcus aureus cells. Finally, in vitro CCK-8 assays showed that KS16 exhibited no cytotoxicity at the minimum inhibitory concentrations used for killing Staphylococcus species. From all our results, novel oxindole compounds directly have lethal action or boost existing antibiotic power with the reduction of doses and toxicity in the treatment of multidrug-resistant Staphylococcus species.

Bacteria's different ways to recycle their own cell wall

The ability to recover components of their own cell wall is a common feature of bacteria. This was initially recognized in the Gram-negative bacterium Escherichia coli, which recycles about half of the peptidoglycan of its cell wall during one cell doubling. Moreover, E. coli was shown to grow on peptidoglycan components provided as nutrients. A distinguished recycling enzyme of E. coli required for both, recovery of the cell wall sugar N-acetylmuramic acid (MurNAc) of the own cell wall and for growth on external MurNAc, is the MurNAc 6-phosphate (MurNAc 6P) lactyl ether hydrolase MurQ. We revealed however, that most Gram-negative bacteria lack a murQ ortholog and instead harbor a pathway, absent in E. coli, that channels MurNAc directly to peptidoglycan biosynthesis. This "anabolic recycling pathway" bypasses the initial steps of peptidoglycan de novo synthesis, including the target of the antibiotic fosfomycin, thus providing intrinsic resistance to the antibiotic. The Gram-negative oral pathogen Tannerella forsythia is auxotrophic for MurNAc and apparently depends on the anabolic recycling pathway to synthesize its own cell wall by scavenging cell wall debris of other bacteria. In contrast, Gram-positive bacteria lack the anabolic recycling genes, but mostly contain one or two murQ orthologs. Quantification of MurNAc 6P accumulation in murQ mutant cells by mass spectrometry allowed us to demonstrate for the first time that Gram-positive bacteria do recycle their own peptidoglycan. This had been questioned earlier, since peptidoglycan turnover products accumulate in the spent media of Gram-positives. We showed, that these fragments are recovered during nutrient limitation, which prolongs starvation survival of Bacillus subtilis and Staphylococcus aureus. Peptidoglycan recycling in these bacteria however differs, as the cell wall is either cleaved exhaustively and monosaccharide building blocks are taken up (B. subtilis) or disaccharides are released and recycled involving a novel phosphomuramidase (MupG; S.aureus). In B. subtilis also the teichoic acids, covalently bound to the peptidoglycan (wall teichoic acids; WTAs), are recycled. During phosphate limitation, the sn-glycerol-3-phosphate phosphodiesterase GlpQ specifically degrades WTAs of B. subtilis. In S. aureus, in contrast, GlpQ is used to scavenge external teichoic acid sources. Thus, although bacteria generally recover their own cell wall, they apparently apply distinct strategies for breakdown and reutilization of cell wall fragments. This review summarizes our work on this topic funded between 2011 and 2019 by the DFG within the collaborative research center SFB766.

Staphylococcal Protein Secretion and Envelope Assembly

The highly cross-linked peptidoglycan represents the rigid layer of the bacterial envelope and protects bacteria from osmotic lysis. In Gram-positive bacteria, peptidoglycan also functions as a scaffold for the immobilization of capsular polysaccharide, wall teichoic acid (WTA), and surface proteins. This chapter captures recent development on the assembly of the envelope of Staphylococcus aureus including mechanisms accounting for immobilization of molecules to peptidoglycan as well as hydrolysis of peptidoglycan for the specific release of bound molecules, facilitation of protein secretion across the envelope and cell division. Peptidoglycan, WTA and capsular polysaccharide are directly synthesized onto undecaprenol. Surface proteins are anchored by Sortase A, a membrane-embedded transpeptidase that scans secreted polypeptides for the C-terminal LPXTG motif of sorting signals. The resulting acyl enzyme intermediate is resolved by lipid II, the undecaprenol-bound peptidoglycan precursor. While these pathways share membrane diffusible undecaprenol, assembly of these molecules occurs either at the cross-walls or the cell poles. In S. aureus, the cross-wall represents the site of de novo peptidoglycan synthesis which is eventually split to complete the cell cycle yielding newly divided daughter cells. Peptidoglycan synthesized at the cross-wall is initially devoid of WTA. Conversely, lipoteichoic acid (LTA) synthesis which does not require bactoprenol is seemingly restricted to septal membranes. Similarly, S. aureus distinguishes two types of surface protein precursors. Polypeptides with canonical signal peptides are deposited at the cell poles, whereas precursors with conserved YSIRK-GXXS motif signal peptides traffic to the cross-wall. A model for protein trafficking in the envelope and uneven distribution of teichoic acids is discussed.

