Staphylococcal cell wall: morphogenesis and fatal variations in the presence of penicillin

Peptidoglycan O-acetylation increases in response to vancomycin treatment in vancomycin-resistant Enterococcus faecalis

Vancomycin resistance is conferred upon vancomycin-resistant enterococci (VRE) through the replacement of peptidoglycan (PG) stem terminal d-Ala-d-Ala with d-Ala-d-Lac. The d-Ala-d-Lac incorporation can affect both the fitness and virulence of VRE. Here we comprehensively investigate the changes to PG composition in vancomycin-resistant Enterococcus faecalis following the growth in presence of vancomycin using liquid chromatography-mass spectrometry. Using high-resolution mass spectrometry, 104 unique muropeptides fragments were identified and the relative abundance of each fragment was accurately quantified by integrating the ion current of a selected ion using extracted-ion chromatogram. The analysis indicates reduced PG cross-linking, increased carboxypeptidase activities, increased N-deacetylation, and increased O-acetylation in VRE when grown in the presence of vancomycin. We found that O-acetylation preferentially occurred on muropeptides fragments with reduced cross-linking with a pentapeptide stem that terminated in d-Ala-d-Lac. These findings show that O-acetylation preferentially occurred in regions of the cell wall with reduced PG cross-linking on PG units that have stems terminating in d-Ala-d-Lac, serving as markers to prevent both the PG-stem modification by carboxypeptidases and the cell wall degradation by autolysins. Accurate quantitative PG composition analysis provided compositional insights into altered cell wall biosynthesis and modification processes in VRE that contribute to lysozyme resistance and enhanced virulence for VRE grown in the presence of vancomycin.

Morphological and ultrastructural changes in bacterial cells as an indicator of antibacterial mechanism of action

Efforts to reduce the global burden of bacterial disease and contend with escalating bacterial resistance are spurring innovation in antibacterial drug and biocide development and related technologies such as photodynamic therapy and photochemical disinfection. Elucidation of the mechanism of action of these new agents and processes can greatly facilitate their development, but it is a complex endeavour. One strategy that has been popular for many years, and which is garnering increasing interest due to recent technological advances in microscopy and a deeper understanding of the molecular events involved, is the examination of treated bacteria for changes to their morphology and ultrastructure. In this review, we take a critical look at this approach. Variables affecting antibacterial-induced alterations are discussed first. These include characteristics of the test organism (e.g. cell wall structure) and incubation conditions (e.g. growth medium osmolarity). The main body of the review then describes the different alterations that can occur. Micrographs depicting these alterations are presented, together with information on agents that induce the change, and the sequence of molecular events that lead to the change. We close by highlighting those morphological and ultrastructural changes which are consistently induced by agents sharing the same mechanism (e.g. spheroplast formation by peptidoglycan synthesis inhibitors) and explaining how changes that are induced by multiple antibacterial classes (e.g. filamentation by DNA synthesis inhibitors, FtsZ disruptors, and other types of agent) can still yield useful mechanistic information. Lastly, recommendations are made regarding future study design and execution.

Effects of Luteolin and Quercetin in Combination with Some Conventional Antibiotics against Methicillin-Resistant Staphylococcus aureus

The present study was designed to evaluate the effects of flavonoids luteolin (L) and quercetin + luteolin (Q + L) in combination with commonly used antibacterial agents against methicillin-resistant Staphylococcus aureus (MRSA) clinical isolates and S. aureus (ATCC 43300). Minimum inhibitory concentrations (MICs) of L and Q + L, as well as the MICs of flavonoids in combination with antibiotics were determined and results showed an increased activity of flavonoids with antibiotics. The synergistic, additive, or antagonistic relationships between flavonoids (L and Q + L) and antibiotics were also evaluated, and additive and synergistic effects were observed for some antibiotic + flavonoid combinations. In addition, some combinations were also found to damage the bacterial cytoplasmic membrane, as assessed through potassium leakage assay. The effects of flavonoids and flavonoids + antibiotics on mecA gene mutations were also tested, and no functional variation was detected in the coding region.

Bacterial physiology is a key modulator of the antibacterial activity of graphene oxide

Carbon-based nanomaterials have a great potential as novel antibacterial agents; however, their interactions with bacteria are not fully understood. This study demonstrates that the antibacterial activity of graphene oxide (GO) depends on the physiological state of cells for both Gram-negative and -positive bacteria. GO susceptibility of bacteria is the highest in the exponential growth phase, which are in growing physiology, and stationary-phase (non-growing) cells are quite resistant against GO. Importantly, the order of GO susceptibility of E. coli with respect to the growth phases (exponential ≫ decline > stationary) correlates well with the changes in the envelope ultrastructures of the cells. Our findings are not only fundamentally important but also particularly critical for practical antimicrobial applications of carbon-based nanomaterials.

Evaluation of antibacterial and antibiofilm mechanisms by usnic acid against methicillin-resistant Staphylococcus aureus

AIM

To evaluate the antibacterial and antibiofilm mechanisms of usnic acid (USN) against methicillin-resistant Staphylococcus aureus from cystic fibrosis patients.

MATERIALS & METHODS

The effects exerted by USN at subinhibitory concentrations on S. aureus Sa3 strain was evaluated by proteomic, real-time PCR and electron microscopy analyses.

RESULTS & CONCLUSION

Proteomic analysis showed that USN caused damage in peptidoglycan synthesis, as confirmed by microscopy. Real-time PCR analysis showed that antibiofilm activity of USN is mainly due to impaired adhesion to the host matrix binding proteins, and decreasing lipase and thermonuclease expression. Our data show that USN exerts anti-staphylococcal effects through multitarget inhibitory effects, thus confirming the rationale for considering it 'lead compound' for the treatment of cystic fibrosis infections.

De Novo Guanine Biosynthesis but Not the Riboswitch-Regulated Purine Salvage Pathway Is Required for Staphylococcus aureus Infection In Vivo

De novo guanine biosynthesis is an evolutionarily conserved pathway that creates sufficient nucleotides to support DNA replication, transcription, and translation. Bacteria can also salvage nutrients from the environment to supplement the de novo pathway, but the relative importance of either pathway during Staphylococcus aureus infection is not known. In S. aureus, genes important for both de novo and salvage pathways are regulated by a guanine riboswitch. Bacterial riboswitches have attracted attention as a novel class of antibacterial drug targets because they have high affinity for small molecules, are absent in humans, and regulate the expression of multiple genes, including those essential for cell viability. Genetic and biophysical methods confirm the existence of a bona fide guanine riboswitch upstream of an operon encoding xanthine phosphoribosyltransferase (xpt), xanthine permease (pbuX), inosine-5'-monophosphate dehydrogenase (guaB), and GMP synthetase (guaA) that represses the expression of these genes in response to guanine. We found that S. aureus guaB and guaA are also transcribed independently of riboswitch control by alternative promoter elements. Deletion of xpt-pbuX-guaB-guaA genes resulted in guanine auxotrophy, failure to grow in human serum, profound abnormalities in cell morphology, and avirulence in mouse infection models, whereas deletion of the purine salvage genes xpt-pbuX had none of these effects. Disruption of guaB or guaA recapitulates the xpt-pbuX-guaB-guaA deletion in vivo In total, the data demonstrate that targeting the guanine riboswitch alone is insufficient to treat S. aureus infections but that inhibition of guaA or guaB could have therapeutic utility.

