Modification of peptidoglycan structure by penicillin action in cell walls of Proteus mirabilis

Peptidoglycan O-Acetylation as a Virulence Factor: Its Effect on Lysozyme in the Innate Immune System

The peptidoglycan sacculus of both Gram-positive and Gram-negative bacteria acts as a protective mesh and provides structural support around the entirety of the cell. The integrity of this structure is of utmost importance for cell viability and so naturally is the first target for attack by the host immune system during bacterial infection. Lysozyme, a muramidase and the first line of defense of the innate immune system, targets the peptidoglycan sacculus hydrolyzing the β-(1→4) linkage between repeating glycan units, causing lysis and the death of the invading bacterium. The O-acetylation of N-acetylmuramoyl residues within peptidoglycan precludes the productive binding of lysozyme, and in doing so renders it inactive. This modification has been shown to be an important virulence factor in pathogens such as Staphylococcus aureus and Neisseria gonorrhoeae and is currently being investigated as a novel target for anti-virulence therapies. This article reviews interactions made between peptidoglycan and the host immune system, specifically with respect to lysozyme, and how the O-acetylation of the peptidoglycan interrupts these interactions, leading to increased pathogenicity.

Mechanistic Pathways for Peptidoglycan O-Acetylation and De-O-Acetylation

The post-synthetic O-acetylation of the essential component of bacterial cell walls, peptidoglycan (PG), is performed by many pathogenic bacteria to help them evade the lytic action of innate immunity responses. Occurring at the C-6 hydroxyl of N-acetylmuramoyl residues, this modification to the glycan backbone of PG sterically blocks the activity of lysozymes. As such, the enzyme responsible for this modification in Gram-positive bacteria is recognized as a virulence factor. With Gram-negative bacteria, the O-acetylation of PG provides a means of control of their autolysins at the substrate level. In this review, we discuss the pathways for PG O-acetylation and de-O-acetylation and the structure and function relationship of the O-acetyltransferases and O-acetylesterases that catalyze these reactions. The current understanding of their mechanisms of action is presented and the prospects of targeting these systems for the development of novel therapeutics are explored.

Peptidoglycan O-acetylation is functionally related to cell wall biosynthesis and cell division in Streptococcus pneumoniae

The peptidoglycan is a rigid matrix required to resist turgor pressure and to maintain the cellular shape. It is formed by linear glycan chains composed of N-acetylmuramic acid-(β-1,4)-N-acetylglucosamine (MurNAc-GlcNAc) disaccharides associated through cross-linked peptide stems. The peptidoglycan is continually remodelled by synthetic and hydrolytic enzymes and by chemical modifications, including O-acetylation of MurNAc residues that occurs in most Gram-positive and Gram-negative bacteria. This modification is a powerful strategy developed by pathogens to resist to lysozyme degradation and thus to escape from the host innate immune system but little is known about its physiological function. In this study, we have investigated to what extend peptidoglycan O-acetylation is involved in cell wall biosynthesis and cell division of Streptococcus pneumoniae. We show that O-acetylation driven by Adr protects the peptidoglycan of dividing cells from cleavage by the major autolysin LytA and occurs at the septal site. Our results support a function for Adr in the formation of robust and mature MurNAc O-acetylated peptidoglycan and infer its role in the division of the pneumococcus.

Structural variation in the glycan strands of bacterial peptidoglycan

The normal, unmodified glycan strands of bacterial peptidoglycan consist of alternating residues of beta-1,4-linked N-acetylmuramic acid and N-acetylglucosamine. In many species the glycan strands become modified after their insertion into the cell wall. This review describes the structure of secondary modifications and of attachment sites of surface polymers in the glycan strands of peptidoglycan. It also provides an overview of the occurrence of these modifications in various bacterial species. Recently, enzymes responsible for the N-deacetylation, N-glycolylation and O-acetylation of the glycan strands were identified. The presence of these modifications affects the hydrolysis of peptidoglycan and its enlargement during cell growth. Glycan strands are frequently deacetylated and/or O-acetylated in pathogenic species. These alterations affect the recognition of bacteria by host factors, and contribute to the resistance of bacteria to host defence factors such as lysozyme.

Molecular basis of resistance to muramidase and cationic antimicrobial peptide activity of lysozyme in staphylococci

It has been shown recently that modification of peptidoglycan by O-acetylation renders pathogenic staphylococci resistant to the muramidase activity of lysozyme. Here, we show that a Staphylococcus aureus double mutant defective in O-acetyltransferase A (OatA), and the glycopeptide resistance-associated two-component system, GraRS, is much more sensitive to lysozyme than S. aureus with the oatA mutation alone. The graRS single mutant was resistant to the muramidase activity of lysozyme, but was sensitive to cationic antimicrobial peptides (CAMPs) such as the human lysozyme-derived peptide ₁₀₇R-A-W-V-A-W-R-N-R₁₁₅ (LP9), polymyxin B, or gallidermin. A comparative transcriptome analysis of wild type and the graRS mutant revealed that GraRS controls 248 genes. It up-regulates global regulators (rot, sarS, or mgrA), various colonization factors, and exotoxin-encoding genes, as well as the ica and dlt operons. A pronounced decrease in the expression of the latter two operons explains why the graRS mutant is also biofilm-negative. The decrease of the dlt transcript in the graRS mutant correlates with a 46.7% decrease in the content of esterified d-alanyl groups in teichoic acids. The oatA/dltA double mutant showed the highest sensitivity to lysozyme; this mutant completely lacks teichoic acid–bound d-alanine esters, which are responsible for the increased susceptibility to CAMPs and peptidoglycan O-acetylation. Our results demonstrate that resistance to lysozyme can be dissected into genes mediating resistance to its muramidase activity (oatA) and genes mediating resistance to CAMPs (graRS and dlt). The two lysozyme activities act synergistically, as the oatA/dltA or oatA/graRS double mutants are much more susceptible to lysozyme than each of the single mutants.

