Living with an imperfect cell wall: compensation of femAB inactivation in Staphylococcus aureus

Spontaneous Genomic Variation as a Survival Strategy of Nosocomial Staphylococcus haemolyticus

Staphylococcus haemolyticus is one of the most important nosocomial human pathogens frequently isolated in bloodstream and medical device-related infections. However, its mechanisms of evolution and adaptation are still poorly explored. To characterize the strategies of genetic and phenotypic diversity in S. haemolyticus, we analyzed an invasive strain for genetic and phenotypic stability after serial passage in vitro in the absence and presence of beta-lactam antibiotics. We performed pulsed-field gel electrophoresis (PFGE) of the culture and analyzed five colonies at seven time points during stability assays for beta-lactam susceptibility, hemolysis, mannitol fermentation, and biofilm production. We compared their whole genomes and performed phylogenetic analysis based on core single-nucleotide polymorphisms (SNPs). We observed a high instability in the PFGE profiles at the different time points in the absence of antibiotic. Analysis of WGS data for individual colonies showed the occurrence of six large-scale genomic deletions within the oriC environ, smaller deletions in non-oriC environ regions, and nonsynonymous mutations in clinically relevant genes. The regions of deletion and point mutations included genes encoding amino acid and metal transporters, resistance to environmental stress and beta-lactams, virulence, mannitol fermentation, metabolic processes, and insertion sequence (IS) elements. Parallel variation was detected in clinically significant phenotypic traits such as mannitol fermentation, hemolysis, and biofilm formation. In the presence of oxacillin, PFGE profiles were overall stable over time and mainly corresponded to a single genomic variant. Our results suggest that S. haemolyticus populations are composed of subpopulations of genetic and phenotypic variants. The maintenance of subpopulations in different physiological states may be a strategy to adapt rapidly to stress situations imposed by the host, particularly in the hospital environment. IMPORTANCE The introduction of medical devices and antibiotics into clinical practice have substantially improved patient quality of life and contributed to extended life expectancy. One of its most cumbersome consequences was the emergence of medical device-associated infections caused by multidrug-resistant and opportunistic bacteria such as Staphylococcus haemolyticus. However, the reason for this bacterium's success is still elusive. We found that in the absence of environmental stresses, S. haemolyticus can spontaneously produce subpopulations of genomic and phenotypic variants with deletions/mutations in clinically relevant genes. However, when exposed to selective pressures, such as the presence of antibiotics, a single genomic variant will be recruited and become dominant. We suggest that the maintenance of these cell subpopulations in different physiological states is an extremely effective strategy to adapt to stresses imposed by the host or the infection environment and might contribute for S. haemolyticus survival and persistence in the hospital.

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

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

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

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

Partition of tRNAGly isoacceptors between protein and cell-wall peptidoglycan synthesis in Staphylococcus aureus

The sequence of tRNAs is submitted to evolutionary constraints imposed by their multiple interactions with aminoacyl-tRNA synthetases, translation elongation factor Tu in complex with GTP (EF-Tu•GTP), and the ribosome, each being essential for accurate and effective decoding of messenger RNAs. In Staphylococcus aureus, an additional constraint is imposed by the participation of tRNA^Gly isoacceptors in the addition of a pentaglycine side chain to cell-wall peptidoglycan precursors by transferases FmhB, FemA and FemB. Three tRNA^Gly isoacceptors poorly interacting with EF-Tu•GTP and the ribosome were previously identified. Here, we show that these ‘non-proteogenic’ tRNAs are preferentially recognized by FmhB based on kinetic analyses and on synthesis of stable aminoacyl-tRNA analogues acting as inhibitors. Synthesis of chimeric tRNAs and of helices mimicking the tRNA acceptor arms revealed that this discrimination involves identity determinants exclusively present in the D and T stems and loops of non-proteogenic tRNAs, which belong to an evolutionary lineage only present in the staphylococci. EF-Tu•GTP competitively inhibited FmhB by sequestration of ‘proteogenic’ aminoacyl-tRNAs in vitro. Together, these results indicate that competition for the Gly-tRNA^Gly pool is restricted by both limited recognition of non-proteogenic tRNAs by EF-Tu•GTP and limited recognition of proteogenic tRNAs by FmhB.