IMPORTANCE

De novo guanine biosynthesis and purine salvage genes were reported to be regulated by a guanine riboswitch in Staphylococcus aureus We demonstrate here that this is not true, because alternative promoter elements that uncouple the de novo pathway from riboswitch regulation were identified. We found that in animal models of infection, the purine salvage pathway is insufficient for S. aureus survival in the absence of de novo guanine biosynthesis. These data suggest targeting the de novo guanine biosynthesis pathway may have therapeutic utility in the treatment of S. aureus infections.

Staphylococcus aureus: the current state of disease, pathophysiology and strategies for prevention

Staphylococcus aureus is both a commensal organism and also an important opportunistic human pathogen, causing a variety of community and hospital-associated pathologies, such as bacteremia-sepsis, endocarditis, pneumonia, osteomyelitis, arthritis and skin diseases. The resurgence of S. aureus during the last decade in many settings has been facilitated not only by bacterial antibiotic resistance mechanisms but also by the emergence of new S. aureus clonal types with increased expression of virulence factors and the capacity to neutralize the host immune response. Prevention of the spread of S. aureus infection relies on the use of contact precautions and adequate procedures for infection control that so far have not been fully effective. Prevention using a prophylactic vaccine would complement these processes, having the potential to bring additional, significant progress toward decreasing invasive disease due to S. aureus.

Synergism of Water Shock and a Biocompatible Block Copolymer Potentiates the Antibacterial Activity of Graphene Oxide

Graphene oxide (GO) is promising in the fight against pathogenic bacteria. However, the antibacterial activity of pristine GO is relatively low and concern over human cytotoxicity further limits its potential. This study demonstrates a general approach to address both issues. The developed approach synergistically combines the water shock treatment (i.e., a sudden decrease in environmental salinity) and the use of a biocompatible block copolymer (Pluronic F-127) as a synergist co-agent. Hypoosmotic stress induced by water shock makes gram-negative pathogens more susceptible to GO. Pluronic forms highly stable nanoassemblies with GO (Pluronic-GO) that can populate around bacterial envelopes favoring the interactions between GO and bacteria. The antibacterial activity of GO at a low concentration (50 μg mL(-1) ) increases from <30% to virtually complete killing (>99%) when complemented with water shock and Pluronic (5 mg mL(-1) ) at ≈2-2.5 h of exposure. Results suggest that the enhanced dispersion of GO and the osmotic pressure generated on bacterial envelopes by polymers together potentiate GO. Pluronic also significantly suppresses the toxicity of GO toward human fibroblast cells. Fundamentally, the results highlight the crucial role of physicochemical milieu in the antibacterial activity of GO. The demonstrated strategy has potentials for daily-life bacterial disinfection applications, as hypotonic Pluronic-GO mixture is both safe and effective.

Excretion of cytoplasmic proteins in Staphylococcus is most likely not due to cell lysis

The excretion of cytoplasmic proteins (ECP) is a long-known phenomenon in bacteria and eukaryotes. So far, it was not possible to associate either a signal peptide-dependent or a signal peptide-independent pathway to ECP. Nevertheless 25% of the proteins found in Staphylococcus aureus supernatants were cytoplasmic proteins. Because the excreted proteins do not possess a common motive, the most widespread opinion is that ECP is due to cell lysis. This explanation seems to be too easy since several indications imply that there exists a yet unknown mechanism for ECP. Certainly, the up-regulation of autolysins as well as decreased peptidoglycan cross-linking increased ECP. However, in recent years, several evidences arose that cell lysis is not the only reason for ECP. It seems that ECP is a part of the normal cell cycle of S. aureus as it turned out that ECP with several model proteins occurs mainly during cell growth. It has common features as proteins secreted via the Sec translocon and finally the excretion site is the cross wall of dividing cells.

SagB Glucosaminidase Is a Determinant of Staphylococcus aureus Glycan Chain Length, Antibiotic Susceptibility, and Protein Secretion

The envelope of Staphylococcus aureus is comprised of peptidoglycan and its attached secondary polymers, teichoic acid, capsular polysaccharide, and protein. Peptidoglycan synthesis involves polymerization of lipid II precursors into glycan strands that are cross-linked at wall peptides. It is not clear whether peptidoglycan structure is principally determined during polymerization or whether processive enzymes affect cell wall structure and function, for example, by generating conduits for protein secretion. We show here that S. aureus lacking SagB, a membrane-associated N-acetylglucosaminidase, displays growth and cell-morphological defects caused by the exaggerated length of peptidoglycan strands. SagB cleaves polymerized glycan strands to their physiological length and modulates antibiotic resistance in methicillin-resistant S. aureus (MRSA). Deletion of sagB perturbs protein trafficking into and across the envelope, conferring defects in cell wall anchoring and secretion, as well as aberrant excretion of cytoplasmic proteins.

IMPORTANCE

Staphylococcus aureus is thought to secrete proteins across the plasma membrane via the Sec pathway; however, protein transport across the cell wall envelope has heretofore not been studied. We report that S. aureus sagB mutants generate elongated peptidoglycan strands and display defects in protein secretion as well as aberrant excretion of cytoplasmic proteins. These results suggest that the thick peptidoglycan layer of staphylococci presents a barrier for protein secretion and that SagB appears to extend the Sec pathway across the cell wall envelope.

Bacterial membrane vesicles, an overlooked environmental colloid: Biology, environmental perspectives and applications

Phospholipid vesicles play important roles in biological systems. Bacteria are one of the most abundant organisms on Earth, and bacterial membrane vesicles (MVs) were first observed 50 years ago. Many bacteria release MVs to the environment that mainly consist of the cell membrane and typically range from 20 to 400 nm in size. Bacterial MVs are involved in several biological functions, such as delivery of cargo, virulence and gene transfer. MVs can be isolated from laboratory culture and directly from the environment, indicating their high abundance in and impact on ecosystems. Many colloidal particles in the environment ranging in size from 1 nm to 1 μm have been reported but not characterized at the molecular level, and MVs remain to be explored. Hence, MVs can be considered terra incognita in environmental colloid research. Although MV biogenesis and biological roles are yet to be fully understood, the accumulation of knowledge has opened new avenues for their applications. Via genetic engineering, the MV yield can be greatly increased, and the components of MVs can be tailored. Recent studies have demonstrated that MVs have promising potential for applications such as drug delivery systems and nanobiocatalysts. For instance, MV vaccines have been extensively studied and have already been approved in Europe. Recent MV studies have evoked great interest in the fields of biology and biotechnology, but fundamental questions, such as their transport in the environment or physicochemical features of MVs, remain to be addressed. In this review, we present the current understanding of bacterial MVs and environmental perspectives and further introduce their applications.