In humans, lysozyme plays an important role in the suppression of bacterial infections. However, some bacterial pathogens, such as Staphylococcus aureus, are completely resistant to lysozyme. Here we demonstrate that lysozyme acts on S. aureus in two ways: as a muramidase (cell wall lytic enzyme) and as a cationic antimicrobial peptide (CAMP). S. aureus has developed resistance mechanisms against both activities by modifying distinct cell wall structures. Modification of the peptidoglycan by O-acetylation (OatA) renders the cells resistant to the muramidase activity. Modification of teichoic acids by d-alanine esterification (Dlt) renders the cells resistant to lysozyme's CAMPs and other CAMPs. Transcriptome analysis of the glycopeptide resistance-associated (GraRS) two-component system revealed that this global regulator controls 248 genes such as other global regulators, colonization factors, or exotoxin-encoding genes. Since GraRS also upregulates the dlt operon, it was not surprising that in the graRS mutant teichoic acid d-alanylation is markedly decreased, which explains its increased sensitivity to CAMPs. By comparative analysis of mutants we were able to dissect genes that were responsive to the dual activities of lysozyme. Here we show how efficiently S. aureus is protected from the human defense system, which enables this pathogen to cause persistent infections.

Why are pathogenic staphylococci so lysozyme resistant? The peptidoglycan O-acetyltransferase OatA is the major determinant for lysozyme resistance of Staphylococcus aureus

Staphylococcus species belong to one of the few bacterial genera that are completely lysozyme resistant, which greatly contributes to their persistence and success in colonizing the skin and mucosal areas of humans and animals. In an attempt to discover the cause of lysozyme resistance, we identified a gene, oatA, in Staphylococcus aureus. The corresponding oatA deletion mutant had an increased sensitivity to lysozyme. HPLC and electrospray ionization tandem mass spectrometry analyses of the cell wall revealed that the muramic acid of peptidoglycan of the wild-type strain was O-acetylated at C6-OH, whereas the muramic acid of the oatA mutant lacked this modification. The complemented oatA mutant was lysozyme resistant. We identified the first bacterial peptidoglycan-specific O-acetyltransferase in S. aureus and showed that OatA, an integral membrane protein, is the molecular basis for the high lysozyme resistance in staphylococci.

Formation of the glycan chains in the synthesis of bacterial peptidoglycan

The main structural features of bacterial peptidoglycan are linear glycan chains interlinked by short peptides. The glycan chains are composed of alternating units of N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc), all linkages between sugars being beta,1-->4. On the outside of the cytoplasmic membrane, two types of activities are involved in the polymerization of the peptidoglycan monomer unit: glycosyltransferases that catalyze the formation of the linear glycan chains and transpeptidases that catalyze the formation of the peptide cross-bridges. Contrary to the transpeptidation step, for which there is an abundant literature that has been regularly reviewed, the transglycosylation step has been studied to a far lesser extent. The aim of the present review is to summarize and evaluate the molecular and cellullar data concerning the formation of the glycan chains in the synthesis of peptidoglycan. Early work concerned the use of various in vivo and in vitro systems for the study of the polymerization steps, the attachment of newly made material to preexisting peptidoglycan, and the mechanism of action of antibiotics. The synthesis of the glycan chains is catalyzed by the N-terminal glycosyltransferase module of class A high-molecular-mass penicillin-binding proteins and by nonpenicillin-binding monofunctional glycosyltransferases. The multiplicity of these activities in a given organism presumably reflects a variety of in vivo functions. The topological localization of the incorporation of nascent peptidoglycan into the cell wall has revealed that bacteria have at least two peptidoglycan-synthesizing systems: one for septation, the other one for elongation or cell wall thickening. Owing to its location on the outside of the cytoplasmic membrane and its specificity, the transglycosylation step is an interesting target for antibacterials. Glycopeptides and moenomycins are the best studied antibiotics known to interfere with this step. Their mode of action and structure-activity relationships have been extensively studied. Attempts to synthesize other specific transglycosylation inhibitors have recently been made.

Analysis of peptidoglycan structure from vegetative cells of Bacillus subtilis 168 and role of PBP 5 in peptidoglycan maturation

The composition and fine structure of the vegetative cell wall peptidoglycan from Bacillus subtilis were determined by analysis of its constituent muropeptides. The structures of 39 muropeptides, representing 97% of the total peptidoglycan, were elucidated. About 99% analyzed muropeptides in B. subtilis vegetative cell peptidoglycan have the free carboxylic group of diaminopimelic acid amidated. Anhydromuropeptides and products missing a glucosamine at the nonreducing terminus account for 0.4 and 1.5%, respectively, of the total muropeptides. These two types of muropeptides are suggested to end glycan strands. An unexpected feature of B. subtilis muropeptides was the occurrence of a glycine residue in position 5 of the peptide side chain on monomers or oligomers, which account for 2.7% of the total muropeptides. This amount is, however, dependent on the composition of the growth media. Potential attachment sites for anionic polymers to peptidoglycan occur on dominant muropeptides and account for 2.1% of the total. B. subtilis peptidoglycan is incompletely digested by lysozyme due to de-N-acetylation of glucosamine, which occurs on 17.3% of muropeptides. The cross-linking index of the polymer changes with the growth phase. It is highest in late stationary phase, with a value of 33.2 or 44% per muramic acid residue, as determined by reverse-phase high-pressure liquid chromatography or gel filtration, respectively. Analysis of the muropeptide composition of a dacA (PBP 5) mutant shows a dramatic decrease of muropeptides with tripeptide side chains and an increase or appearance of muropeptides with pentapeptide side chains in monomers or oligomers. The total muropeptides with pentapeptide side chains accounts for almost 82% in the dacA mutant. This major low-molecular-weight PBP (DD-carboxypeptidase) is suggested to play a role in peptidoglycan maturation.