Noncanonical inputs and outputs of tRNA aminoacylation

The aminoacylation reaction is one of most extensively studied cellular processes. The so-called "canonical" reaction is carried out by direct charging of an amino acid (aa) onto its corresponding transfer RNA (tRNA) by the cognate aminoacyl-tRNA synthetase (aaRS), and the canonical usage of the aminoacylated tRNA (aa-tRNA) is to translate a messenger RNA codon in a translating ribosome. However, four out of the 22 genetically-encoded aa are made "noncanonically" through a two-step or indirect route that usually compensate for a missing aaRS. Additionally, from the 22 proteinogenic aa, 13 are noncanonically used, by serving as substrates for the tRNA- or aa-tRNA-dependent synthesis of other cellular components. These nontranslational processes range from lipid aminoacylation, and heme, aa, antibiotic and peptidoglycan synthesis to protein degradation. This chapter focuses on these noncanonical usages of aa-tRNAs and the ways of generating them, and also highlights the strategies that cells have evolved to balance the use of aa-tRNAs between protein synthesis and synthesis of other cellular components.

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.

Extracellular electron transfer features of Gram-positive bacteria

Electroactive microorganisms possess the unique ability to transfer electrons to or from solid phase electron conductors, e.g., electrodes or minerals, through various physiological mechanisms. The processes are commonly known as extracellular electron transfer and broadly harnessed in microbial electrochemical systems, such as microbial biosensors, microbial electrosynthesis, or microbial fuel cells. Apart from a few model microorganisms, the nature of the microbe-electrode conductive interaction is poorly understood for most of the electroactive species. The interaction determines the efficiency and a potential scaling up of bioelectrochemical systems. Gram-positive bacteria generally have a thick electron non-conductive cell wall and are believed to exhibit weak extracellular electron shuttling activity. This review highlights reported research accomplishments on electroactive Gram-positive bacteria. The use of electron-conducting polymers as mediators is considered as one promising strategy to enhance the electron transfer efficiency up to application scale. In view of the recent progress in understanding the molecular aspects of the extracellular electron transfer mechanisms of Enterococcus faecalis, the electron transfer properties of this bacterium are especially focused on. Fundamental knowledge on the nature of microbial extracellular electron transfer and its possibilities can provide insight in interspecies electron transfer and biogeochemical cycling of elements in nature. Additionally, a comprehensive understanding of cell-electrode interactions may help in overcoming insufficient electron transfer and restricted operational performance of various bioelectrochemical systems and facilitate their practical applications.

The pentaglycine bridges of Staphylococcus aureus peptidoglycan are essential for cell integrity

Bacterial cells are surrounded by cell wall, whose main component is peptidoglycan (PG), a macromolecule that withstands the internal turgor of the cell. PG composition can vary considerably between species. The Gram-positive pathogen Staphylococcus aureus possesses highly crosslinked PG due to the presence of cross bridges containing five glycines, which are synthesised by the FemXAB protein family. FemX adds the first glycine of the cross bridge, while FemA and FemB add the second and the third, and the fourth and the fifth glycines, respectively. Of these, FemX was reported to be essential. To investigate the essentiality of FemAB, we constructed a conditional S. aureus mutant of the femAB operon. Depletion of femAB was lethal, with cells appearing as pseudomulticellular forms that eventually lyse due to extensive membrane rupture. This deleterious effect was mitigated by drastically increasing the osmolarity of the medium, indicating that pentaglycine crosslinks are required for S. aureus cells to withstand internal turgor. Despite the absence of canonical membrane targeting domains, FemA has been shown to localise at the membrane. To study its mechanism of localisation, we constructed mutants in key residues present in the putative transferase pocket and the α6 helix of FemA, possibly involved in tRNA binding. Mutations in the α6 helix led to a sharp decrease in protein activity in vivo and in vitro but did not impair correct membrane localisation, indicating that FemA activity is not required for localisation. Our data indicates that, contrarily to what was previously thought, S. aureus cells do not survive in the absence of a pentaglycine cross bridge.

Raf-kinase inhibitor GW5074 shows antibacterial activity against methicillin-resistant Staphylococcus aureus and potentiates the activity of gentamicin

AIM

Increasing antimicrobial resistance has compromised the effectiveness of many antibiotics, including those used to treat staphylococcal infections like methicillin-resistant Staphylococcus aureus. The development of combination therapies, where antimicrobial agents are used with compounds that inhibit resistance pathways is a promising strategy. Results/methodology: The Raf kinase inhibitor GW5074 exhibited selective in vitro activity against Gram-positive bacteria, including clinical isolates of S. aureus with a minimum inhibitory concentration (MIC) of 2-8 µg/ml. GW5074 was effective in vivo in the Galleria mellonella infection model. The compound showed synergy with gentamicin by lowering MIC by fourfold, compared with gentamicin MIC alone.