The Staphylococcal Cell Wall

Dating back to the 1960s, initial studies on the staphylococcal cell wall were driven by the need to clarify the mode of action of the first antibiotics and the resistance mechanisms developed by the bacteria. During the following decades, the elucidation of the biosynthetic path and primary composition of staphylococcal cell walls was propelled by advances in microbial cell biology, specifically, the introduction of high-resolution analytical techniques and molecular genetic approaches. The field of staphylococcal cell wall gradually gained its own significance as the complexity of its chemical structure and involvement in numerous cellular processes became evident, namely its versatile role in host interactions, coordination of cell division and environmental stress signaling.This chapter includes an updated description of the anatomy of staphylococcal cell walls, paying particular attention to information from the last decade, under four headings: high-resolution analysis of the Staphylococcus aureus peptidoglycan; variations in peptidoglycan composition; genetic determinants and enzymes in cell wall synthesis; and complex functions of cell walls. The latest contributions to a more precise picture of the staphylococcal cell envelope were possible due to recently developed state-of-the-art microscopy and spectroscopy techniques and to a wide combination of -omics approaches, that are allowing to obtain a more integrative view of this highly dynamic structure.

Cell Wall Hydrolases in Bacteria: Insight on the Diversity of Cell Wall Amidases, Glycosidases and Peptidases Toward Peptidoglycan

The cell wall (CW) of bacteria is an intricate arrangement of macromolecules, at least constituted of peptidoglycan (PG) but also of (lipo)teichoic acids, various polysaccharides, polyglutamate and/or proteins. During bacterial growth and division, there is a constant balance between CW degradation and biosynthesis. The CW is remodeled by bacterial hydrolases, whose activities are carefully regulated to maintain cell integrity or lead to bacterial death. Each cell wall hydrolase (CWH) has a specific role regarding the PG: (i) cell wall amidase (CWA) cleaves the amide bond between N-acetylmuramic acid and L-alanine residue at the N-terminal of the stem peptide, (ii) cell wall glycosidase (CWG) catalyses the hydrolysis of the glycosidic linkages, whereas (iii) cell wall peptidase (CWP) cleaves amide bonds between amino acids within the PG chain. After an exhaustive overview of all known conserved catalytic domains responsible for CWA, CWG, and CWP activities, this review stresses that the CWHs frequently display a modular architecture combining multiple and/or different catalytic domains, including some lytic transglycosylases as well as CW binding domains. From there, direct physiological and collateral roles of CWHs in bacterial cells are further discussed.

Pyrrolomycins as antimicrobial agents. Microwave-assisted organic synthesis and insights into their antimicrobial mechanism of action

New compounds able to counteract staphylococcal biofilm formation are needed. In this study we investigate the mechanism of action of pyrrolomycins, whose potential as antimicrobial agents has been demonstrated. We performed a new efficient and easy method to use microwave organic synthesis suitable for obtaining pyrrolomycins in good yields and in suitable amount for their in vitro in-depth investigation. We evaluate the inhibitory activity towards Sortase A (SrtA), a transpeptidase responsible for covalent anchoring in Gram-positive peptidoglycan of many surface proteins involved in adhesion and in biofilm formation. All compounds show a good inhibitory activity toward SrtA, having IC₅₀ values ranging from 130 to 300 µM comparable to berberine hydrochloride. Of note compound 1d shows a good affinity in docking experiment to SrtA and exhibits the highest capability to interfere with biofilm formation of S. aureus showing an IC₅₀ of 3.4 nM. This compound is also effective in altering S. aureus murein hydrolase activity that is known to be responsible for degradation, turnover, and maturation of bacterial peptidoglycan and involved in the initial stages of S. aureus biofilm formation.

