A new mercury-penicillin V derivative as a probe for ultrastructural localization of penicillin-binding proteins in Escherichia coli.

The precise ultrastructural localization of penicillin-binding protein (PBP)-antibiotic complexes in Escherichia coli JM101, JM101 (pBS96), and JM101(pPH116) was investigated by high-resolution electron microscopy. We used mercury-penicillin V (Hg-pen V) as a heavy-metal-labeled, electron-dense probe for accurately localizing PBPs in situ in single bacterial cells grown to exponential growth phase. Biochemical data derived from susceptibility tests and bacteriolysis experiments revealed no significant differences between Hg-pen V and the parent compound, penicillin V, or between strains. Both antibiotics revealed differences in the binding affinities for PBPs of all strains. Deacylation rates for PBPs were slow despite the relatively low binding affinities of antibiotics. Cells bound most of the Hg-pen V added to cultures, and the antibiotic-PBP complex could readily be seen by electron microscopy of unstained whole mounts as distinct, randomly situated electron-dense particles. Fifty to 60% of the antibiotic was retained by cells during processing for conventional embedding so that thin sections could also be examined. These revealed similar electron-dense particles located predominantly on the plasma membrane and less frequently in the cytoplasm. Particles positioned on the plasma membranes were occasionally shown to protrude into the periplasmic space, thereby reflecting the high resolution of the Hg-pen V probe. Moreover, some particles were observed free in the periplasm, suggesting, for the first time, that a proportion of PBPs may not be restricted to the plasma membrane but may be tightly associated with the peptidoglycan for higher efficiency of peptidoglycan assembly. All controls were devoid of the electron-dense particles. The presence of electron-dense particles in cells of the wild-type JM101, demonstrated that our probe could identify PBPs in naturally occurring strains without inducing PBP overproduction.     

Fosfomycin enhances the expression of penicillin-binding protein 2 in methicillin-sensitive and methicillin-resistant Staphylococcus aureus strains

The effects of fosfomycin on penicillin-binding proteins (PBPs) were studied on the methicillin-resistant Staphylococcus aureus strain CIP (Collection de l'Institut Pasteur, Paris, France) 65-25 and on a methicillin-susceptible S. aureus strain CIP 65-6. The combinations of fosfomycin and oxacillin were synergistic, additive or antagonistic, depending on antibiotic concentrations. Fosfomycin induced modifications of the PBP profile of the two strains studied. In particular, it increased the expression of PBP2. This suggested that this protein is inducible; the only PBP not affected by fosfomycin was PBP3.     

Peptidoglycan composition of a highly methicillin-resistant Staphylococcus aureus strain. The role of penicillin binding protein 2A.

All clinical isolates of methicillin-resistant Staphylococcus aureus contain an extra penicillin binding protein (PBP) 2A in addition to four PBPs present in all staphylococcal strains. This extra PBP is thought to be a transpeptidase essential for the continued cell wall synthesis and growth in the presence of beta-lactam antibiotics. As an approach of testing this hypothesis we compared the muropeptide composition of cell walls of a highly methicillin-resistant S. aureus strain containing PBP2A and its isogenic Tn551 derivative with reduced methicillin resistance, which contained no PBP2A because of the insertional inactivation of the PBP2A gene. Purified cell walls were hydrolyzed into muropeptides which were subsequently resolved by reversed-phase high-performance liquid chromatography and identified by chemical and mass spectrometric analysis. The peptidoglycan composition of the two strains were identical. Both peptidoglycans were highly cross-linked mainly through pentaglycine cross-bridges, although other, chemically distinct peptide cross-bridges were also present including mono-, tri-, and tetraglycine; alanine; and alanyl-tetraglycine. Our experiments provided no experimental data for a unique transpeptidase activity associated with PBP2A.     