CONCLUSION

This work demonstrates the antimicrobial properties of GW5074 and supports further investigation of the kinase inhibitors as antibiotic adjuvants.

Osmolyte transport in Staphylococcus aureus and the role in pathogenesis

Osmolyte transport is a pivotal part of bacterial life, particularly in high salt environments. Several low and high affinity osmolyte transport systems have been identified in various bacterial species. A lot of research has centered on characterizing the osmolyte transport systems of Gram-negative bacteria, but less has been done to characterize the same transport systems in Gram-positive bacteria. This review will focus on the previous work that has been done to understand the osmolyte transport systems in the species Staphylococcus aureus and how these transporters may serve dual functions in allowing the bacteria to survive and grow in a variety of environments, including on the surface or within humans or other animals.

A glyS T-box riboswitch with species-specific structural features responding to both proteinogenic and nonproteinogenic tRNAGly isoacceptors

In Staphylococcus aureus, a T-box riboswitch exists upstream of the glyS gene to regulate transcription of the sole glycyl-tRNA synthetase, which aminoacylates five tRNA(Gly) isoacceptors bearing GCC or UCC anticodons. Subsequently, the glycylated tRNAs serve as substrates for decoding glycine codons during translation, and also as glycine donors for exoribosomal synthesis of pentaglycine peptides during cell wall formation. Probing of the predicted T-box structure revealed a long stem I, lacking features previously described for similar T-boxes. Moreover, the antiterminator stem includes a 42-nt long intervening sequence, which is staphylococci-specific. Finally, the terminator conformation adopts a rigid two-stem structure, where the intervening sequence forms the first stem followed by the second stem, which includes the more conserved residues. Interestingly, all five tRNA(Gly) isoacceptors interact with S. aureus glyS T-box with different binding affinities and they all induce transcription readthrough at different levels. The ability of both GCC and UCC anticodons to interact with the specifier loop indicates ambiguity during the specifier triplet reading, similar to the unconventional reading of glycine codons during protein synthesis. The S. aureus glyS T-box structure is consistent with the recent crystallographic and NMR studies, despite apparent differences, and highlights the phylogenetic variability of T-boxes when studied in a genome-dependent context. Our data suggest that the S. aureus glyS T-box exhibits differential tRNA selectivity, which possibly contributes toward the regulation and synchronization of ribosomal and exoribosomal peptide synthesis, two essential but metabolically unrelated pathways.

High-Resolution Mass Spectrometry Based Proteomic Analysis of the Response to Vancomycin-Induced Cell Wall Stress in Streptomyces coelicolor A3(2)

Understanding how bacteria survive periods of cell wall stress is of fundamental interest and can help generate ideas for improved antibacterial treatments. In this study we use tandem mass tagging to characterize the proteomic response of vancomycin resistant Streptomyces coelicolor to the exposure to sublethal levels of the antibiotic. A common set of 804 proteins were identified in triplicate experiments. Contrasting changes in the abundance of proteins closely associated with the cytoplasmic membrane with those taking place in the cytosol identified aspects of protein spatial localization that are associated with the response to vancomycin. Enzymes for peptidoglycan precursor, mycothiol, ectoine and menaquinone biosynthesis together with a multisubunit nitrate reductase were recruited to the membrane following vancomycin treatment. Many proteins with regulatory functions (including sensor protein kinases) also exhibited significant changes in abundance exclusively in the membrane-associated protein fraction. Several enzymes predicted to be involved in extracellular peptidoglycan crossbridge formation became significantly depleted from the membrane. A comparison with data previously acquired on the changes in gene transcription following vancomycin treatment identified a common high-confidence set of changes in gene expression. Generalized changes in protein abundance indicate roles for proteolysis, the pentose phosphate pathway and a reorganization of amino acid biosynthesis in the stress response.

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.

RNA-Seq reveals differential gene expression in Staphylococcus aureus with single-nucleotide resolution

Staphylococcus aureus is a gram-positive cocci and an important human commensal bacteria and pathogen. S. aureus infections are increasingly difficult to treat because of the emergence of highly resistant MRSA (methicillin-resistant S. aureus) strains. Here we present a method to study differential gene expression in S. aureus using high-throughput RNA-sequencing (RNA-seq). We used RNA-seq to examine gene expression in S. aureus RN4220 cells containing an exogenously expressed transcription factor and between two S. aureus strains (RN4220 and NCTC8325-4). We investigated the sequence and gene expression differences between RN4220 and NCTC8325-4 and used the RNA-seq data to identify S. aureus promoters suitable for in vitro analysis. We used RNA-seq to describe, on a genome wide scale, genes positively and negatively regulated by the phage encoded transcription factor gp67. RNA-seq offers the ability to study differential gene expression with single-nucleotide resolution, and is a considerable improvement over the predominant genome-wide transcriptome technologies used in S. aureus.