Characterisation of Staphylococcus felis isolated from cats using whole genome sequencing

This study used phenotypic tests and whole genome sequencing to characterise a collection of 37 clinical Staphylococcus felis isolates from cats. Samples were isolated from a range of diseases including feline lower urinary tract disease (n = 15), otitis externa (n = 13), and ocular disease (n = 2). Isolates were identified using MALDI-TOF MS and by BLASTn analysis of S. felis-specific 16S rRNA, rpoB and nuc genes in whole genome sequence-based contigs. Phenotypic antimicrobial resistance was determined using disk diffusion and broth microdilution. Coagulase activity was assessed using feline and rabbit plasma. Genomes were screened for putative virulence and antimicrobial resistance genes using the sequences of known genes from other staphylococci as homologous references. Phylogenetic relationships were inferred using single nucleotide polymorphisms. One isolate was coagulase-positive when tested with feline plasma but all isolates were rabbit plasma coagulase-negative. No genetic determinant of coagulase activity was identified in this isolate. A range of putative virulence genes were found amongst isolates including genes associated with adhesion, toxin production and immune evasion. Ninety two percent of isolates were fully susceptible to all antimicrobials tested, which was reflected by a general absence of resistance genes. Clustering within the phylogenetic tree suggested a multiclonal population structure; this clustering did not correlate with disease syndrome or geographic origin of the isolate. Future studies of veterinary staphylococci will benefit from the publicly available S. felis draft genomes that were generated in this study.

Phenotypic signatures and genetic determinants of oxacillin tolerance in a laboratory mutant of Staphylococcus aureus

Addition of β-lactam antibiotics to growing cultures of bacteria inhibit synthesis of the bacterial cell wall peptidoglycan accompanied by killing (loss of viable titer) and lysis (physical disintegration) of the cells. However, it has also been well established that these antibiotics are not effective in killing non-growing or slow-growing bacteria and the mechanism of this "antibiotic tolerance" is not well understood. In this study, we report on the genetic basis and phenotypic properties of an antibiotic tolerant derivative of the methicillin susceptible S. aureus strain 27s. Cultures were exposed to "pulses" of high concentrations of oxacillin followed by outgrowth of the surviving bacteria. This procedure quickly selected for antibiotic tolerant mutants with an increased ability to survive antibiotic treatment without increase in the MIC value for the antibiotic. Such mutants also exhibited longer lag phase, decreased lysis, virtually no change in antibiotic susceptibilities, cross tolerance to D-cycloserine and vancomycin, and increase in biofilm formation in the presence of high concentrations of oxacillin. Whole genome sequencing showed that these altered properties were linked to mutations in the atl and gdpP genes.

Exposure of Staphylococcus aureus to Targocil Blocks Translocation of the Major Autolysin Atl across the Membrane, Resulting in a Significant Decrease in Autolysis