Evidence that the extracytoplasmic function sigma factor sigmaE is required for normal cell wall structure in Streptomyces coelicolor A3(2)

The sigE gene of Streptomyces coelicolor A3(2) encodes an RNA polymerase sigma factor belonging to the extracytoplasmic function (ECF) subfamily. Constructed sigE deletion and disruption mutants were more sensitive than the parent to muramidases such as hen egg white lysozyme and to the CwlA amidase from Bacillus subtilis. This correlated with an altered muropeptide profile, as determined by reverse-phase high-performance liquid chromatography analysis of lytic digests of purified peptidoglycan. The sigE mutants required high levels of magnesium for normal growth and sporulation, overproducing the antibiotic actinorhodin and forming crenellated colonies in its absence. Together, these data suggest that sigE is required for normal cell wall structure. The role of sigmaE was further investigated by analyzing the expression of hrdD, which is partially sigE dependent. The hrdD gene, which encodes the sigmaHrdD subunit of RNA polymerase, is transcribed from two promoters, hrdDp1 and hrdDp2, both similar to promoters recognized by other ECF sigma factors. The activities of hrdDp1 and hrdDp2 were reduced 20- and 3-fold, respectively, in sigE mutants, although only hrdDp1 was recognized by EsigmaE in vitro. Growth on media deficient in magnesium caused the induction of both hrdD promoters in a sigE-dependent manner.

Peptidoglycan structure of Lactobacillus casei, a species highly resistant to glycopeptide antibiotics

The structure of the peptidoglycan of Lactobacillus casei ATCC 393, a species highly resistant to glycopeptide antibiotics, was examined. After digestion, 23 muropeptides were identified; monomers represented 44.7% of all muropeptides, with monomer tetrapeptides being the major ones. Fifty-nine percent of the peptidoglycan was O-acetylated. The cross-bridge between D-alanine and L-lysine consisted of one asparagine, although aspartate could be found in minor quantities. Since UDP-MurNAc-tetrapeptide-D-lactate is the normal cytoplasmic precursor found in this species, monomer tetrapeptide-lactate was expected to be found. However, such a monomer was found only after exposure to penicillin, suggesting that penicillin-sensitive D,D-carboxypeptidases were very active in normal growing cells.

Extent of peptidoglycan O acetylation in the tribe Proteeae

The degree of peptidoglycan O acetylation in 18 strains of the different genera of the tribe Proteeae (Proteus, Providencia, and Morganella) has been determined by high-performance liquid chromatography-based organic acid analysis of mild-base-released acetic acid and quantitation of peptidoglycan concentrations by simultaneous amino sugar-amino acid analysis using high-performance anion-exchange chromatography with pulsed amperometric detection. The N,O-diacetylmuramyl content of all isolated and purified peptidoglycans was greater than 29% and ranged up to 57% relative to total muramic acid concentration. Each of the O-acetylated peptidoglycans was found to be resistant to solubilization by hen egg white lysozyme.

In vitro synthesis and O acetylation of peptidoglycan by permeabilized cells of Proteus mirabilis

The synthesis and O acetylation in vitro of peptidoglycan by Proteus mirabilis was studied in microorganisms made permeable to specifically radiolabelled nucleotide precursors by treatment with either diethyl ether or toluene. Optimum synthesis occurred with cells permeabilized by 1% (vol/vol) toluene in 30 mM MgCl2 in in vitro experiments with 50 mM Tris-HCl buffer (pH 6.80). Acetate recovered by mild base hydrolysis from sodium dodecyl sulfate-insoluble peptidoglycan synthesized in the presence of UDP-[acetyl-1-14C]N-acetyl-D-glucosamine was found to be radioactive. Radioactivity was not retained by peptidoglycan synthesized when UDP-[acetyl-1-14C]N-acetyl-D-glucosamine was replaced with both unlabelled nucleotide and either [acetyl-3H]N-acetyl-D-glucosamine or [glucosamine-1,6-3H]N-acetyl-D-glucosamine. In addition, no radioactive acetate was detected in the mild base hydrolysates of peptidoglycan synthesized in vitro with UDP-[glucosamine-6-3H]N-acetyl-D-glucosamine as the radiolabel. Chasing UDP-[acetyl-1-14C]N-acetyl-D-glucosamine with unlabelled material served to increase the yield of O-linked [14C]acetate, whereas penicillin G blocked both peptidoglycan synthesis and [14C]acetate transfer. These results support the hypothesis that the base-labile O-linked acetate is derived directly from N-acetylglucosamine incorporated into insoluble peptidoglycan via N----O transacetylation and not from the catabolism of the supplemented peptidoglycan precursors followed by subsequent reactivation of acetate.

Evidence for N----O acetyl migration as the mechanism for O acetylation of peptidoglycan in Proteus mirabilis

O-acetylated peptidoglycan was purified from Proteus mirabilis grown in the presence of specifically radiolabelled glucosamine derivatives, and the migration of the radiolabel was monitored. Mild-base hydrolysis of the isolated peptidoglycan (to release ester-linked acetate) from cells grown in the presence of 40 microM [acetyl-3H]N-acetyl-D-glucosamine resulted in the release of [3H]acetate, as detected by high-pressure liquid chromatography. The inclusion of either acetate, pyruvate, or acetyl phosphate, each at 1 mM final concentration, did not result in a diminution of mild-base-released [3H]acetate levels. No such release of [3H]acetate was observed with peptidoglycan isolated from either Escherichia coli incubated with the same radiolabel or P. mirabilis grown with [1,6-3H]N-acetyl-D-glucosamine or D-[1-14C]glucosamine. These observations support a hypothesis that O acetylation occurs by N----O acetyl transfer within the sacculus. A decrease in [3H]acetate release by mild-base hydrolysis was observed with the peptidoglycan of P. mirabilis cultures incubated in the presence of antagonists of peptidoglycan biosynthesis, penicillin G and D-cycloserine. The absence of free-amino sugars in the peptidoglycan of P. mirabilis but the detection of glucosamine in spent culture broths implies that N----O transacetylation is intimately associated with peptidoglycan turnover.