Direct Quantitation of the Numbers of Individual Penicillin-Binding Proteins per Cell in Staphylococcus aureus

The penicillin-binding proteins (PBPs) are a set of enzymes that participate in bacterial peptidoglycan assembly. The absolute numbers of each PBP were determined by direct measurement and have been reported for two Staphylococcus aureus strains, RN4220 (methicillin-sensitive S. aureus) and RN450M (methicillin-resistant S. aureus). From the specific activity of the labeled penicillin and the absolute number of disintegrations per minute, and from the number of CFU per milliliter calculated from proteins and optical density, a determination of the number of PBPs per cell was made. These numbers ranged from approximately 150 to 825 PBPs/cell and represent the first direct determination of absolute numbers of PBPs in S. aureus.     

Low-affinity penicillin-binding protein associated with beta-lactam resistance in Staphylococcus aureus

Methicillin resistance in Staphylococcus aureus has been associated with alterations in the penicillin-binding proteins (PBPs). An intriguing property of all methicillin-resistant staphylococci is the dependence of resistance on the pH value of the growth medium. Growth of such bacteria at pH 5.2 completely suppressed the expression of methicillin resistance. We have examined the PBP patterns of methicillin-resistant staphylococci grown at pH 7.0. We detected a high-molecular-weight PBP (PBP-2a; approximate size, 78,000 daltons) that was only present in the resistant bacteria but not in the isogenic sensitive strain. In cultures grown at pH 5.2, the extra PBP was not detectable.     

Genetic characterization of the natural SigB variants found in clinical isolates of Staphylococcus aureus

The SigB concentrations in clinical isolates of Staphylococcus aureus were measured to examine their correlation with the antibiotic resistance. The SigB concentrations in methicillin-resistant S. aureus (MRSA) were higher than in the control strain, N315, and many of methicillin-susceptible S. aureus (MSSA). Sequencing analyses of the sigB genes revealed that the strains exhibiting the high SigB concentrations have three amino acid substitutions in SigB: I11V, D141N, and Q256K. Further analysis using isogenic mutants demonstrated that D141N (or both D141N and Q256K) is essential to maintain the high SigB concentration. These substitutions affected the UV tolerance, but had no effect on the antibiotic resistance. The SigB activity was affected by these substitutions toward the stationary phase, but not during the transient heat shock response.     

Studies on the mechanism of intrinsic resistance to beta-lactam antibiotics in group D streptococci

Six penicillin-binding proteins (PBPs) were detected in clinical isolates of each one of three group D streptococci: Streptococcus bovis, S. faecalis and S. faecium. When examined in whole organisms, the PBPs of S. faecium, the most penicillin-resistant species of group D streptococci, generally had lower affinities for the antibiotic than those of S. faecalis (intermediate penicillin resistance), which in turn were of lower affinity than those of S. bovis (penicillin-sensitive). On the other hand, no quantitative correlation could be established between the binding of penicillin to any one PBP or group of PBPs, and the penicillin MIC value for the corresponding micro-organism. Examination of the amounts of antibiotic bound and the rates of binding to PBPs of equal numbers of protoplasts and whole bacteria of S. faecalis and S. faecium, indicated that there was no permeability barrier to benzylpenicillin in the cell walls of these species. The lower antibacterial effectiveness of cephalothin compared with ampicillin in group D streptococci was paralleled by the higher concentrations of cephalothin needed in competition assays to inhibit the lower molecular size PBPs of these bacteria.     

Benzylpenicillin-induced filament formation of Clostridium perfringens

Growth of Clostridium perfringens with low concentrations of benzylpenicillin inhibited septum formation and division of the organisms. This resulted in continued growth of the organisms as aseptate filaments. The effect was reversed on removal of the antibiotic. The composition of walls isolated from organisms grown with the antibiotic was similar to that of walls from untreated bacteria. In addition, both contained non-N-acetylated glucosamine residues in their peptidoglycan. No differences were detected in the degree of cross-linkage of peptidoglycan. Clostridium perfringens contains six membrane-associated penicillin-binding proteins (PBPs) which have different affinities for [3H]benzylpenicillin. Concentrations of the antibiotic which were sufficient to cause filamentation of apparently all organisms in a culture caused almost complete saturation of PBPs 3, 4, 5 and 6. At these concentrations there was no measurable interaction with PBPs 1 and 2. Thus interaction of the antibiotic with the lower molecular weight PBPs is correlated with the inhibition of septum formation in C. perfringens.     

