Improved high-performance liquid chromatographic separation of peptidoglycan isolated from various Staphylococcus aureus strains for mass spectrometric characterization

FmhA and FmhC of Staphylococcus aureus incorporate serine residues into peptidoglycan cross-bridges

Staphylococcal peptidoglycan is characterized by pentaglycine cross-bridges that are cross-linked between adjacent wall peptides by penicillin-binding proteins to confer robustness and flexibility. In Staphylococcus aureus, pentaglycine cross-bridges are synthesized by three proteins: FemX adds the first glycine, and the homodimers FemA and FemB sequentially add two Gly-Gly dipeptides. Occasionally, serine residues are also incorporated into the cross-bridges by enzymes that have heretofore not been identified. Here, we show that the FemA/FemB homologues FmhA and FmhC pair with FemA and FemB to incorporate Gly-Ser dipeptides into cross-bridges and to confer resistance to lysostaphin, a secreted bacteriocin that cleaves the pentaglycine cross-bridge. FmhA incorporates serine residues at positions 3 and 5 of the cross-bridge. In contrast, FmhC incorporates a single serine at position 5. Serine incorporation also lowers resistance toward oxacillin, an antibiotic that targets penicillin-binding proteins, in both methicillin-sensitive and methicillin-resistant strains of S. aureus FmhC is encoded by a gene immediately adjacent to lytN, which specifies a hydrolase that cleaves the bond between the fifth glycine of cross-bridges and the alanine of the adjacent stem peptide. In this manner, LytN facilitates the separation of daughter cells. Cell wall damage induced upon lytN overexpression can be alleviated by overexpression of fmhC. Together, these observations suggest that FmhA and FmhC generate peptidoglycan cross-bridges with unique serine patterns that provide protection from endogenous murein hydrolases governing cell division and from bacteriocins produced by microbial competitors.

Effect of gamma radiation and oregano essential oil on murein and ATP concentration of Staphylococcus aureus

The study was carried out to evaluate the effects of gamma-irradiation alone or in combination with oregano essential oil on murein composition of Staphylococcus aureus and on the intracellular and extracellular concentration of ATP. The bacterial strain was treated with 3 irradiation doses: 1.2 kGy to induce cell damage, 2.9 kGy to obtain a viable but nonculturable state, and 3.5 kGy to cause cell death. Oregano essential oil was used at 0.010% and 0.013% (w/v), which is the minimum inhibitory concentration (MIC). All treatments had a significant effect (P < or = 0.05) on the murein composition, although some muropeptides did not seem to be affected by the treatment. Each treatment influenced differently the relative percentage and number of muropeptides. There was a significant (P < or = 0.05) correlation between the reduction of intracellular ATP and increase in extracellular ATP following treatment of the cells with oregano oil. The reduction of intracellular ATP was even more important when essential oil was combined with irradiation. Also, irradiation alone of S. aureus induced a significant decrease (P < or = 0.05) of the internal ATP and a significant increase (P < or = 0.05) of the external ATP. However, no significant difference (P > 0.05) was observed in ATP concentrations between different radiation doses. Transmission electron microscopic observation revealed that oregano oil and irradiation have an effect on cell wall structure.

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.

The beta-lactam-resistance modifier (-)-epicatechin gallate alters the architecture of the cell wall of Staphylococcus aureus

(-)-Epicatechin gallate (ECg), a component of green tea, sensitizes meticillin-resistant Staphylococcus aureus (MRSA) to beta-lactam antibiotics, promotes staphylococcal cell aggregation and increases cell-wall thickness. The potentiation of beta-lactam activity against MRSA by ECg was not due to decreased bacterial penicillin-binding protein (PBP) 2a expression or ECg binding to peptidoglycan. A 5-10 % reduction in peptidoglycan cross-linking was observed. Reduced cross-linking was insufficient to compromise the integrity of the cell wall and no evidence of PBP2a activity was detected in the muropeptide composition of ECg-grown cells. ECg increased the quantity of autolysins associated with the cell wall, even though the cells were less susceptible to Triton X-100-induced autolysis than cells grown in the absence of ECg. ECg promoted increased lysostaphin resistance that was not due to alteration of the pentaglycine cross-bridge configuration or inhibition of lysostaphin activity. Rather, decreased lysostaphin susceptibility was associated with structural changes to wall teichoic acid (WTA), an acid-labile component of peptidoglycan. ECg also promoted lipoteichoic acid (LTA) release from the cytoplasmic membrane. It is proposed that ECg reduces beta-lactam resistance in MRSA either by binding to PBPs at sites distinct from the penicillin-binding site or by intercalation into the cytoplasmic membrane, displacing LTA from the phospholipid palisade. Thus, ECg-mediated alterations to the physical nature of the bilayer will elicit structural changes to WTA that result in modulation of the cell-surface properties necessary to maintain the beta-lactam-resistant phenotype.