Cell wall assembly in Bacillus megaterium: incorporation of new peptidoglycan by a monomer addition process

The pattern of cross-linking in the peptidoglycan of Bacillus megaterium has been studied by the pulsed addition of radiolabeled diaminopimelic acid. The distribution of label in muropeptides, generated by digestion with Chalaropsis muramidase and separated by high-performance liquid chromatography, stabilized after 0.15 of a generation time. The proportion of label in the acceptor and donor positions of isolated muropeptide dimers stabilized over the same period of time. The results have led to the formulation a new model for the assembly of peptidoglycan into the cylindrical wall of B. megaterium by a monomer addition process. Single nascent glycan peptide strands form cross-linkages only with material at the inner surface of the wall. Maturation is a direct consequence of subsequent incorporation of further new glycan peptide strands, and there is no secondary cross-linking process. The initial distribution of muropeptides is constant. It follows that the final pattern of cross-linking in the wall is determined solely by, and can be forecast from, this repetitive pattern of incorporation. In a modified form, this model can also be applied to assembly of cell walls in rod-shaped gram-negative bacteria.

Dependence of lysozyme-catalysed solubilization of Proteus mirabilis peptidoglycan on the extent of O-acetylation

The degree of peptidoglycan O-acetylation in 14 strains of Proteus mirabilis has been accurately determined by a procedure which employs the quantitation of mild-base-released acetic acid by HPLC, and the estimation of peptidoglycan concentration by cation-exchange amino acid analysis. The beta-D-N,6-O-diacetylmuramyl content of all isolated and purified peptidoglycans was ranged 20-52.8%, relative to the total muramic acid concentration. Each of the O-acetylated peptidoglycans was found to be resistant to solubilization by both human and hen egg-white lysozymes and for hen egg-white lysozyme, the extent of this resistance was dependent upon the degree of O-acetylation. The steady-state parameters, Km and V, for the hen-egg-white-lysozyme-catalysed solubilization of various peptidoglycan preparations were determined at pH 6.61 and 25 degrees C. Values of Km for the different peptidoglycan samples were found to increase with increasing O-acetylation, whereas with V no such relationship appeared to exist. An increase in the overall change in the standard Gibbs free energy of activation [delta(delta G#)], a consequence of increasing O-acetylation, was observed, and is shown to result from the weaker affinity of the enzyme for the modified substrates.

Peptidoglycan cross-linking in Staphylococcus aureus. An apparent random polymerisation process

The peptidoglycan of Staphylococcus aureus contains relatively short glycan chains and is highly cross-linked via its peptide chains. The material from wild-type (strain H) and mutants H28, H4B and MR-1 was freed from the teichoic-acid-linked component and then hydrolysed by Chalaropsis muramidase to yield disaccharide-repeating units of the glycan with attached peptides either non-cross-linked (monomer) or joined to similar units by one (dimer), two (trimer) or more (oligomer) peptide cross links. The resulting fragments were separated by high-resolution HPLC so that distinguishable components as large as nonamer could be identified. Extrapolation showed that, in S. aureus H, H28 and MR-1, oligomers at least as large as eicosamer formed part of the smooth distribution of oligomer fragments, whereas in strain H4B (PBP4-) the maximum size was around dodecamer. The oligomer distribution profile was related to the polymerization theories of Flory, which allow a distinction to be made between a monomer addition model, whereby each oligomer can only be synthesized by the addition of a single monomer unit to its next lower homologue, and a random addition model, in which an oligomer can be formed by linkage of any combination of its constituent smaller units. In S. aureus close approximation to the random addition model for oligomer synthesis and hence for peptidoglycan cross-linking was observed, both in PBP4+ and PBP4- mutants. The implications for secondary cross-linking in S. aureus cell wall formation are inescapable, although the possibility of an endopeptidase/transpeptidase providing later modification of the peptidoglycan is not completely ruled out.

Penicillin-induced changes in the cell wall composition of Staphylococcus aureus before the onset of bacteriolysis

To analyze if chemical cell wall alterations contribute to penicillin-induced bacteriolysis, changes in the amount, stability, and chemical composition of staphylococcal cell walls were investigated. All analyses were performed before onset of bacteriolysis i.e. during the first 60 min following addition of different penicillin G doses. Only a slight reduction of the amount of cell wall material incorporated after penicillin addition at the optimal lytic concentration was observed as compared to control cells. However, the presence of higher penicillin G concentrations reduced the incorporation of wall material progressively without bacteriolysis. Losses of wall material during isolation of dodecylsulfate insoluble cell walls were monitored to assess the stability of the wall material following penicillin addition. Wall material grown at the lytic penicillin concentration was least stable but about 30% of the newly incorporated wall material withstood even the harsh conditions of mechanical breakage and dodecylsulfate treatment. Dodecylsulfate insoluble cell walls were used for chemical analyses. While peptidoglycan chain length was unaffected in the presence of penicillin, other wall parameters were considerably altered: peptide cross-linking was reduced in the wall material synthesized after addition of penicillin; reductions from approx. 85% in controls to about 60% were similar for lytic and also for very high penicillin concentrations leading to nonlytic death. O-acetylation was also reduced after treatment with penicillin; this effect paralleled the occurrence of subsequent bacteriolysis at different drug concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Reactivation of peptidoglycan synthesis in ether-permeabilized Escherichia coli after inhibition by beta-lactam antibiotics