High level oxacillin and vancomycin resistance and altered cell wall composition in Staphylococcus aureus carrying the staphylococcal mecA and the enterococcal vanA gene complex

Recently, for the first time in the history of this bacterial species, methicillin-resistant Staphylococcus aureus (MRSA) carrying the enterococcal vanA gene complex and expressing high level resistance to vancomycin was identified in clinical specimens (CDC (2002) MMWR 51, 565-567). The purpose of our studies was to understand how vanA is expressed in the heterologous background of S. aureus and how it interacts with the mecA-based resistance mechanism, which is also present in these strains and is targeted on cell wall biosynthesis. The vanA-containing staphylococcal plasmid was transferred from the clinical vancomycin-resistant S. aureus (VRSA) strain HIP11714 (CDC (2002) MMWR 51, 565-567) to the methicillin-resistant S. aureus (MRSA) strain COL for which extensive genetic and biochemical information is available on staphylococcal cell wall biochemistry and drug resistance mechanisms. The transconjugant named COLVA showed high and homogeneous resistance to both oxacillin and vancomycin. COLVA grown in vancomycin-containing medium produced an abnormal peptidoglycan: all pentapeptides were replaced by tetrapeptides, and the peptidoglycan contained at least 22 novel muropeptide species that frequently showed a deficit or complete absence of pentaglycine branches. The UDP-MurNAc-pentapeptide, the major component of the cell wall precursor pool in vancomycin-sensitive cells was replaced by UDP-MurNAc-depsipeptide and UDP-MurNAc-tetrapeptide. Transposon inactivation of the beta-lactam resistance gene mecA caused complete loss of beta-lactam resistance but had no effect on the expression of vancomycin resistance. The two major antibiotic resistance mechanisms encoded by mecA and vanA residing in the same S. aureus appear to use different sets of enzymes for the assembly of cell walls.     

Identification of a fmtA-like gene that has similarity to other PBPs and beta-lactamases in Staphylococcus aureus

We identified a gene from Staphylococcus aureus, flp (fmtA-like protein), encoding a protein of 489 amino acid residues with a molecular mass of 56.4 kDa. The deduced amino acid sequence shows similarity to previously characterized penicillin binding proteins (PBPs) and FmtA of S. aureus (one of the factors which affect methicillin resistance). FLP protein has three motifs, which are conserved in PBPs and beta-lactamases, suggesting that it might be associated with cell wall synthesis. Recombinant FLP protein, however, lacks penicillin binding activity, and the inactivation of flp in two methicillin-resistant S. aureus strains did not cause a reduction in the methicillin resistance.     

Mercury and tetracycline resistance genes and flanking repeats associated with methicillin resistance on the chromosome of Staphylococcus aureus

Sections of a cloned 27 kb segment of chromosomal DNA, associated with resistance to four antimicrobial agents in a clinical isolate of methicillin-resistant Staphylococcus aureus (MRSA), were tested for their ability to determine resistance when transformed into a sensitive laboratory strain of S. aureus. This was achieved by inserting the sections into a newly constructed shuttle vector, amplifying the recombinant DNA in E. coli, and transforming protoplasts of the sensitive S. aureus strain. Two sections of the cloned DNA were found to determine resistance separately to mercuric ion and to tetracycline, in both S. aureus and Escherichia coli.     

Simultaneous release of penicilloic acid and phenylacetyl glycine by penicillin-binding proteins 5 and 6 of Escherichia coli.

Penicillin-binding proteins (PBPs) 5 and 6 of Escherichia coli released the bound penicilloyl moiety at an intermediate rate relative to, e.g., Staphylococcus aureus PBPs 4 (rapid) and 1 or 2 (slow). Each of these E. coli PBPs released the bound penicilloyl moiety as both penicilloic acid (hydrolysis) and phenylacetyl glycine (scission of the C-5--C-6 bond followed by hydrolysis).     