Identification and molecular characterization of an N-Acetylmuraminidase, Aml, involved in Streptococcus mutans cell separation

We previously demonstrated Streptococcus mutans produces two bacteriolytic enzymes of 100 kDa and 80 kDa (G. Yoshimura et al. Microbiol. Immunol. 48, 465-469, 2004). Here, we identified the protein sequence of these enzymes and found they come from a single gene product designated as automutanolysin (Aml). Aml has a modular design where the N-terminus contains five 13-amino-acid repeats and a C-terminal enzyme active domain. Aml selectively lyses S. mutans and S. sobrinus but no other oral streptococci. This suggests Aml possesses strong substrate specificity towards cariogenic bacteria present in the human oral cavity. Analysis of S. mutans peptidoglycan fragments released by Aml shows the enzyme is an N-acetylmuraminidase. We found Ca(2+) enhances the activity; and EGTA, EDTA and iodoacetic acid inhibit the activity. The optimum pH range for lytic activity was 6 to 7. Disruption of the aml gene in S. mutans results in the formation of a longer bacterial cell chain length that was dispersed by the addition of a low concentration of Aml. This suggests Aml is involved in S. mutans cell separation.

Identification and molecular characterization of anN-acetylmuramyl-l-alanine amidase Sle1 involved in cell separation ofStaphylococcus aureus

We purified a peptidoglycan hydrolase involved in cell separation from a Staphylococcus aureus atl null mutant and identified its gene. Characterization of the gene product shows a 32 kDa N-acetylmuramyl-L-alanine amidase that we designated Sle1. Analysis of peptidoglycan digests showed Sle1 preferentially cleaved N-acetylmuramyl-L-Ala bonds in dimeric cross-bridges that interlink the two murein strands in the peptidoglycan. An insertion mutation of sle1 impaired cell separation and induced S. aureus to form clusters suggesting Sle1 is involved in cell separation of S. aureus. The Sle1 mutant revealed a significant decrease in pathogenesis using an acute infection mouse model. Atl is the major autolysin of S. aureus, which has been implicated in cell separation of S. aureus. Generation of an atl/sle1 double mutant revealed that the mutant cell separation was heavily impaired suggesting that S. aureus uses two peptidoglycan hydrolases, Atl and Sle1, for cell separation. Unlike Atl, Sle1 is not directly involved in autolysis of S. aureus.

In vitro assembly of a complete, pentaglycine interpeptide bridge containing cell wall precursor (lipid II-Gly5) of Staphylococcus aureus

Staphylococcus aureus peptidoglycan is cross-linked via a characteristic pentaglycine interpeptide bridge. Genetic analysis had identified three peptidyltransferases, FemA, FemB and FemX, to catalyse the formation of the interpeptide bridge, using glycyl t-RNA as Gly donor. To analyse the pentaglycine bridge formation in vitro, we purified the potential substrates for FemA, FemB and FemX, UDP-MurNAc-pentapeptide, lipid I and lipid II and the staphylococcal t-RNA pool, as well as His-tagged Gly-tRNA-synthetase and His-tagged FemA, FemB and FemX. We found that FemX used lipid II exclusively as acceptor for the first Gly residue. Addition of Gly 2,3 and of Gly 4,5 was catalysed by FemA and FemB, respectively, and both enzymes were specific for lipid II-Gly1 and lipid II-Gly3 as acceptors. None of the FemABX enzymes required the presence of one or two of the other Fem proteins for activity; rather, bridge formation was delayed in the in vitro system when all three enzymes were present. The in vitro assembly system described here will enable detailed analysis of late, membrane-associated steps of S. aureus peptidoglycan biosynthesis.