The recovery of peptidoglycan-synthesizing activity after inhibition by beta-lactam antibiotics was investigated in ether-permeabilized cells of Escherichia coli B. Such cells synthesize sodium dodecyl sulfate-insoluble peptidoglycan when provided with UDP-linked precursors and Mg2+. The ability of beta-lactam antibiotics to inhibit the synthesis of peptidoglycan was correlated with their affinity for penicillin-binding proteins 1A and 1Bs. Penicillin-binding protein 1Bs is thought to be the major peptidoglycan synthetase in E. coli and is a major lethal target for beta-lactam antibiotics. Ether-treated bacteria were preincubated with concentrations of beta-lactams sufficient to completely inhibit peptidoglycan synthesis and then treated with beta-lactamases to inactivate free antibiotic prior to measurement of peptidoglycan synthesis. At 40 min after beta-lactamase treatment, the rate of peptidoglycan synthesis was about 74% of the control rate in cells pretreated with ampicillin, but only 15% of the control in cells pretreated with penicillin G or azlocillin. Reversal of inhibition by several other antibiotics fell between these extremes. When cross-linking of peptidoglycan was measured specifically, reversal of inhibition by ampicillin also occurred more readily than that by penicillin G. Reactivation of peptidoglycan synthesis was not due to de novo synthesis of penicillin-binding proteins since it occurred under conditions that did not allow incorporation of [14C]leucine. We conclude that there is considerable variation in the stability of the inactive acyl enzymes formed between various beta-lactams and penicillin-binding protein 1Bs, with those formed by penicillin G being relatively long-lived.

Somnogenic activity of O-acetylated and dimeric muramyl peptides

Slow-wave sleep-promoting factors in brain and urine were identified as muramyl peptides (MPs), the building blocks of bacterial cell wall peptidoglycan. In this study, structural variations of MPs that occur naturally in bacterial peptidoglycan were investigated for somnogenic activity. Monomeric and dimeric MPs were isolated and purified from Neisseria gonorrhoeae and Actinomadura sp. strain R39. The structures of these MPs were verified by fast atom bombardment mass spectroscopy and tandem mass spectroscopy. After intracerebroventricular administration of MPs, electroencephalograms and brain temperatures of rabbits were recorded for 6 h and were analyzed to determine durations of slow-wave sleep, rapid-eye-movement sleep, and wakefulness. The 6-O acetylation of muramic acid enhanced the somnogenic effects of certain monomeric MPs relative to their non-O-acetylated (but otherwise identical) counterparts. Two monomeric MPs containing an unsubstituted amide (i.e., Iso-Gln) were inactive, thus confirming previous results showing that amidation of a variety of MPs can block somnogenic activity. Two peptide-cross-linked MP dimers tested had no effect on slow-wave sleep, although a third peptide-cross-linked MP containing a 1,6-anhydro muramyl end on one of its monomeric subunits, a structure that enhances somnogenic potency of un-cross-linked monomers, was somnogenic. Two dimers connected by glycosidic bonds and containing an Iso-Gln moiety were inactive. Two other glycosidically linked dimers that also contained an Iso-Gln moiety, but were of lower molecular weight, were somnogenic. In summary, 6-O acetylation of muramic acid in somnogenic MPs enhances activity, and as a class, peptide-linked dimeric MPs tend to be less active than their constituent monomers.

Antibacterial properties of (2,3)-alpha- and (2,3)-beta-methylene analogs of penicillin G

The penam nucleus can assume two conformations; these are designated open and closed. The synthetic (2,3)-alpha- and (2,3)-beta-methylenepenams can be regarded as analogs of the open and closed conformations, respectively. It has been shown that the beta-methylenepenams are essentially inactive, suggesting that the closed conformation of penams is also inactive. In this study, we investigated a series of beta-lactams, all of which contained phenylacetamido side chains: penicillin G, the (2,3)-alpha- and (2,3)-beta-methylenepenams, and the 3-acetoxymethyl- and 3-methylcephalosporins. The alpha-methylenepenam and penicillin G were the most active compounds, while the beta-methylene isomer was only poorly active. Results with permeability mutants suggested that the alpha-methylene compound penetrated the outer membrane somewhat more readily than penicillin G did. The intrinsic potency of the alpha-methylenepenam appeared to be similar to that of penicillin G, on the basis of their affinities for penicillin-binding proteins and their abilities to inhibit peptidoglycan synthesis in ether-permeabilized Escherichia coli, while the beta-methylene analog had very poor intrinsic potency. The alpha-methylene analog was about 10-fold more efficient (Vmax/Km) than penicillin G as a substrate for the cephalosporinases from Enterobacter cloacae and Proteus vulgaris, but it was about 40-fold less efficient with penicillinase from Staphylococcus aureus. These results strongly support the hypothesis that the active conformation of penams is the open conformation and suggest that the position in space of the carboxyl group relative to the beta-lactam carbonyl is an important determinant of cephalosporinlike character, as distinct from penicillinlike character.