Activity of Genital Tract Secretions and Synthetic Antimicrobial Peptides against Group B Streptococcus

PROBLEM

Genital tract secretions inhibit Escherichia coli (E. coli) through antimicrobial peptides (AMP) secreted by the host and vaginal microbiota. However, there are limited data against group B Streptococcus (GBS).

METHOD OF STUDY

Group B Streptococcus were incubated with cervico-vaginal lavage (CVL) samples from healthy non-pregnant women (n = 12) or synthetic AMP and monitored for bacterial growth using a turbidimetric approach. E. coli inhibitory activity was determined by a colony-forming unit assay.

RESULTS

None of the CVL samples inhibited GBS. The human neutrophil peptide-1 and human defensin 5 inhibited GBS growth by ≥80% at concentrations ≥20 μg/mL and ≥50 μg/mL, respectively, while human beta-defensin 2 and LL-37 did not inhibit at highest concentration tested (100 μg/mL). In contrast, all AMP inhibited E. coli.

CONCLUSIONS

Antimicrobial peptides may protect against E. coli colonization but have more limited activity against GBS. Future studies will focus on augmenting host defense with specific AMP to prevent genitourinary infection with these pathogenic organisms.

Cell shape dynamics during the staphylococcal cell cycle

Staphylococcus aureus is an aggressive pathogen and a model organism to study cell division in sequential orthogonal planes in spherical bacteria. However, the small size of staphylococcal cells has impaired analysis of changes in morphology during the cell cycle. Here we use super-resolution microscopy and determine that S. aureus cells are not spherical throughout the cell cycle, but elongate during specific time windows, through peptidoglycan synthesis and remodelling. Both peptidoglycan hydrolysis and turgor pressure are required during division for reshaping the flat division septum into a curved surface. In this process, the septum generates less than one hemisphere of each daughter cell, a trait we show is common to other cocci. Therefore, cell surface scars of previous divisions do not divide the cells in quadrants, generating asymmetry in the daughter cells. Our results introduce a need to reassess the models for division plane selection in cocci.

Staphylococci are spherical bacteria that divide in sequential orthogonal planes. Here, the authors use super-resolution microscopy to show that staphylococcal cells elongate before dividing, and that the division septum generates less than one hemisphere of each daughter cell, generating asymmetry.

Chemical probes reveal an extraseptal mode of cross-linking in Staphylococcus aureus

Staphylococcus aureus is an important human pathogen and a model organism for studying cell wall synthesis in Gram-positive cocci. The prevailing model of cell wall biogenesis in cocci holds that peptidoglycan synthesis (i.e., transglycosylation and cross-linking) is restricted spatially to the septal cross-wall and temporally to cell division. Previously, we developed a method for visualizing cross-linking in S. aureus using fluorescently tagged mimics of the endogenous substrate of penicillin-binding proteins (PBPs). These probes are incorporated into the cell wall of S. aureus specifically by PBP4, allowing localization of the enzyme's cross-linking activity in vivo with precise spatial and temporal resolution. Here, using this methodology, we have discovered that PBP4 is active not only at the septum, but unexpectedly at the peripheral wall as well. These results challenge the long-held belief that peptidoglycan synthesis is restricted to the septum in spherical bacteria, and instead indicate the presence of two spatiotemporally distinct modes of cross-linking in S. aureus: one at the septum during cell division, and another at the peripheral wall between divisions.

Mechanisms of Methicillin Resistance in Staphylococcus aureus

Staphylococcus aureus is a major human and veterinary pathogen worldwide. Methicillin-resistant S. aureus (MRSA) poses a significant and enduring problem to the treatment of infection by such strains. Resistance is usually conferred by the acquisition of a nonnative gene encoding a penicillin-binding protein (PBP2a), with significantly lower affinity for β-lactams. This resistance allows cell-wall biosynthesis, the target of β-lactams, to continue even in the presence of typically inhibitory concentrations of antibiotic. PBP2a is encoded by the mecA gene, which is carried on a distinct mobile genetic element (SCCmec), the expression of which is controlled through a proteolytic signal transduction pathway comprising a sensor protein (MecR1) and a repressor (MecI). Many of the molecular and biochemical mechanisms underlying methicillin resistance in S. aureus have been elucidated, including regulatory events and the structure of key proteins. Here we review recent advances in this area.

The Essential WalK Histidine Kinase and WalR Regulator Differentially Mediate Autolysis of Staphylococcus aureus RN4220

The two-component regulatory system, WalR/WalK is necessary for growth of different gram-positive bacteria, including Staphylococcus aureus. In present study, we confirmed the essentiality of both the histidine kinase protein WalK and the response regulator WalR for growth using S. aureus RN4220 strain and demonstrated that the histidine kinase protein WalK and the response regulator WalR function differently in regulation of staphylococcal autolysis. The down-regulation of walR expression effectively inhibited Triton X-100-induced lysis and had a weak impact on bacterial tolerance to penicillin induced cell lysis. In contrast, the down-regulation of walK expression had no influence on either Triton X-100- or penicillin-caused autolysis. Moreover, we determined the effect of WalR and WalK on bacterial hydrolase activity using a zymogram analysis. The results showed that the cell lysate of down-regulated walR expression mutant displayed several bands of decreased cell wall hydrolytic activities; however, the down-regulation of WalK had no dramatic impact on the hydrolytic activities. Furthermore, we examined the impact of WalR on the transcription of cidA associated with staphylococcal autolysis, and the results showed that the down-regulation of WalR led to decreased transcription of cidA in the log phase of growth. Taken together, the above results suggest that the essential WalR response regulator and the essential WalK histidine kinase might differently control bacterial lysis in RN4220 strain.

Staphylococcus aureus Survives with a Minimal Peptidoglycan Synthesis Machine but Sacrifices Virulence and Antibiotic Resistance

Many important cellular processes are performed by molecular machines, composed of multiple proteins that physically interact to execute biological functions. An example is the bacterial peptidoglycan (PG) synthesis machine, responsible for the synthesis of the main component of the cell wall and the target of many contemporary antibiotics. One approach for the identification of essential components of a cellular machine involves the determination of its minimal protein composition. Staphylococcus aureus is a Gram-positive pathogen, renowned for its resistance to many commonly used antibiotics and prevalence in hospitals. Its genome encodes a low number of proteins with PG synthesis activity (9 proteins), when compared to other model organisms, and is therefore a good model for the study of a minimal PG synthesis machine. We deleted seven of the nine genes encoding PG synthesis enzymes from the S. aureus genome without affecting normal growth or cell morphology, generating a strain capable of PG biosynthesis catalyzed only by two penicillin-binding proteins, PBP1 and the bi-functional PBP2. However, multiple PBPs are important in clinically relevant environments, as bacteria with a minimal PG synthesis machinery became highly susceptible to cell wall-targeting antibiotics, host lytic enzymes and displayed impaired virulence in a Drosophila infection model which is dependent on the presence of specific peptidoglycan receptor proteins, namely PGRP-SA. The fact that S. aureus can grow and divide with only two active PG synthesizing enzymes shows that most of these enzymes are redundant in vitro and identifies the minimal PG synthesis machinery of S. aureus. However a complex molecular machine is important in environments other than in vitro growth as the expendable PG synthesis enzymes play an important role in the pathogenicity and antibiotic resistance of S. aureus.