Reduced content of lysyl-phosphatidylglycerol in the cytoplasmic membrane affects susceptibility to moenomycin, as well as vancomycin, gentamicin, and antimicrobial peptides, in Staphylococcus aureus

An association between moenomycin resistance and vancomycin intermediate resistance in Staphylococcus aureus was demonstrated previously. Thus, to elucidate the mechanism of vancomycin intermediate resistance, we searched for factors contributing to moenomycin resistance. Random Tn551 insertional mutagenesis of methicillin-resistant S. aureus strain COL yielded three mutants with decreased susceptibilities to moenomycin. Correspondingly, these mutants also exhibited slightly decreased susceptibilities to vancomycin. Genetic analysis revealed that two of the mutants had Tn551 insertions in the fmtC (mprF) gene, which is associated with the synthesis of lysyl-phosphatidylglycerol. The third Tn551 insertion was located in the lysC gene, which is involved in the biosynthesis of lysine from aspartic acid. Consequently, mutations in both of these loci reduced the lysyl-phosphatidylglycerol content in the cell membrane, giving it a more negative net charge. The positively charged antibiotic gentamicin and cationic antimicrobial peptides such as beta-defensins and CAP18 were more effective against the mutants. The levels of moenomycin and vancomycin binding to intact cells was also greater in the mutants than in the wild type, while the binding affinity was not altered when cells boiled in sodium dodecyl sulfate were used, indicating that both agents had higher affinities for the negatively charged membranes of the mutants. Therefore, the membrane charge of S. aureus appears to influence the efficacies of moenomycin, vancomycin, and other cationic antimicrobial agents.

Cell wall composition and decreased autolytic activity and lysostaphin susceptibility of glycopeptide-intermediate Staphylococcus aureus

The cell wall composition and autolytic properties of passage-selected glycopeptide-intermediate Staphylococcus aureus (GISA) isolates and their parent strains were studied in order to investigate the mechanism of decreased vancomycin susceptibility. GISA had relatively modest changes in peptidoglycan composition involving peptidoglycan interpeptide bridges and somewhat decreased cross-linking compared to that of parent strains. The cell wall phosphorus content of GISA strains was lower than that of susceptible parent strains, indicating somewhat lower wall teichoic acid levels in the GISA strains. Similar to whole cells, isolated crude cell walls retaining autolytic activity of GISA had drastically reduced autolytic activity compared to that of parent strains, and this arose early in the development of the GISA phenotype. This was due to an alteration in the autolytic enzymes of GISA as revealed by normal susceptibility of GISA-purified cell walls to parental strain autolysin extract and lower activity and altered peptidoglycan hydrolase activity profiles in GISA autolysin extracts compared to those of parent strains. Northern blot analysis indicated that expression of atl, the major autolysin gene, was significantly downregulated in a GISA strain compared to that of its parent strain. In contrast to whole cells, which showed decreased lysostaphin susceptibility, purified cell walls of GISA showed increased susceptibility to lysostaphin. We suggest that in our GISA strains, decreased autolytic activity is involved in the tolerance of vancomycin and the activities of endogenous autolysins are important in conferring sensitivity to lysostaphin on whole cells.

Zymographic characterization of bacteriolytic enzymes produced by oral streptococci

Zymographic analysis was performed to know the bacteriolytic enzyme profiles of 4% SDS extracts of oral streptococci, Streptococcus mutans, S. sobrinus, S. sanguis, S. mitis and S. salivarius. We investigated the five strains in each species and found that the profile was very similar among strains of the same species except for S. salivarius(the profile was classified into two types). On the other hand, the profile was considerably different among species. Two major bacteriolytic enzymes of S. mutans showing molecular mass of 80 and 100 kDa were found using SDS-boiled S. mutans or S. sobrinus cells as substrate. These bacteriolytic activities were less apparent in the gel containing S. mitis or S. salivarius, and also not detectable in the gel containing S. sanguis. S. sobrinus extract showed only one bacteriolytic band (78 kDa) as strong activity using S. sobrinus cells as substrate. S. sanguis extract showed no bacteriolytic bands using any streptococcal cells. Extracts of either S. mitis or S. salivarius showed weak activity by using respective strains as substrate.

Increased glycan chain length distribution and decreased susceptibility to moenomycin in a vancomycin-resistant Staphylococcus aureus mutant

A vancomycin-resistant Staphylococcus aureus mutant, COL-VR1 (MIC, 16 microg/ml), was isolated from methicillin-resistant S. aureus COL by exposure to vancomycin. COL-VR1 also showed decreased susceptibility to teicoplanin (8-fold), methicillin (2-fold), macarbomycin (8-fold), and moenomycin (16-fold). Macarbomycin and moenomycin are thought to directly inhibit transglycosylase activity. Characterization of the mutant revealed a thickened cell wall and suppression of penicillin-induced lysis, although the amounts of the five penicillin-binding proteins (PBPs 1, 2, 3, 4, and 2') and the profiles of peptidoglycan hydrolases were not altered. Analysis of muropeptide profile and glycan chain length distribution by reversed-phase high-pressure liquid chromatography revealed slightly decreased peptide cross-linking and an increased average glycan chain length compared to those of the parent. These results together suggest that a transglycosylase activity was enhanced in the mutant. This may represent a novel mechanism of glycopeptide resistance in S. aureus.