Murein biosynthesis in synchronized cells of Proteus mirabilis. Quantitative analysis of O-acetylated murein subunits and of chain terminators incorporated into the sacculus during the cell cycle

Cells of Proteus mirabilis, synchronized by sucrose density gradient centrifugation, were grown in complex medium containing radioactive N-acetylglucosamine. At various times, labelled murein sacculi were isolated and digested with endo-N,O-acetylmuramidase from Chalaropsis. The murein fragments thus obtained were separated into disaccharide peptides as the monomeric subunits and into peptide-cross-linked subunits by gel filtration. The subunits were further differentiated into O-acetylated and non-O-acetylated species, and into subunits containing anhydro-N-acetylmuramic acid which were glycan chain terminators in the native sacculi. Quantification of the subunit species gave the following results. At specific times during the cell cycle, murein subunits were lost from the polymer and a transient decrease in cross-linkage was observed. The overall degree of cross-linkage in mature murein, i.e. the ratio of peptide-cross-linked subunits versus uncross-linked subunits, was 1.15 as determined by regression analysis. Anhydro-N-acetylmuramic-acid-containing murein subunits representing glycan chain terminators were found either peptide-cross-linked or uncross-linked as monomers. Since these two subunit species were recovered in a defined ratio of 1.6, mature murein consisted of at least two different types of glycan chains. On average, each chain contained 15.4 murein subunits. About 60% of the murein subunits in mature murein were O-acetylated and showed a higher degree of cross-linkage than the non-O-acetylated portion. Finally, following the composition of the sacculus during the cell cycle revealed a complex precursor-product relationship between non-O-acetylated and O-acetylated subunits during murein maturation. The data allowed us to deduce several features of the assembly process of murein sacculi.

Effects of growth of methicillin-resistant and -susceptible Staphylococcus aureus in the presence of beta-lactams on peptidoglycan structure and susceptibility to lytic enzymes

Growth of methicillin-resistant Staphylococcus aureus DU4916 in the presence of methicillin yielded crude cell walls that showed an increased rate of autolysis and purified cell walls (PCW) and peptidoglycan (PG) that had increased susceptibilities to autolysin extracted with LiCl and to lysozyme. The PG of cells grown in the presence of methicillin had markedly decreased cross-linking and O acetylation. Growth of the methicillin-susceptible strain H in the presence of subinhibitory concentrations of cefoxitin, a specific inhibitor of penicillin-binding protein (PBP) 4, caused a substantial decrease in PG cross-linking and O acetylation and increased susceptibilities of PCW and PG to LiCl-extracted autolysin and to lysozyme. Strain DU4916 cells grown in the presence of methicillin did not show an increased rate of autolysis or an increased susceptibility to vancomycin- or D-cycloserine-induced lysis, even though their PG was hypo-cross-linked. This implies that the potential for increased autolysis is controlled in intact cells and that this regulation may be involved in the methicillin resistance phenomenon. Growth of the methicillin-susceptible strain DU4916S in the presence of methicillin yielded PCW and PG that showed small increases in susceptibilities to LiCl-extracted autolysin and to lysozyme and a small decrease in PG cross-linking. Comparison of the PBPs of a penicillinase-nonproducing derivative of strain DU4916 (DU4916-K7) with those of strain DU4916S in intact cells and isolated membranes revealed that PBPs 1 to 4 had similar high beta-lactam antibiotic affinities in both strains and identified an additional PBP, PBP2(1), with low beta-lactam affinity in the methicillin-resistant strain DU4916-K7. The low degree of cross-linking of PG in strain DU4916 cells grown with methicillin was probably due mainly to inhibition of the secondary cross-linking function of PBP 4.

Cell cycle parameters of Proteus mirabilis: interdependence of the biosynthetic cell cycle and the interdivision cycle

We investigated the time periods of DNA replication, lateral cell wall extension, and septum formation within the cell cycle of Proteus mirabilis. Cells were cultivated under three different conditions, yielding interdivision times of approximately 55, 57, and 160 min, respectively. Synchrony was achieved by sucrose density gradient centrifugation. The time periods were estimated by division inhibition studies with cephalexin, mecillinam, and nalidixic acid. In addition, DNA replication was measured by thymidine incorporation, and murein biosynthesis was measured by incorporation of N-acetylglucosamine into sodium dodecyl sulfate-insoluble murein sacculi. At interdivision times of 55 to 57 min murein biosynthesis for reproduction of a unit cell lasted longer than the interdivision time itself, whereas DNA replication finished within 40 min. Surprisingly, inhibition of DNA replication by nalidixic acid did not inhibit the subsequent cell division but rather the one after that. Because P. mirabilis fails to express several reactions of the recA-dependent SOS functions known from Escherichia coli, the drug allowed us to determine which DNA replication period actually governed which cell division. Taken together, the results indicate that at an interdivision time of 55 to 57 min, the biosynthetic cell cycle of P. mirabilis lasts approximately 120 min. To achieve the observed interdivision time, it is necessary that two subsequent biosynthetic cell cycles be tightly interlocked. The implications of these findings for the regulation of the cell cycle are discussed.

Involvement of a change in penicillin target and peptidoglycan structure in low-level resistance to beta-lactam antibiotics in Neisseria gonorrhoeae

A penicillin-susceptible gonococcus and its low-level resistant penA transformant were examined with regard to their penicillin-binding proteins (PBPs) and their peptidoglycan structures. Treatment of the susceptible strain with its MIC of penicillin (0.01 microgram/ml) led to significant binding to PBPs 2 and 3 and a substantial decrease in the O-acetyl modification on the peptidoglycan. Peptidoglycan synthesis gradually ceased over an extended time. When the penA strain was treated with the same concentration of penicillin, only binding to PBP 3 was observed and there was no O-acetylation decrease, with continued peptidoglycan synthesis. This suggested that PBP 2 was the primary target in penicillin-susceptible gonococci and that this protein participated in the O-acetylation of peptidoglycan. Penicillin concentrations representing the MIC for the penA transformant (0.06 microgram/ml) caused significant binding to PBPs 1, 2, and 3 in the susceptible strain and PBPs 1 and 3 in the penA strain. In both strains the rate of peptidoglycan synthesis and the cross-linkage of the peptidoglycan made declined sharply, suggesting that significant inhibition of PBP 1 interfered with transpeptidation. A model for low-level resistance is proposed in which a decreased PBP 2 affinity leads to assumption of the role of primary target in the resistant transformant by PBP 1. The differences observed in peptidoglycan metabolism are a direct consequence of this change.