Peptidoglycan forms the stress-bearing sacculus that prevents lysis of bacteria due to turgor pressure. The integrity of peptidoglycan is therefore essential for bacterial survival and its synthesis is the target of many important antibiotics, such as penicillin. The final steps of peptidoglycan synthesis are catalyzed by penicillin-binding proteins, enzymes that are proposed to work in multi-enzyme complexes. We show that seven of the nine genes encoding peptidoglycan synthesis enzymes can be deleted from the Staphylococcus aureus genome without affecting normal growth and cell morphology in vitro, identifying the minimal peptidoglycan synthesis machinery of this organism. Identification of minimal machineries is key for synthetic biology efforts towards the design of systems with reduced complexity. However, the non-essential peptidoglycan synthetic proteins are important for survival of S. aureus in more challenging environments, such as in the presence of antibiotics that target cell wall synthesis or within the host, as shown by the inability of the mutant strain to establish a successful infection and kill Drosophila flies.

Carvacrol codrugs: a new approach in the antimicrobial plan

Objective

The increasing prevalence of antibiotic-resistant bacterial infections led to identify alternative strategies for a novel therapeutic approach. In this study, we synthesized ten carvacrol codrugs – obtained linking the carvacrol hydroxyl group to the carboxyl moiety of sulphur-containing amino acids via an ester bond – to develop novel compounds with improved antimicrobial and antibiofilm activities and reduced toxicity respect to carvacrol alone.

Method

All carvacrol codrugs were screened against a representative panel of Gram positive (S. aureus and S. epidermidis), Gram negative (E. coli and P. aeruginosa) bacterial strains and C. albicans, using broth microdilution assays.

Findings

Results showed that carvacrol codrug 4 possesses the most notable enhancement in the anti-bacterial activity displaying MIC and MBC values equal to 2.5 mg/mL for all bacterial strains, except for P. aeruginosa ATCC 9027 (MIC and MBC values equal to 5 mg/mL and 10 mg/mL, respectively). All carvacrol codrugs 1-10 revealed good antifungal activity against C. albicans ATCC 10231. The cytotoxicity assay showed that the novel carvacrol codrugs did not produce human blood hemolysis at their MIC values except for codrugs 8 and 9. In particular, deepened experiments performed on carvacrol codrug 4 showed an interesting antimicrobial effect on the mature biofilm produced by E. coli ATCC 8739, respect to the carvacrol alone. The antimicrobial effects of carvacrol codrug 4 were also analyzed by TEM evidencing morphological modifications in S. aureus, E. coli, and C. albicans.

Conclusion

The current study presents an insight into the use of codrug strategy for developing carvacrol derivatives with antibacterial and antibiofilm potentials, and reduced cytotoxicity.

Mechanical and cell-to-cell adhesive properties of aggregated Methanosarcina

The mechanical and adhesive properties as well as the turgor pressure of microbes play an important role in cell growth and aggregation. By applying AFM together with finite element modelling, one can determine the cell wall structural homogeneity, mechanical and cell-to-cell adhesive properties for aggregated Methanosarcina barkeri cells. This also allows a novel approach to determine in-aggregate turgor pressure determination. Analyzing the AFM force-indentation response of the aggregates under loads less than 10 nN, our study reveals structural inhomogeneity of the polymeric part of the cell wall material and suggests that the cell wall consists of two layers of methanochondroitin (external: with a thickness of 3 ± 1 nm and internal: with a thickness of 169 ± 30 nm). On average, the hyperelastic finite element model showed that the internal layer is more rigid (μ = 14 ± 4 MPa) than the external layer (μ = 2.8 ± 0.9 MPa). To determine the turgor pressure and adhesiveness of the cells, a specific mode of indentation (under a load of 45 nN), aimed towards the centre of the individual aggregate, was performed. By modelling the AFM induced decohesion of the aggregate, the turgor pressure and the cell-to-cell adhesive interface properties could be determined. On average, the turgor pressure is estimated to be 59 ± 22 kPa, the interface strength is 78 ± 12 kPa and the polymer network extensibility is 2.8 ± 0.9 nm. We predict that internal cell wall comprised highly compressed methanochondroitin chains and we are able to identify a conceptual model for stress dependent inner cell wall growth.

Peptidoglycan architecture of Gram-positive bacteria by solid-state NMR

Peptidoglycan is an essential component of cell wall in Gram-positive bacteria with unknown architecture. In this review, we summarize solid-state NMR approaches to address some of the unknowns in the Gram-positive bacteria peptidoglycan architecture: 1) peptidoglycan backbone conformation, 2) PG-lattice structure, 3) variations in the peptidoglycan architecture and composition, 4) the effects of peptidoglycan bridge-length on the peptidoglycan architecture in Fem mutants, 5) the orientation of glycan strands with respect to the membrane, and 6) the relationship between the peptidoglycan structure and the glycopeptide antibiotic mode of action. Solid-state NMR analyses of Staphylococcus aureus cell wall show that peptidoglycan chains are surprisingly ordered and densely packed. The peptidoglycan disaccharide backbone adopts 4-fold screw helical symmetry with the disaccharide unit periodicity of 40Å. Peptidoglycan lattice in the S. aureus cell wall is formed by cross-linked PG stems that have parallel orientations. The structural characterization of Fem-mutants of S. aureus with varying lengths of bridge structures suggests that the PG-bridge length is an important determining factor for the PG architecture.

Controlling mechanisms in directional growth of aggregated archaeal cells

Members of the family Methanosarcinaceae are important archaeal representatives due to their broad functionality, ubiquitous presence, and functionality in harsh environments. A key characteristic is their multicellular (packet) morphology represented by aggregates of spatially confined cells. This morphology is driven by directed growth of cells in confinement with sequential variation in growth direction. To further understand why spatially confined Methanosarcina cells (and in general, confined prokaryotes) change their direction of growth during consecutive growth-division stages, and how a particular cell senses its wall topology and responds to changes on it a theoretical model for stress dependent growth of aggregated archaeal cells was developed. The model utilizes a confined elastic shell representation of aggregated archaeal cell and is derived based on a work-energy principle. The growth law takes into account the fine structure of archaeal cell wall, polymeric nature of methanochondroitin layer, molecular-biochemical processes and is based on thermodynamic laws. The developed model has been applied to three typical configurations of aggregated cell in 3D. The developed model predicted a geometry response with delayed growth of aggregated archaeal cells explained from mechanistic principles, as well as continuous changes in direction of growth during the consecutive growth-division stages. This means that cell wall topology sensing and growth anisotropy can be predicted using simple cellular mechanisms without the need for dedicated cellular machinery.