Negative and positive ion matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and positive ion nano-electrospray ionization quadrupole ion trap mass spectrometry of peptidoglycan fragments isolated from various Bacillus species

A general approach for the detailed characterization of sodium borohydride-reduced peptidoglycan fragments (syn. muropeptides), produced by muramidase digestion of the purified sacculus isolated from Bacillus subtilis (vegetative cell form of the wild type and a dacA mutant) and Bacillus megaterium (endospore form), is outlined based on UV matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) and nano-electrospray ionization (nESI) quadrupole ion trap (QIT) mass spectrometry (MS). After enzymatic digestion and reduction of the resulting muropeptides, the complex glycopeptide mixture was separated and fractionated by reversed-phase high-performance liquid chromatography. Prior to mass spectrometric analysis, the muropeptide samples were subjected to a desalting step and an aliquot was taken for amino acid analysis. Initial molecular mass determination of these peptidoglycan fragments (ranging from monomeric to tetrameric muropeptides) was performed by positive and negative ion MALDI-MS using the thin-layer technique with the matrix alpha-cyano-4-hydroxycinnamic acid. The results demonstrated that for the fast molecular mass determination of large sample numbers in the 0.8-10 pmol range and with a mass accuracy of +/-0.07%, negative ion MALDI-MS in the linear TOF mode is the method of choice. After this kind of muropeptide screening often a detailed primary structural analysis is required owing to ambiguous data. Structural data could be obtained from peptidoglycan monomers by post-source decay (PSD) fragment ion analysis, but not from dimers or higher oligomers and not with the necessary sensitivity. Multistage collision-induced dissociation (CID) experiments performed on an nESI-QIT instrument were found to be the superior method for structural characterization of not only monomeric but also of dimeric and trimeric muropeptides. Up to MS4 experiments were sometimes necessary to obtain unambiguous structural information. Three examples are presented: (a) CID MSn (n = 2-4) of a peptidoglycan monomer (disaccharide-tripeptide) isolated from B. subtilis (wild type, vegetative cell form), (b) CID MSn (n = 2-4) of a peptidoglycan dimer (bis-disaccharide-tetrapentapeptide) obtained from a B. subtilis mutant (vegetative cell form) and (c) CID MS2 of a peptidoglycan trimer (a linear hexasaccharide with two peptide side chains) isolated from the spore cortex of B. megaterium. All MS(n) experiments were performed on singly charged precursor ions and the MS2 spectra were dominated by fragments derived from interglycosidic bond cleavages. MS3 and MS4 spectra exhibited mainly peptide moiety fragment ions. In case of the bis-disaccharide-tetrapentapeptide, the peptide branching point could be determined based on MS3 and MS4 spectra. The results demonstrate the utility of nESI-QIT-MS towards the facile determination of the glycan sequence, the peptide linkage and the peptide sequence and branching of purified muropeptides (monomeric up to trimeric forms). The wealth of structural information generated by nESI-QIT-MSn is unsurpassed by any other individual technique.

A spectrum of changes occurs in peptidoglycan composition of glycopeptide-intermediate clinical Staphylococcus aureus isolates

The mechanism of glycopeptide resistance in Staphylococcus aureus is not known with certainty. Because the target of vancomycin is the D-Ala-D-Ala terminus of the stem peptide of the peptidoglycan precursor, by subjecting muropeptides to reversed-phase high-performance liquid chromatography, we investigated peptidoglycan obtained from glycopeptide-intermediate S. aureus (GISA) isolates for changes in composition and evaluated whether any peptidoglycan structural change was a consistent feature of clinical GISA isolates. GISA isolates Mu50 and Mu3 from Japan had the large glutamate-containing monomeric peak demonstrated previously, although strain H1, a vancomycin-susceptible MRSA isolate from Japan that was clonally related to Mu3 and Mu50, and a femC mutant that we studied, did also. For the U.S. GISA isolates, strain NJ had a large monomeric peak with a retention time identical to that described for the glutamate-containing monomer in strains H1, Mu3, and Mu50. However, a much smaller corresponding peak was seen in GISA MI, and this peak was absent from both GISA PC and a recent GISA isolate obtained from an adult patient in Illinois (strain IL). These data suggest that a uniform alteration in peptidoglycan composition cannot be discerned among the GISA isolates and indicate that a single genetic or biochemical change is unlikely to account for the glycopeptide resistance phenotype in the clinical GISA isolates observed to date. Furthermore, a large monomeric glutamate-containing peak is not sufficient to confer the resistance phenotype.