Neither an enhancement of autolytic wall degradation nor an inhibition of the incorporation of cell wall material are pre-requisites for penicillin-induced bacteriolysis in staphylococci

In contrast to what has been postulated, penicillin G at its optimal lytic concentration of 0.1 microgram per ml did not lead to a detectable activation of autolytic wall processes in staphylococci in terms of the release of uniformly labelled wall fragments from cells pretreated with the drug for 1 h. Rather a considerable inhibition of this release was observed. A similarly profound inhibition of the release of peptidoglycan fragments occurred when staphylococci pretreated for 1 h with 0.1 microgram penicillin per ml acted as a source of crude autolysins on peptidoglycan isolated from labelled normal cells of the same strain. This clearly demonstrated that the overall inhibition of autolytic wall processes caused by penicillin was mainly due to a decreased total autolysin action rather than to an altered wall structure. Furthermore, no substantial penicillin-induced inhibition of the incorporation of 14C-N-acetylglucosamine into the staphylococcal wall could be observed before bacteriolysis started, i.e., approximately during the first 80 min of penicillin action. These results are not consistent with any of the models hitherto proposed for the action of penicillin.

In vitro synthesis of peptidoglycan by spheroplasts of Proteus mirabilis grown in the presence of penicillin

Spheroplasts of the unstable L-form of Proteus mirabilis with fragile, shape defective cell walls grown in medium containing 120 mg/l penicillin G and then killed and permeabilized by ether treatment, were capable of in vitro synthesis of peptidoglycan from the precursors UDP-GlcNAc and UDP-MurNAc-L-Ala-D-Glu(ms-A2pm-D-Ala-D-Ala). The in vitro peptidoglycan was extensively peptide-crosslinked, indicating a continuing function of peptidoglycan transpeptidase in the spheroplasts. The seven penicillin-binding proteins (PBPs) of P. mirabilis with their functions as multiple peptidoglycan transpeptidases were shown to be saturated in the spheroplasts and thereby functionally inactivated by the penicillin of the growth medium to a very different degree. Complete or almost complete saturation occurred with the PBPs 1A, 1B, and 3, for which functions as indispensable transpeptidases in Escherichia coli have been postulated. In contrast, PBPs 5 and 6 were not saturated in the L-form spheroplasts. Transpeptidase function has been described previously in PBP 5 of P. mirabilis. The working hypothesis is proposed that synthesis of the functionally defective peptidoglycan of L-form spheroplasts in the presence of penicillin takes place with transpeptidase function of PBP 5.

Strain distribution in extents of lysozyme resistance and O-acetylation of gonococcal peptidoglycan determined by high-performance liquid chromatography

The extent of lysozyme resistance and O-acetylation of purified peptidoglycan (PG) from 20 strains of Neisseria gonorrhoeae was examined to determine how widespread these properties are among various subsets of gonococcal isolates. To determine digestibility by lysozyme, we treated [3H]- or [14C]glucosamine-labeled PG with hen egg white lysozyme (HEW-LZ) and determined the size distribution of HEW-LZ soluble PG at the completion of the reaction by molecular-sieve high-performance liquid chromatography, using a Varian TSK SW2000 column, a method that proved considerably more efficient than traditional chromatography for fractionating low-molecular-weight PG fragments solely on the basis of size. The extent of HEW-LZ resistance was expressed as the percentage of PG that was larger in size than disaccharide peptide tetramers (including insoluble PG removed by centrifugation). The percent O-acetylation was determined by converting insoluble PG totally to uncross-linked monomers by the combined action of Chalaropsis B muramidase followed by Escherichia coli endopeptidase and then quantitating radioactivity in O-acetylated and non-O-acetylated monomers after paper chromatography. The PG of the vast majority (19 of 20) of gonococcal strains examined was extensively HEW-LZ resistant (range, 40 to 60% larger than tetramers) and extensively O-acetylated (range, 34 to 52%). Only the PG of strain RD5 (highest rate of PG turnover among gonococci so far examined and the prototype of gonococci having O-acetyl-deficient PG) had greatly reduced O-acetylation (15%) and exhibited virtually no HEW-LZ resistance (2% larger than tetramers). Extensive HEW-LZ resistance and O-acetylation were apparently not associated specifically with (i) a given type of colonial variant (piliated versus nonpiliated or opaque versus transparent), (ii) a given type of clinical isolate (local versus disseminated), (iii) the extent of laboratory passage, or (iv) (with the possible exception of penicillin-resistant strain FA102) the presence of one or more genetic loci governing antibiotic resistance among members of an isogenic set of gonococci. From this survey, we conclude that lysozyme resistance and extensive O-acetylation of PG are widespread among gonococci and, thus, that most strains are potential sources of hydrolase-resistant PG that conceivably could persist as macromolecular fragments in vivo.

Reduced degradability by lysozyme of staphylococcal cell walls after chloramphenicol treatment

The effectiveness of host defence against staphylococcal infections depends on the capability of phagocytes to degrade the bacterial cell walls. Treatment with bacteriostatic agents like chloramphenicol could cause problems since under these drugs staphylococcal walls will be substantially thickened. This study presents evidence that the additional wall material built in the presence of chloramphenicol could moreover be rendered more resistant to lysosomal enzymes: In vitro at pH 5.6, lysozyme from hen egg-white proved to degrade the chloramphenicol-wall material at a velocity reduced to 20% of that of the normal wall. Thus, during the degradation of chloramphenicol-treated staphylococcal cell walls the phagocytes have to deal not only with a quantitative but also with a qualitative problem. Possible reasons for the reduced degradability as to chloramphenicol-induced alterations of the wall composition as well as to activating effects of lysozyme on cell wall autolysins are discussed in view of microbiological and medical implications.