The interplay between cell wall mechanical properties and the cell cycle in Staphylococcus aureus

The nanoscale mechanical properties of live Staphylococcus aureus cells during different phases of growth were studied by atomic force microscopy. Indentation to different depths provided access to both local cell wall mechanical properties and whole-cell properties, including a component related to cell turgor pressure. Local cell wall properties were found to change in a characteristic manner throughout the division cycle. Splitting of the cell into two daughter cells followed a local softening of the cell wall along the division circumference, with the cell wall on either side of the division circumference becoming stiffer. Once exposed, the newly formed septum was found to be stiffer than the surrounding, older cell wall. Deeper indentations, which were affected by cell turgor pressure, did not show a change in stiffness throughout the division cycle, implying that enzymatic cell wall remodeling and local variations in wall properties are responsible for the evolution of cell shape through division.

LytR-CpsA-Psr enzymes as determinants of Bacillus anthracis secondary cell wall polysaccharide assembly

Bacillus anthracis, the causative agent of anthrax, replicates as chains of vegetative cells by regulating the separation of septal peptidoglycan. Surface (S)-layer proteins and associated proteins (BSLs) function as chain length determinants and bind to the secondary cell wall polysaccharide (SCWP). In this study, we identified the B. anthracis lcpD mutant, which displays increased chain length and S-layer assembly defects due to diminished SCWP attachment to peptidoglycan. In contrast, the B. anthracis lcpB3 variant displayed reduced cell size and chain length, which could be attributed to increased deposition of BSLs. In other bacteria, LytR-CpsA-Psr (LCP) proteins attach wall teichoic acid (WTA) and polysaccharide capsule to peptidoglycan. B. anthracis does not synthesize these polymers, yet its genome encodes six LCP homologues, which, when expressed in S. aureus, promote WTA attachment. We propose a model whereby B. anthracis LCPs promote attachment of SCWP precursors to discrete locations in the peptidoglycan, enabling BSL assembly and regulated separation of septal peptidoglycan.

The effect of skin fatty acids on Staphylococcus aureus

Staphylococcus aureus is a commensal of the human nose and skin. Human skin fatty acids, in particular cis-6-hexadecenoic acid (C-6-H), have high antistaphylococcal activity and can inhibit virulence determinant production. Here, we show that sub-MIC levels of C-6-H result in induction of increased resistance. The mechanism(s) of C-6-H activity was investigated by combined transcriptome and proteome analyses. Proteome analysis demonstrated a pleiotropic effect of C-6-H on virulence determinant production. In response to C-6-H, transcriptomics revealed altered expression of over 500 genes, involved in many aspects of virulence and cellular physiology. The expression of toxins (hla, hlb, hlgBC) was reduced, whereas that of host defence evasion components (cap, sspAB, katA) was increased. In particular, members of the SaeRS regulon had highly reduced expression, and the use of specific mutants revealed that the effect on toxin production is likely mediated via SaeRS.

Sec-secretion and sortase-mediated anchoring of proteins in Gram-positive bacteria

Signal peptide-driven secretion of precursor proteins directs polypeptides across the plasma membrane of bacteria. Two pathways, Sec- and SRP-dependent, converge at the SecYEG translocon to thread unfolded precursor proteins across the membrane, whereas folded preproteins are routed via the Tat secretion pathway. Gram-positive bacteria lack an outer membrane and are surrounded by a rigid layer of peptidoglycan. Interactions with their environment are mediated by proteins that are retained in the cell wall, often through covalent attachment to the peptidoglycan. In this review, we describe the mechanisms for both Sec-dependent secretion and sortase-dependent assembly of proteins in the envelope of Gram-positive bacteria. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

Genetic manipulation of Staphylococcus aureus

Staphylococcus aureus is a facultative anaerobic Gram-positive coccus and a member of the normal skin flora, as well as that of the nasal passages of humans. However, S. aureus can also gain entry into the host and cause life-threatening infections or persist as disease foci that develop into suppurative abscesses. While genetically tractable, the manipulation of S. aureus remains challenging. This unit describes methods developed in our laboratory for gene disruption by allelic replacement and transposition. We also provide a protocol for bacteriophage-mediated transduction of mutants marked with selectable alleles and describe plasmid utilization for complementation studies.

Different walls for rods and balls: the diversity of peptidoglycan

Peptidoglycan performs the essential role of resisting turgor in the cell walls of most bacteria. It determines cell shape, and its biosynthesis is the target for many important antibiotics. The fundamental chemical building blocks of peptidoglycan are conserved: repeating disaccharides cross-linked by peptides. However, these blocks come in many varieties and can be assembled in different ways. So beyond the fundamental similarity, prodigious chemical, organizational and architectural diversity is revealed. Here, we track the evolution of our current understanding of peptidoglycan and underpinning technical and methodological developments. The origin and function of chemical diversity is discussed with respect to some well-studied example species. We then explore how this chemistry is manifested in elegant and complex peptidoglycan organization and how this is interpreted in different and sometimes controversial architectural models. We contend that emerging technology brings about the possibility of achieving a complete understanding of peptidoglycan chemistry, through architecture, to the way in which diverse species and populations of cells meet the challenges of maintaining viability and growth within their environmental niches, by exploiting the bioengineering versatility of peptidoglycan.

Release of protein A from the cell wall of Staphylococcus aureus

Staphylococcal protein A (SpA) is anchored to the cell wall envelope of Staphylococcus aureus by sortase A, which links the threonyl (T) of its C-terminal LPXTG motif to peptidoglycan cross-bridges (i.e., Gly5). SpA binds the Fcγ domains of IgG and protects staphylococci from opsonophagocytic clearance. Moreover, SpA cross-links B-cell receptors to modify host adaptive immune responses. The mechanisms whereby SpA is released from the bacterial surface to access the host's immune system are not known. Here we demonstrate that SpA is released with murein tetrapeptide-tetraglycyl [L-Ala-D-iGln-(SpA-Gly5)L-Lys-D-Ala-Gly4] linked to its C-terminal threonyl. LytN, a cross-wall murein hydrolase, contributes to the release of SpA by removing amino sugars [i.e., N-acetylmuramic acid-N-acetylglucosamine (MurNAc-GlcNAc)] from attached peptidoglycan, whereas LytM, a pentaglycyl-endopeptidase, triggers polypeptide release from the bacterial envelope. A model is proposed whereby murein hydrolases cleave the anchor structure of released SpA to modify host immune responses.