Peptidoglycan (PG) and wall teichoic acid (WTA) are the major staphylococcal cell wall components, and WTA biosynthesis has recently been explored for drug development. Targocil is a novel agent that targets the TarG subunit of the WTA translocase (TarGH) that transports WTA across the membrane to the wall. Previously we showed that targocil treatment of a methicillin-susceptible Staphylococcus aureus strain led to a rapid shut down of cellular autolysis. Targocil II, which targets the TarH subunit of TarGH, also resulted in a drastic decrease in autolysis. Here, we address the mechanism of targocil-mediated decreased autolysis. The mechanism is WTA dependent since targocil treatment decreased autolysis in methicillin-resistant strains but not in a WTA-deficient mutant. Similar to cellular autolysis, autolysin-retaining crude cell walls isolated from targocil-treated cells had vastly decreased autolytic activity compared to those from untreated cells. Purified cell walls from control and targocil-treated cells, which lack autolytic activity, were similarly susceptible to lysozyme and lysostaphin and had similar O-acetyl contents, indicating that targocil treatment did not grossly alter PG structure and chemistry. Purified cell walls from targocil-treated cells were highly susceptible to autolysin extracts, supporting the notion that targocil treatment led to decreased autolysin in the crude cell walls. Quantitative real-time PCR analysis revealed that the decrease in autolysis in the targocil-exposed cells was not due to transcriptional repression of the autolysin genes atl, lytM, lytN, and sle1 Zymographic analysis of peptidoglycan hydrolase profiles showed a deficiency of cell surface autolysins in targocil-treated cells but higher activity in cell membrane fractions. Here, we propose that the untranslocated WTA molecules in the targocil-exposed cells sequester Atl at the membrane, resulting in significantly decreased autolysis.

Immunotherapy synergizes with debridement and antibiotic therapy in a murine 1-stage exchange model of MRSA implant-associated osteomyelitis

Methicillin-resistant Staphylococcus aureus (MRSA) reinfection following revision surgery remains a major orthopaedic problem. Toward the development of immunotherapy with anti-glucosaminidase monoclonal antibodies (anti-Gmd), we aimed to: (i) develop a murine 1-stage exchange model of bioluminescent MRSA (USA300LAC::lux) contaminated femoral implants; and (ii) utilize this model to demonstrate the synergistic effects of combination vancomycin and anti-Gmd therapy on reinfection and bone healing. Following an infection surgery, the original plate and two screws were removed on day 7, and exchanged with sterile implants. Mice were randomized to five groups: (i) no infection control; (ii) infected placebo; (iii) anti-Gmd; (iv) vancomycin; and (v) combination therapy. Bioluminescent imaging (BLI) was performed on days 0, 1, 3, 5, 7, 8, 10, 12, and 14. Mice were euthanized on day 14 (day 7 post-revision), and efficacy was assessed via colony forming units (CFU) on explanted hardware, micro-CT, and histology. As monotherapies, anti-Gmd inhibited Staphylococcus abscess communities, and vancomycin reduced CFU on the implants. However, only combination therapy prevented increased BLI post-revision surgery, with a significant 6.5-fold reduction on day 10 (p < 0.05 vs. placebo), and achieved sterile implant levels by day 12. Synergistic effects were also apparent from reduced osteolysis and increased new bone formation around the screws only observed following combination therapy. Taken together, we find that: (i) this murine femoral plate 1-stage revision model can efficiently evaluate therapies to prevent reinfection; and (ii) immunotherapy plays a distinct role from antibiotics to reduce reinfection following revision surgery, such that synergy to achieve osseointegration is possible. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1590-1598, 2018.

Colonization of medical devices by staphylococci

The use of medical devices in modern medicine is constantly increasing. Despite the multiple precautionary strategies that are being employed in hospitals, which include increased hygiene and sterilization measures, bacterial infections on these devices still happen frequently. Staphylococci are among the major causes of medical device infection. This is mostly due to the strong capacity of those bacteria to form device-associated biofilms, which provide resistance to chemical and physical treatments as well as attacks by the host's immune system. Biofilm development is a multistep process with specific factors participating in each step. It is tightly regulated to provide a balance between biofilm expansion and detachment. Detachment from a biofilm on a medical device can lead to severe systemic infection, such as bacteremia and sepsis. While our understanding of staphylococcal biofilm formation has increased significantly and staphylococcal biofilm formation on medical devices is among the best understood biofilm-associated infections, the extensive effort put in preclinical studies with the goal to find novel therapies against staphylococcal device-associated infections has not yet resulted in efficient, applicable therapeutic options for that difficult-to-treat type of disease.