An insight into pleiotropic regulators Agr and Sar: molecular probes paving the new way for antivirulent therapy

Staphylococcus aureus pathogenesis is an intricate process involving a diverse array of extracellular proteins, biofilm and cell wall components that are coordinately expressed in different stages of infection. The expression of two divergent loci, agr and sar, is increasingly recognized as a key regulator of virulence in S. aureus, and there is mounting evidence for the role of these loci in staphylococcal infections. The functional agr regulon is critical for the production of virulence factors, including α, β and δ hemolysins. The sar locus encodes SarA protein, which regulates the expression of cell wall-associated and certain extracellular proteins in agr-dependent and agr-independent pathways. Multidrug-resistant S. aureus is a leading cause of morbidity and mortality in the world and its management, especially in community-acquired methicillin-resistant S. aureus infections, has evolved comparatively little. In particular, no novel targets have been incorporated into its treatment to date. Hence, these loci appear to be the most significant and are currently at the attention of intense investigation regarding their therapeutic prospects.

The Staphylococcus aureus Membrane Protein SA2056 Interacts with Peptidoglycan Synthesis Enzymes

The yet uncharacterized membrane protein SA2056 belongs to the ubiquitous RND (Resistance-Nodulation-cell Division) family of transmembrane efflux transporters. The sa2056 gene is located downstream of femX, the gene encoding the essential, non-ribosomal peptidyl-transferase adding the first glycine in the staphylococcal cell wall pentaglycine interpeptide. Due to its proximity to and weak co-transcription with femX, we assumed that sa2056 may somehow be involved in peptidoglycan synthesis. Specific antibodies against SA2056 showed that this protein is expressed during growth and present in the membrane fraction of cell preparations. Using a bacterial two hybrid system, SA2056 was shown to interact (i) with itself, (ii) with FemB, which adds glycines 4 and 5 to the peptidoglycan interpeptide and (iii) with the essential penicillin binding proteins, PBP1 and PBP2, required for cell division and incorporation of the peptidoglycan into the cell wall. Unexpectedly, deletion of sa2056 led to no phenotype regarding growth, antibiotic resistances or cell morphology; nor did sa2056 deletion in combination with femB inactivation alter β-lactam and lysostaphin sensitivity and resistance, respectively, pointing to possible redundancy in the cell wall synthesis pathway. These results suggest an accessory role of SA2056 in S. aureus peptidoglycan synthesis, broadening the range of biological functions of RND proteins.

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.

The bacterial cell envelope

The bacteria cell envelope is a complex multilayered structure that serves to protect these organisms from their unpredictable and often hostile environment. The cell envelopes of most bacteria fall into one of two major groups. Gram-negative bacteria are surrounded by a thin peptidoglycan cell wall, which itself is surrounded by an outer membrane containing lipopolysaccharide. Gram-positive bacteria lack an outer membrane but are surrounded by layers of peptidoglycan many times thicker than is found in the gram-negatives. Threading through these layers of peptidoglycan are long anionic polymers, called teichoic acids. The composition and organization of these envelope layers and recent insights into the mechanisms of cell envelope assembly are discussed.

Detection of mutations in the FemXAB protein family in oxacillin-susceptible mecA-positive Staphylococcus aureus clinical isolates

Objectives Methicillin-resistant Staphylococcus aureus (MRSA) strains that express the mecA gene but are oxacillin susceptible (OS-MRSA; oxacillin MIC </=2 mg/L) are increasingly reported. To gain molecular and functional insights on this observation, we focused on additional factors possibly contributing to phenotypic susceptibility. Methods The nucleotide content of mecA, femA, femB and femX genes, which are considered essential for methicillin resistance, was determined in four OS-MRSA clinical isolates and a genetically similar low-level MRSA control (oxacillin MIC 6 mg/L). Gene expression was quantified compared with the low- and a high-level MRSA (MIC 256 mg/L) control. The tertiary structure of Fem proteins was predicted based on protein structure homology modelling, using web-based automated comparative protein modelling. Growth kinetics were tested for the study and control isolates, to determine whether FemXAB mutations lead to reduced fitness. Results Genes mecA, femA, femB and femX were expressed similarly in the study and the control isolates. Mutations in the gene mecA were not present in any isolate. However, several mutations leading to amino acid substitutions in positions possibly affecting Fem enzyme activity were detected in all fem genes. Two OS-MRSA that had no oxacillin heteroresistance had more mutations in the Fem proteins compared with the remaining isolates that were heteroresistant. The low-level MRSA control had considerably fewer mutations. No differences between growth rates of the OS-MRSA and the MRSA controls were observed. Conclusions Accumulation of amino acid changes in Fem proteins might affect intact cell wall synthesis, even though not causing reduced viability, thus contributing to atypical oxacillin responsiveness.