The giant protein Ebh is a determinant of Staphylococcus aureus cell size and complement resistance

Staphylococcus aureus USA300, the clonal type associated with epidemic community-acquired methicillin-resistant S. aureus (MRSA) infections, displays the giant protein Ebh on its surface. Mutations that disrupt the ebh reading frame increase the volume of staphylococcal cells and alter the cross wall, a membrane-enclosed peptidoglycan synthesis and assembly compartment. S. aureus ebh variants display increased sensitivity to oxacillin (methicillin) as well as susceptibility to complement-mediated killing. Mutations in ebh are associated with reduced survival of mutant staphylococci in blood and diminished virulence in mice. We propose that Ebh, following its secretion into the cross wall, contributes to the characteristic cell growth and envelope assembly pathways of S. aureus, thereby enabling complement resistance and the pathogenesis of staphylococcal infections.

Functional and structural analysis of the major amidase (Atl) in Staphylococcus

The cytoplasmic membrane of most bacteria is surrounded by a more or less thick murein layer (peptidoglycan) that protects the protoplast from mechanical damage, osmotic rupture and lysis. When bacteria are dividing processes are initiated stepwise that involve DNA replication, constriction of the membranes, cell growth, biosynthesis of new murein, and finally the generation of two daughter cells. As the daughter cells are still covalently interlinked by the murein network they must be separated by specific peptidoglycan hydrolases, also referred to as autolysins. In staphylococci, the major autolysin (Atl) and its processed products N-acetylmuramoyl-l-alanine amidase (AM) and endo-β-N-acetylglucosaminidase (GL) have been in the research focus for long time. This review addresses phenotypic consequences of atl mutants, impact of Atl in virulence, the mechanism of targeting to the septum region, regulation of atl, the structure of the amidase and the repeat regions, as well as the phylogeny of Atl and its use in Staphylococcus genus and species typing.

Growth and laboratory maintenance of Staphylococcus aureus

Staphylococcus aureus is a facultative anaerobic Gram-positive coccus and a member of the normal skin flora as well as the nasal passages of humans. S. aureus is also the etiological agent of suppurative abscesses, as first described by Sir Alexander Ogston in 1880. Ever since, studies on S. aureus have focused on the complex battery of virulence factors and regulators that allow for its swift transition between commensalism and pathogenic states and escape from host immune defenses. The success of this pathogen is further evidenced by its ability to acquire antibiotic resistance traits through mechanisms that often remain poorly understood.

Staphylococcus aureus peptidoglycan stem packing by rotational-echo double resonance NMR spectroscopy

Staphylococcus aureus grown in the presence of an alanine-racemase inhibitor was labeled with d-[1-(13)C]alanine and l-[(15)N]alanine to characterize some details of the peptidoglycan tertiary structure. Rotational-echo double-resonance NMR of intact whole cells was used to measure internuclear distances between (13)C and (15)N of labeled amino acids incorporated in the peptidoglycan, and from those labels to (19)F of a glycopeptide drug specifically bound to the peptidoglycan. The observed (13)C-(15)N average distance of 4.1-4.4 Å between d- and l-alanines in nearest-neighbor peptide stems is consistent with a local, tightly packed, parallel-stem architecture for a repeating structural motif within the peptidoglycan of S. aureus.

Resistance mechanisms in Campylobacter jejuni

Campylobacter jejuni is a major cause of food-borne gastroenteritis worldwide. While mortality is low, morbidity imparted by post-infectious sequelae such as Guillain-Barré syndrome, Reiter syndrome/reactive arthritis and irritable bowel syndrome is significant. In addition, the economic cost is high due to lost productivity. Food animals, particularly poultry, are the main reservoirs of C. jejuni. The over-use of antibiotics in the human population and in animal husbandry has led to an increase in antibiotic-resistant infections, particularly with fluoroquinolones. This is problematic because C. jejuni gastroenteritis is clinically indistinguishable from that caused by other bacterial pathogens, and such illnesses are usually treated empirically with fluoroquinolones. Since C. jejuni is naturally transformable, acquisition of additional genes imparting antibiotic resistance is likely. Therefore, an understanding of the antibiotic resistance mechanisms in C. jejuni is needed to provide proper therapy both to the veterinary and human populations.

Uniformity of glycyl bridge lengths in the mature cell walls of fem mutants of methicillin-resistant Staphylococcus aureus

Peptidoglycan (PG) composition in intact cells of methicillin-resistant Staphylococcus aureus (MRSA) and its isogenic Fem mutants has been characterized by measuring the glycine content of PG bridge structures by solid-state nuclear magnetic resonance (NMR). The glycine content estimated from integrated intensities (rather than peak heights) in the cell walls of whole cells was increased by approximately 30% for the FemA mutant and was reduced by 25% for the FemB mutant relative to expected values for homogeneous structures. In contrast, the expected compositions were observed in isolated cell walls of the same mutants. For FemA mutant whole cells, the increase was due to the presence of triglycyl bridge PG units (confirmed directly by mass spectrometric analysis), which constituted 10% of the total PG. These species were coalesced in some sort of a lattice or aggregate with spatial proximity to other PG bridges. This result suggests that the triglycyl-bridged PG units form a PG-like structure that is not incorporated into the mature cell wall.

Anti-infectious agents against MRSA

Clinically useful antibiotics, β-lactams and vancomycin, are known to inhibit bacterial cell wall peptidoglycan synthesis. Methicillin-resistant Staphylococcus aureus (MRSA) has a unique cell wall structure consisting of peptidoglycan and wall teichoic acid. In recent years, new anti-infectious agents (spirohexaline, tripropeptin C, DMPI, CDFI, cyslabdan, 1835F03, and BPH-652) targeting MRSA cell wall biosynthesis have been discovered using unique screening methods. These agents were found to inhibit important enzymes involved in cell wall biosynthesis such as undecaprenyl pyrophosphate (UPP) synthase, FemA, flippase, or UPP phosphatase. In this review, the discovery, the mechanism of action, and the future of these anti-infectious agents are described.

Interplay of antibiotics and bacterial inoculum on suture-associated biofilms

BACKGROUND

Biofilms are often antibiotic resistant, and it is unclear if prophylactic antibiotics can effectively prevent biofilm formation. Experiments were designed to test the ability of high (bactericidal) concentrations of ampicillin (AMP), vancomycin (VAN), and oxacillin (OXA) to prevent formation of suture-associated biofilms initiated with low (10(4)) and high (10(7)) numbers of Staphylococcus aureus.

MATERIALS AND METHODS

S. aureus biofilms were cultivated overnight on silk suture incubated in biofilm growth medium supplemented with bactericidal concentrations of AMP, VAN, or OXA. Standard microbiological methods were used to quantify total numbers of viable suture-associated S. aureus. Crystal violet staining followed by spectroscopy was used to quantify biofilm biomass, which includes bacterial cells plus matrix components. To observe the effects of antibiotics on the microscopic appearance of biofilm formation, biofilms were cultivated on glass slides, then stained with fluorescent dyes, and observed by confocal microscopy.