Autolysin mediated adherence of Staphylococcus aureus with Fibronectin, Gelatin and Heparin

Major autolysin (Atl) of Staphylococcus aureusis a cell surface associated peptidoglycan hydrolase with amidase and glucosaminidase domains. Atl enzymes (amidase and glucosaminidase) are known to participate in biofilm formation and also can bind with host matrices. Earlier studies demonstrated the binding of Atlwithfibronectin, thrombospondin 1, vitronectin and heat shock cognate protein Hsc70. Here, we have shown, Atl mediates attachment of S.aureus to heparin and gelatine as well. The atl mutant strain demonstrated around 2.5 fold decreased adherence with fibronectin, gelatin and heparin coated microtiter plates. The microscopic studies confirmed the reduced binding of atl mutant with them compared to its parental wild type and complemented mutant strains. Amidase and glucosaminidase were expressed as N-terminal histidine tagged proteins from Escherichia coli, purified and refolded. We found refolded amidase bind with fibronectin, gelatin and heparin; whereas refolded glucosaminidase binds with only fibronectin and heparin but not gelatin. These results reemphasize Atl as one of the crucial proteins from Staphylococcus that facilitate their binding with multiple host cellular components during colonization and infection.

Luciferase Reporter Gene System to Detect Cell Wall Stress Stimulon Induction in Staphylococcus aureus

Luciferase reporter gene fusions provide an extremely rapid and sensitive tool for measuring the induction or repression of stress responses in bacteria. Staphylococcus aureus activates the expression of a cell wall stress stimulon (CWSS) in response to the inhibition or disruption of cell wall synthesis. The highly sensitive promoter-reporter gene fusion construct psas016 p-luc+ can be used to quantify and compare any changes in CWSS expression levels and induction kinetics. Potential uses of this system include identifying and characterizing novel cell wall-targeting antibacterial agents, identifying genomic loci influencing cell envelope synthesis and detecting changes in CWSS expression that could be linked to decreased antibiotic susceptibility profiles in clinical isolates.

Bacterial size matters: Multiple mechanisms controlling septum cleavage and diplococcus formation are critical for the virulence of the opportunistic pathogen Enterococcus faecalis

Enterococcus faecalis is an opportunistic pathogen frequently isolated in clinical settings. This organism is intrinsically resistant to several clinically relevant antibiotics and can transfer resistance to other pathogens. Although E. faecalis has emerged as a major nosocomial pathogen, the mechanisms underlying the virulence of this organism remain elusive. We studied the regulation of daughter cell separation during growth and explored the impact of this process on pathogenesis. We demonstrate that the activity of the AtlA peptidoglycan hydrolase, an enzyme dedicated to septum cleavage, is controlled by several mechanisms, including glycosylation and recognition of the peptidoglycan substrate. We show that the long cell chains of E. faecalis mutants are more susceptible to phagocytosis and are no longer able to cause lethality in the zebrafish model of infection. Altogether, this work indicates that control of cell separation during division underpins the pathogenesis of E. faecalis infections and represents a novel enterococcal virulence factor. We propose that inhibition of septum cleavage during division represents an attractive therapeutic strategy to control infections.

Enterococcus faecalis is a commensal bacterium that colonizes the gastrointestinal tract of humans. This organism is an opportunistic pathogen that can cause a wide range of life-threatening infections in hospital settings. Despite the identification of several virulence factors, the mechanisms by which E. faecalis evades host immunity and causes infections remains poorly understood. Here, we explore how the formation of diplococci and short cell chains, a distinctive property of E. faecalis, contributes to pathogenesis. We describe several mechanisms that control the activity of AtlA, the enzyme dedicated to septum cleavage during division. Using a combination of in vitro assays and flow cytometry analyses of E. faecalis mutants, we show that AtlA activity is regulated by several mechanisms. We reveal that during pathogenesis, AtlA activity is critical for overcoming the host immune response. In the absence of AtlA, the long cell chains of E. faecalis mutants are more susceptible to phagocytosis and can no longer cause lethality in the zebrafish model of infection, thus indicating that control of cell chain length is a novel virulence factor in E. faecalis. This work highlights a link between cell division and pathogenesis and suggests that cell separation represents a step that can be targeted to control bacterial infections.