Involvement of multiple genetic loci in Staphylococcus aureus teicoplanin resistance

Teicoplanin resistance was transformed from a teicoplanin-resistant Staphylococcus aureus into the susceptible strain BB255 to give strain BB938. The cell wall composition, amidation of the iD-glutamate, and peptide crosslinking were identical in BB938 as in BB255 except for a 60% increased length of the glycan chain. Transductional crosses revealed that at least two distinct loci contributed in a cumulative fashion to teicoplanin resistance. One of these loci correlated with a mutation inactivating the anti-sigma factor RsbW. This mutation must have occurred during transformation and selection for teicoplanin resistance in BB938. Genetic manipulations involving the sigB operon showed that transcription factor SigB contributed to decreased teicoplanin susceptibility.

Characterization of fmtA, a gene that modulates the expression of methicillin resistance in Staphylococcus aureus

FmtA is a factor which affects the methicillin resistance level in methicillin-resistant Staphylococcus aureus. Since FmtA has two of three conserved motifs which are typically found in penicillin-binding proteins (PBPs) and beta-lactamases, we investigated the penicillin-binding activity of recombinant FmtA and found no such activity. Immunoblotting analysis revealed that FmtA localizes in the membrane fraction. To investigate the function of FmtA, high-pressure liquid chromatography analysis of cell wall muropeptides was performed with an fmtA-inactivated mutant and its parent. The mutant showed a reduced cross-linking and partially reduced amidation of glutamate residues in the peptidoglycan of the mutant. The transcription of fmtA was dose dependently increased by the addition of beta-lactam antibiotics, fosfomycin, and bacitracin, while its transcription was not changed by the addition of vancomycin or tetracycline. These results reveal that Fmt is a membrane-located, non-penicillin-binding protein and that mutation of fmtA affects the cell wall structure, although its precise function is still unknown.

The essential Staphylococcus aureus gene fmhB is involved in the first step of peptidoglycan pentaglycine interpeptide formation

The factor catalyzing the first step in the synthesis of the characteristic pentaglycine interpeptide in Staphylococcus aureus peptidoglycan was found to be encoded by the essential gene fmhB. We have analyzed murein composition and structure synthesized when fmhB expression is reduced. The endogenous fmhB promoter was substituted with the xylose regulon from Staphylococcus xylosus, which allowed glucose-controlled repression of fmhB transcription. Repression of fmhB reduced growth and triggered a drastic accumulation of uncrosslinked, unmodified muropeptide monomer precursors at the expense of the oligomeric fraction, leading to a substantial decrease in overall peptidoglycan crosslinking. The composition of the predominant muropeptide was confirmed by MS to be N-acetylglucosamine-(beta-1,4)-N-acetylmuramic acid(-L-Ala-D-iGln-L-Lys-D-Ala-D-Ala), proving that FmhB is involved in the attachment of the first glycine to the pentaglycine interpeptide. This interpeptide plays an important role in crosslinking and stability of the S. aureus cell wall, acts as an anchor for cell wall-associated proteins, determinants of pathogenicity, and is essential for the expression of methicillin resistance. Any shortening of the pentaglycine side chain reduces or even abolishes methicillin resistance, as occurred with fmhB repression. Because of its key role FmhB is a potential target for novel antibacterial agents that could control the threat of emerging multiresistant S. aureus.

Identification of three additional femAB-like open reading frames in Staphylococcus aureus

Three new proteins, FmhA, FmhB and FmhC, with significant identities to FemA and FemB were identified in the Staphylococcus aureus (ATCC 55748) genome database. They were mapped to the SmaI-C, SmaI-H and SmaI-A fragments of the S. aureus 8325 chromosome, respectively. Whereas insertional inactivation of fmhA and fmhC had no effects on growth, antibiotic susceptibility, lysostaphin resistance, or peptidoglycan composition of the strains, fmhB could not be inactivated, strongly suggesting that fmhB may be an essential gene. As deduced from the functions of FemA and FemB which are involved in the synthesis of the peptidoglycan pentaglycine interpeptide, FmhB may be a candidate for the postulated FemX thought to add the first glycine to the nascent interpeptide.