Effect of a glucose impulse on the CcpA regulon in Staphylococcus aureus

Background

The catabolite control protein A (CcpA) is a member of the LacI/GalR family of transcriptional regulators controlling carbon-metabolism pathways in low-GC Gram-positive bacteria. It functions as a catabolite repressor or activator, allowing the bacteria to utilize the preferred carbon source over secondary carbon sources. This study is the first CcpA-dependent transcriptome and proteome analysis in Staphylococcus aureus, focussing on short-time effects of glucose under stable pH conditions.

Results

The addition of glucose to exponentially growing S. aureus increased the expression of genes and enzymes of the glycolytic pathway, while genes and proteins of the tricarboxylic acid (TCA) cycle, required for the complete oxidation of glucose, were repressed via CcpA. Phosphotransacetylase and acetate kinase, converting acetyl-CoA to acetate with a concomitant substrate-level phosphorylation, were neither regulated by glucose nor by CcpA. CcpA directly repressed genes involved in utilization of amino acids as secondary carbon sources. Interestingly, the expression of a larger number of genes was found to be affected by ccpA inactivation in the absence of glucose than after glucose addition, suggesting that glucose-independent effects due to CcpA may have a particular impact in S. aureus. In the presence of glucose, CcpA was found to regulate the expression of genes involved in metabolism, but also that of genes coding for virulence determinants.

Conclusion

This study describes the CcpA regulon of exponentially growing S. aureus cells. As in other bacteria, CcpA of S. aureus seems to control a large regulon that comprises metabolic genes as well as virulence determinants that are affected in their expression by CcpA in a glucose-dependent as well as -independent manner.

Characterization of peptidoglycan in fem-deletion mutants of methicillin-resistant Staphylococcus aureus by solid-state NMR

Compositional analysis of the peptidoglycan (PG) of a wild-type methicillin-resistant Staphylococcus aureus and its fem-deletion mutants has been performed on whole cells and cell walls using stable-isotope labeling and rotational-echo double-resonance NMR. The labels included [1-(13)C,(15)N]glycine and l-[epsilon-(15)N]lysine (for a direct measure of the number of glycyl residues in the bridging segment), [1-(13)C]glycine and l-[epsilon-(15)N]lysine (concentration of bridge links), and d-[1-(13)C]alanine and [(15)N]glycine (concentrations of cross-links and wall teichoic acids). The bridging segment length changed from 5.0 glycyl residues (wild-type strain) to 2.5 +/- 0.1 (FemB) with modest changes in cross-link and bridge-link concentrations. This accurate in situ measurement for the FemB mutant indicates a heterogeneous PG structure with 25% monoglycyl and 75% triglycyl bridges. When the bridging segment was reduced to a single glycyl residue 1.0 +/- 0.1 (FemA), the level of cross-linking decreased by more than 20%, resulting in a high concentration of open N-terminal glycyl segments.

In vitro and in vivo evaluations of oxacillin efficiency against mecA-positive oxacillin-susceptible Staphylococcus aureus

Community-type Staphylococcus aureus strains that are positive for mecA and PBP2a but appear phenotypically susceptible to oxacillin are increasingly reported worldwide. Four S. aureus clinical isolates carrying the mecA gene with oxacillin MICs of <2 microg/ml were tested for oxacillin efficiency by population analyses and experimental thigh infections. These isolates harbored staphylococcal cassette chromosome mec type IV and belonged to two genotypes. Two of the four isolates were found by population analysis to be truly oxacillin susceptible. All four isolates exhibited significant reductions in the numbers of colonies grown after dicloxacillin treatment of experimental thigh infections, as also did a mecA-negative S. aureus control strain. These observations indicate that some of the phenotypically oxacillin susceptible mecA-positive Staphylococcus aureus isolates may be at least partially responsive to oxacillin.