RESULTS

In the presence of a relatively low inoculum (10(4)) of S. aureus cells, bactericidal concentrations of AMP, VAN, or OXA were effective in preventing development of suture-associated biofilms. However, similar concentrations of these antibiotics were typically ineffective in preventing biofilm development on sutures inoculated with 10(7)S. aureus, a concentration relevant to contaminated skin. Confocal microscopy confirmed that bactericidal concentrations of AMP, VAN, or OXA inhibited, but did not prevent, development of S. aureus biofilms.

CONCLUSION

Bactericidal concentrations of AMP, VAN, or OXA inhibited formation of suture-associated biofilms initiated with low numbers (10(4)), but not high numbers (10(7)), of S. aureus cells.

Protein secretion and surface display in Gram-positive bacteria

The cell wall peptidoglycan of Gram-positive bacteria functions as a surface organelle for the transport and assembly of proteins that interact with the environment, in particular, the tissues of an infected host. Signal peptide-bearing precursor proteins are secreted across the plasma membrane of Gram-positive bacteria. Some precursors carry C-terminal sorting signals with unique sequence motifs that are cleaved by sortase enzymes and linked to the cell wall peptidoglycan of vegetative forms or spores. The sorting signals of pilin precursors are cleaved by pilus-specific sortases, which generate covalent bonds between proteins leading to the assembly of fimbrial structures. Other precursors harbour surface (S)-layer homology domains (SLH), which fold into a three-pronged spindle structure and bind secondary cell wall polysaccharides, thereby associating with the surface of specific Gram-positive microbes. Type VII secretion is a non-canonical secretion pathway for WXG100 family proteins in mycobacteria. Gram-positive bacteria also secrete WXG100 proteins and carry unique genes that either contribute to discrete steps in secretion or represent distinctive substrates for protein transport reactions.

Determinants of murein hydrolase targeting to cross-wall of Staphylococcus aureus peptidoglycan

Cells of eukaryotic or prokaryotic origin express proteins with LysM domains that associate with the cell wall envelope of bacteria. The molecular properties that enable LysM domains to interact with microbial cell walls are not yet established. Staphylococcus aureus, a spherical microbe, secretes two murein hydrolases with LysM domains, Sle1 and LytN. We show here that the LysM domains of Sle1 and LytN direct murein hydrolases to the staphylococcal envelope in the vicinity of the cross-wall, the mid-cell compartment for peptidoglycan synthesis. LysM domains associate with the repeating disaccharide β-N-acetylmuramic acid, (1→4)-β-N-acetylglucosamine of staphylococcal peptidoglycan. Modification of N-acetylmuramic acid with wall teichoic acid, a ribitol-phosphate polymer tethered to murein linkage units, prevents the LysM domain from binding to peptidoglycan. The localization of LytN and Sle1 to the cross-wall is abolished in staphylococcal tagO mutants, which are defective for wall teichoic acid synthesis. We propose a model whereby the LysM domain ensures septal localization of LytN and Sle1 followed by processive cleavage of peptidoglycan, thereby exposing new LysM binding sites in the cross-wall and separating bacterial cells.

Impact of sub-inhibitory antibiotics on fibronectin-mediated host cell adhesion and invasion by Staphylococcus aureus

Background

Staphylococcus aureus is a well-armed pathogen prevalent in severe infections such as endocarditis and osteomyelitis. Fibronectin-binding proteins A and B, encoded by fnbA/B, are major pathogenesis determinants in these infections through their involvement in S. aureus adhesion to and invasion of host cells. Sub-minimum inhibitory concentrations (sub-MICs) of antibiotics, frequently occurring in vivo because of impaired drug diffusion at the infection site, can alter S. aureus phenotype. We therefore investigated their impact on S. aureus fibronectin-mediated adhesiveness and invasiveness.

Methods

After in vitro challenge of S. aureus 8325-4 and clinical isolates with sub-MICs of major anti-staphylococcal agents, we explored fnbA/B transcription levels, bacterial adhesiveness to immobilised human fibronectin and human osteoblasts in culture, and bacterial invasion of human osteoblasts.

Results

Oxacillin, moxifloxacin and linezolid led to the development of a hyper-adhesive phenotype in the fibronectin adhesion assay that was consistent with an increase in fnbA/B transcription. Conversely, rifampin treatment decreased fibronectin binding in all strains tested without affecting fnbA/B transcription. Gentamicin and vancomycin had no impact on fibronectin binding or fnbA/B transcription levels. Only oxacillin-treated S. aureus displayed a significantly increased adhesion to cultured osteoblasts, but its invasiveness did not differ from that of untreated controls.

Conclusion

Our findings demonstrate that several antibiotics at sub-MICs modulate fibronectin binding in S. aureus in a drug-specific fashion. However, hyper- and hypo- adhesive phenotypes observed in controlled in vitro conditions were not fully confirmed in whole cell infection assays. The relevance of adhesion modulation during in vivo infections is thus still uncertain and requires further investigations.

Role of ArlRS in autolysis in methicillin-sensitive and methicillin-resistant Staphylococcus aureus strains

Autolysis plays an essential role in bacterial cell division and lysis with β-lactam antibiotics. Accordingly, the expression of autolysins is tightly regulated by several endogenous regulators, including ArlRS, a two component regulatory system that has been shown to negatively regulate autolysis in methicillin-sensitive Staphylococcus aureus (MSSA) strains. In this study, we found that inactivation of arlRS does not play a role in autolysis of methicillin-resistant S. aureus (MRSA) strains, such as community-acquired (CA)-MRSA strains USA300 and MW2 or the hospital-acquired (HA)-MRSA strain COL. This contrasts with MSSA strains, including Newman, SH1000, RN6390, and 8325-4, where autolysis is affected by ArlRS. We further demonstrated that the striking difference in the roles of arlRS between MSSA and MRSA strains is not due to the methicillin resistance determinant mecA. Among known autolysins and their regulators, we found that arlRS represses lytN, while no effect was seen on atl, lytM, and lytH expression in both CA- and HA-MRSA strains. Transcriptional-fusion assays showed that the agr transcripts, RNAII and RNAIII, were significantly more downregulated in the arlRS mutant of MW2 than the MSSA strain Newman. Importantly, provision of agr RNAIII in trans to the MW2 arlRS mutant via a multicopy plasmid induced autolysis in this MRSA strain. Also, the autolytic phenotype in the arlRS mutant of MSSA strain Newman could be rescued by a mutation in either atl or lytM. Together, these data showed that ArlRS impacts autolysis differently in MSSA and MRSA strains.

Transcriptional and functional analysis of the effects of magnolol: inhibition of autolysis and biofilms in Staphylococcus aureus

BACKGROUND

The targeting of Staphylococcus aureus biofilm structures are now gaining interest as an alternative strategy for developing new types of antimicrobial agents. Magnolol (MOL) shows inhibitory activity against S. aureus biofilms and Triton X-100-induced autolysis in vitro, although there are no data regarding the molecular mechanisms of MOL action in bacteria.