Gradual alterations in cell wall structure and metabolism in vancomycin-resistant mutants of Staphylococcus aureus

In five vancomycin-resistant laboratory step mutants selected from the highly and homogeneously methicillin-resistant Staphylococcus aureus strain COL (MIC of methicillin, 800 microg/ml; MIC of vancomycin, 1.5 microg/ml), the gradually increasing levels of resistance to vancomycin were accompanied by parallel decreases in the levels of methicillin resistance and abnormalities in cell wall metabolism. The latter included a gradual reduction in the proportion of highly cross-linked muropeptide species in peptidoglycan, down-regulation of the production of penicillin-binding protein 2A (PBP2A) and PBP4, and hypersensitivity to beta-lactam antibiotics each with a relatively selective affinity for the various staphylococcal PBPs; the PBP2-specific inhibitor ceftizoxime was particularly effective.     

Structural basis for the beta lactam resistance of PBP2a from methicillin-resistant Staphylococcus aureus

The multiple antibiotic resistance of methicillin-resistant strains of Staphylococcus aureus (MRSA) has become a major clinical problem worldwide. The key determinant of the broad-spectrum beta-lactam resistance in MRSA strains is the penicillin-binding protein 2a (PBP2a). Because of its low affinity for beta-lactams, PBP2a provides transpeptidase activity to allow cell wall synthesis at beta-lactam concentrations that inhibit the beta-lactam-sensitive PBPs normally produced by S. aureus. The crystal structure of a soluble derivative of PBP2a has been determined to 1.8 A resolution and provides the highest resolution structure for a high molecular mass PBP. Additionally, structures of the acyl-PBP complexes of PBP2a with nitrocefin, penicillin G and methicillin allow, for the first time, a comparison of an apo and acylated resistant PBP. An analysis of the PBP2a active site in these forms reveals the structural basis of its resistance and identifies features in newly developed beta-lactams that are likely important for high affinity binding.     

Triton X-100 alters the resistance level of methicillin-resistant Staphylococcus aureus to oxacillin

Abstract We used population analysis to examine the effects of Triton X-100 on the level of resistance to oxacillin of 18 methicillin-resistant Staphylococcus aureus . In the presence of 0.02% Triton X-100, 17 formerly methicillin-resistiant strains exhibited enhanced sensitivity to oxacillin. One homogeneous isolate, KSAF1 was barely affected by the Triton X-100. Sensitivities of lysostaphin, 51 kDa N -acetylglucosaminidase and 62 kDa N -acetylmuramoyl-l-alanine amidase to heat-inactivated cells were not affected when the bacteria were grown in 0.02% Triton X-100. Our data, together with those of a previous study, suggested that Triton X-100 alters the resistance level of methicillin-resistant S. aureus by influencing a factor(s) other than PBPs, bacteriolytic enzymes, or femAB products.     

High-level (beta)-lactam resistance and cell wall synthesis catalyzed by the mecA homologue of Staphylococcus sciuri introduced into Staphylococcus aureus

A close homologue of mecA, the determinant of broad-spectrum beta-lactam resistance in Staphylococcus aureus was recently identified as a native gene in the animal commensal species Staphylococcus sciuri. Introduction of the mecA homologue from a methicillin-resistant strain of S. sciuri into a susceptible strain of S. aureus caused an increase in drug resistance and allowed continued growth and cell wall synthesis of the bacteria in the presence of high concentrations of antibiotic. We determined the muropeptide composition of the S. sciuri cell wall by using a combination of high-performance liquid chromatography, mass spectrometric analysis, and Edman degradation. Several major differences between the cell walls of S. aureus and S. sciuri were noted. The pentapeptide branches in S. sciuri were composed of one alanine and four glycine residues in contrast to the pentaglycine units in S. aureus. The S. sciuri wall but not the wall of S. aureus contained tri- and tetrapeptide units, suggesting the presence of dd- and ld-carboxypeptidase activity. Most interestingly, S. aureus carrying the S. sciuri mecA and growing in methicillin-containing medium produced a cell wall typical of S. aureus and not S. sciuri, in spite of the fact that wall synthesis under these conditions had an absolute dependence on the heterologous S. sciuri gene product. The protein product of the S. sciuri mecA can efficiently participate in cell wall biosynthesis and build a cell wall using the cell wall precursors characteristic of the S. aureus host.     