METHODOLOGY/PRINCIPAL FINDINGS

The molecular basis of the markedly reduced autolytic phenotype and biofilm inhibition triggered by MOL were explored using transcriptomic analysis, and the transcription of important genes were verified by real-time RT-PCR. The inhibition of autolysis by MOL was evaluated using quantitative bacteriolytic assays and zymographic analysis, and antibiofilm activity assays and confocal laser scanning microscopy were used to elucidate the inhibition of biofilm formation caused by MOL in 20 clinical isolates or standard strains. The reduction in cidA, atl, sle1, and lytN transcript levels following MOL treatment was consistent with the induced expression of their autolytic repressors lrgA, lrgB, arlR, and sarA. MOL generally inhibited or reversed the expression of most of the genes involved in biofilm production. The growth of S. aureus strain ATCC 25923 in the presence of MOL dose-dependently led to decreases in Triton X-100-induced autolysis, extracellular murein hydrolase activity, and the amount of extracellular DNA (eDNA). MOL may impede biofilm formation by reducing the expression of cidA, a murein hydrolase regulator, to inhibit autolysis and eDNA release, or MOL may directly repress biofilm formation.

CONCLUSIONS/SIGNIFICANCE

MOL shows in vitro antimicrobial activity against clinical and standard S. aureus strains grown in planktonic and biofilm cultures, suggesting that the structure of MOL may potentially be used as a basis for the development of drugs targeting biofilms.

LytN, a murein hydrolase in the cross-wall compartment of Staphylococcus aureus, is involved in proper bacterial growth and envelope assembly

Cell cycle progression for the spherical microbe Staphylococcus aureus requires the coordinated synthesis and remodeling of peptidoglycan. The majority of these rearrangements takes place at the mid-cell, in a compartment designated the cross-wall. Secreted polypeptides endowed with a YSIRK-G/S signal peptide are directly delivered to the cross-wall compartment. One such YSIRK-containing protein is the murein hydrolase LytN. lytN mutations precipitate structural damage to the cross-wall and interfere with staphylococcal growth. Overexpression of lytN also affects growth and triggers rupture of the cross-wall. The lytN phenotype can be reversed by the controlled expression of lytN but not by adding purified LytN to staphylococcal cultures. LytN harbors LysM and CHAP domains, the latter of which functions as both an N-acetylmuramoyl-L-alanine amidase and D-alanyl-glycine endopeptidase. Thus, LytN secretion into the cross-wall promotes peptidoglycan separation and completion of the staphylococcal cell cycle.

AFM study of the differential inhibitory effects of the green tea polyphenol (-)-epigallocatechin-3-gallate (EGCG) against Gram-positive and Gram-negative bacteria

(-)-Epigallocatechin-3-gallate (EGCG), a main constituent of tea catechins, affects Gram-positive and Gram-negative bacteria differently; however, the underlying mechanisms are not clearly understood. Atomic force microscopy (AFM) was used to compare morphological alterations in Gram-positive and Gram-negative bacteria induced by EGCG and by H(2)O(2) at sub-minimum inhibitory concentrations (MICs). EGCG initially induced aggregates in the cell envelopes of Staphylococcus aureus and eventually caused cell lysis, which was not observed in cells treated with H(2)O(2). It initially induced nanoscale perforations or microscale grooves in the cell envelopes of Escherichia coli O157:H7 which eventually disappeared, similar to E. coli cells treated with H(2)O(2). An E. coli O157:H7 tpx mutant, with a defect in thioredoxin-dependent thiol peroxidase (Tpx), was more severely damaged by EGCG when compared with its wild type. Similar differing effects were observed in other Gram-positive and Gram-negative bacteria when exposed to EGCG; it caused aggregated in Streptococcus mutans, while it caused grooves in Pseudomonas aeruginosa. AFM results suggest that the major morphological changes of Gram-negative bacterial cell walls induced by EGCG depend on H(2)O(2) release. This is not the case for Gram-positive bacteria. Oxidative stress in Gram-negative bacteria induced by EGCG was confirmed by flow cytometry.

Structural and functional analyses reveal that Staphylococcus aureus antibiotic resistance factor HmrA is a zinc-dependent endopeptidase

HmrA is an antibiotic resistance factor of methicillin-resistant Staphylococcus aureus. Molecular analysis of this protein revealed that it is not a muramidase or β-lactamase but a nonspecific double-zinc endopeptidase consisting of a catalytic domain and an inserted oligomerization domain, which probably undergo a relative interdomain hinge rotation upon substrate binding. The active-site cleft is located at the domain interface. Four HmrA protomers assemble to a large ∼170-kDa homotetrameric complex of 125 Å. All four active sites are fully accessible and ∼50-70 Å apart, far enough apart to act on a large meshwork substrate independently but simultaneously. In vivo studies with four S. aureus strains of variable resistance levels revealed that the extracellular addition of HmrA protects against loss of viability in the presence of oxacillin and that this protection depends on proteolytic activity. All of these results indicate that HmrA is a peptidase that participates in resistance mechanisms in vivo in the presence of β-lactams. Furthermore, our results have implications for most S. aureus strains of known genomic sequences and several other cocci and bacilli, which harbor close orthologs. This suggests that HmrA may be a new widespread antibiotic resistance factor in bacteria.

The requirement for pneumococcal MreC and MreD is relieved by inactivation of the gene encoding PBP1a

MreC and MreD, along with the actin homologue MreB, are required to maintain the shape of rod-shaped bacteria. The depletion of MreCD in rod-shaped bacteria leads to the formation of spherical cells and the accumulation of suppressor mutations. Ovococcus bacteria, such as Streptococcus pneumoniae, lack MreB homologues, and the functions of the S. pneumoniae MreCD (MreCD(Spn)) proteins are unknown. mreCD are located upstream from the pcsB cell division gene in most Streptococcus species, but we found that mreCD and pcsB are transcribed independently. Similarly to rod-shaped bacteria, we show that mreCD are essential in the virulent serotype 2 D39 strain of S. pneumoniae, and the depletion of MreCD results in cell rounding and lysis. In contrast, laboratory strain R6 contains suppressors that allow the growth of ΔmreCD mutants, and bypass suppressors accumulate in D39 ΔmreCD mutants. One class of suppressors eliminates the function of class A penicillin binding protein 1a (PBP1a). Unencapsulated Δpbp1a D39 mutants have smaller diameters than their pbp1a(+) parent or Δpbp2a and Δpbp1b mutants, which lack other class A PBPs and do not show the suppression of ΔmreCD mutations. Suppressed ΔmreCD Δpbp1a double mutants form aberrantly shaped cells, some with misplaced peptidoglycan (PG) biosynthesis compared to that of single Δpbp1a mutants. Quantitative Western blotting showed that MreC(Spn) is abundant (≈8,500 dimers per cell), and immunofluorescent microscopy (IFM) located MreCD(Spn) to the equators and septa of dividing cells, similarly to the PBPs and PG pentapeptides indicative of PG synthesis. These combined results are consistent with a model in which MreCD(Spn) direct peripheral PG synthesis and control PBP1a localization or activity.