Cross-linking and O-acetylation of peptidoglycan in Staphylococcus aureus (strains H and MR-1) grown in the presence of sub-growth-inhibitory concentrations of beta-lactam antibiotics

Staphylococcus aureus H was grown for 4 generation times with various sub-growth-inhibitory concentrations of beta-lactam antibiotics specific for particular penicillin-binding proteins (PBPs) - PBP2, clavulanic acid; PBP3, methicillin; PBP4, cefoxitin - and also with the non-specific benzylpenicillin. Isolated cell walls were digested with Chalaropsis muramidase and the resulting peptidoglycan fragments were fractionated by HPLC into disaccharide-peptide monomers and cross-linked dimers, trimers, tetramers and greater oligomers. The pattern of relative fragment concentrations with increasing amounts of drug was roughly the same regardless of the antibiotic used, monomers and dimers increasing while trimers and tetramers changed little and oligomers decreased rapidly. The patterns resembled closely those predicted by the 'random addition' model for multiple cross-link formation and not at all those predicted by the 'monomer addition' model. The O-acetylation of the peptidoglycan remained essentially unaffected under all these conditions. S. aureus MR-1, a constitutive producer of PBP2', gave similar results when treated with methicillin.     

Transcriptional analysis of antibiotic resistance and virulence genes in multiresistant hospital-acquired MRSA

The staphylococcal chromosome cassette mec cannot solely explain the multiresistance phenotype or the relatively mild virulence profile of hospital-acquired methicillin-resistant Staphylococcus aureus (HA-MRSA). This study reports that several multiresistant HA-MRSA strains differently expressed genes that may support antibiotic resistance, modify the bacterial surface and influence the pathogenic process. Genes encoding efflux pumps (norA, arsB, emrB) and the macrolide resistance gene ermA were found to be commonly expressed by HA-MRSA strains, but not in the archetypal MRSA strain COL. At equivalent cell density, the agr system was considerably less activated in all MRSA strains (including COL) in comparison with a prototypic antibiotic-susceptible strain. These results are in contrast to those observed in recent community-acquired MRSA isolates and may partly explain how multiresistant HA-MRSA persist in the hospital setting.     

Complete genome sequence of Staphylococcus aureus M013, a pvl-positive, ST59-SCCmec type V strain isolated in Taiwan

We report the complete genome sequence of M013, a representative strain of a pvl-positive, sequence type 59-staphylococcal cassette chromosome mec type V (ST59-SCCmec type V) community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) clone in Taiwan. Comparison of M013 with the genomes of two CA-MRSA strains in the United States revealed major differences in the regions covering several genomic islands and prophages.     

Characterisation of Staphylococcus aureus nasal and skin carriage among patients undergoing haemodialysis treatment

The aim of the study was to investigate the rate of Staphylococcus aureus nasal and skin carriage in patients undergoing haemodialysis. The cultured staphylococcal isolates were subsequently characterized by molecular methods. The study group comprised 43 haemodialysed patients from whom nasal and skin swabs from the vascular access sites were collected. The identification of staphylococcal isolates and antibiotic susceptibility testing were performed on the basis of conventional diagnostic procedures. The staphylococci were further characterized using Pulsed-Field Gel Electrophoresis (PFGE). S. aureus was cultured from 12 (27.9%) patients. Only one (8.3%) patient was colonized with the microorganism both in the anterior nares and the vascular access site representing a single strain, as evidenced by PFGE analysis. Antibiotic susceptibility testing identified one (7.6%) methicillin-resistant S. aureus (MRSA) strain. PFGE typing identified several S. aureus genotypes with the lack of one specific strain responsible for colonization. However, it should be noted that among two (A and D) PFGE patterns genetically indistinguishable and closely related isolates (two isolates for each pattern) were identified. The obtained results revealed a relatively low rate of S. aureus carriage accompanied by low methicillin resistance rate and a significant genetic diversity of cultured isolates with the lack of one predominant strain responsible for colonization.