Peptidoglycan biosynthesis in Neisseria gonorrhoeae strains sensitive and intrinsically resistant to beta-lactam antibiotics

Treatment of penicillin-sensitive and intrinsically resistant Neisseria gonorrhoeae strains with their respective inhibitory concentrations of penicillin caused rapid cell death. When the peptidoglycan syntheses of these two strains were examined in the presence of penicillin, the sensitive strain continued to make this cell wall polymer for an extended time, whereas the resistant strain underwent a rapid and marked depression in synthesis. Examination of the labeled sodium dodecyl sulfate-insoluble peptidoglycan made in the presence of inhibitory concentrations of penicillin revealed further differences. The primary effect on the penicillin-sensitive gonococcus was a slight change in peptide cross-linking and a sharp decline in the degree of O-acetylation. In contrast, the resistant strain exhibited a substantial decline in cross-linking, with a very moderate change in O-acetylation. The degree of saturation of the individual penicillin-binding proteins (PBPs) was assessed under these conditions. PBP 2, which exhibits a reduced affinity for penicillin in the resistant strain, appeared to be related to O-acetylation, whereas PBP 1 was implicated in the transpeptidation reaction.

Strain-related differences in lysozyme sensitivity and extent of O-acetylation of gonococcal peptidoglycan

Peptidoglycan from Neisseria gonorrhoeae RD5 was completely degraded by hen egg white lysozyme and was not extensively O-acetylated. In contrast, peptidoglycans from gonococcal strains FA19 and FA102 (a penicillin-resistant mutant derived from FA19), were markedly resistant to digestion by hen egg white lysozyme and were extensively O-acetylated.

Effects of beta-lactam antibiotics on peptidoglycan synthesis in growing Neisseria gonorrhoeae, including changes in the degree of O-acetylation

Low concentrations of beta-lactam antibiotics caused an increased uptake of radioactive glucosamine into the sodium dodecyl sulfate-insoluble peptidoglycan of growing Neisseria gonorrhoeae. There was no appreciable change in the (small) amount of sodium dodecyl sulfate-soluble polymer present in the cultures. The sodium dodecyl sulfate-insoluble product in control cells was only partially dissolved by egg-white lysozyme (about 40%), but could all be released by the Chalaropsis B muramidase. In cells exposed to beta-lactams the proportion of labeled peptidoglycan susceptible to lysozyme increased to 60%. Examination of the Chalaropsis B digests by thin-layer chromatography showed that they contained disaccharide-peptide monomers with and without O-acetylation and bis-disaccharide-peptide dimers with one or two O-acetyl groups, or with none. beta-Lactam antibiotics caused a decrease in the degree of O-acetylation but did not greatly affect the amount of peptidoglycan cross-linking. They also had the effect of enlarging the bacteria and conserving and thickening the septa that could be observed in thin sections under the electron microscope. The relationship between these results and the effects of beta-lactams on in vitro synthesis of peptidoglycan by ether-treated N. gonorrhoeae is discussed.

Evidence from a carbohydrate incorporation assay for direct activation of bone marrow myelopoietic precursor cells by bacterial cell wall constitutents

The stimulation of incorporation of [3H]galactose into membrane glycoconjugates, measured in a precipitation test, was used as a criterion for activation of bone marrow cells. In this assay, purified bacterial lipopolysaccharide, lipoprotein, and murein monomer and dimer fragments all activated rat bone marrow cells in vitro. The response was dose dependent, followed a defined time course, and was not serum dependent. O-Acetylated murein dimer fragments from Proteus mirabilis were much less active than their unsubstituted counterparts, indicating a structural specificity for murein activation. Removal of adherent and phagocytizing cells from the marrow suspensions did not alter these results. The labeled, activated cells constituted a distinct population of buoyant density 1.064 to 1.069 g/cm3 when centrifuged on a continuous gradient of Percoll. Enrichment of the target cell population was achieved by a combination of adherent cell removal and discontinuous density gradient centrifugation to remove granulocytes and erythropoietic cells. It was concluded that a population of myelopoietic precursors could be activated by direct contact with bacterial cell wall constituents. The stimulation of galactose incorporation was not coupled to active deoxyribonucleic acid synthesis in the marrow cells. Thus, the activation was interpreted as an induction of differentiation rather than a mitotic event.

Inhibition of Bacillus subtilis aminopeptidase D by beta-lactam antibiotics

Penicillins and cephalosporins inhibit the hydrolysis of D-alanyl-beta-naphthylamide by aminopeptidase D (EC 3.4.11.-) from Bacillus subtilis. The inhibition is predominantly non-competitive, although the more effective inhibitors, i.e., those with the lower Ki values, seem to exhibit a tendency toward competitive kinetics, while penicillin V, the antibiotic with the largest Ki, exhibits a strong tendency toward uncompetitive kinetics. The removal of antibiotic from the enzyme by gel filtration on Sephadex G-25 restores 100% activity to the enzyme, and suggests that the inhibition does not derive from a covalent antibiotic-enzyme complex. The antibiotics which have been studied, with their respective Ki values (mM) and inhibition type are: methicillin (1.01 +/- 0.34, mixed non-competitive-competitive); cephaloridine (2.43 +/- 0.17, non-competitive); cloxacillin (3.19 +/- 1.29, mixed non-competitive-competitive); oxacillin (6.88 +/- 0.77, non-competitive); cephalothin (8.12 +/- 0.99, non-competitive); penicillin G (20.0 +/- 3.21, non-competitive) and penicillin V (32.1 +/- 4.90, mixed non-competitive-uncompetitive). An empirical correlation exists between the Ki value and the freedom of rotation about the bond between the phenyl ring and the atom alpha to the ring in the variable side chain portion of the penicillin molecule.

Alteration of Escherichia coli murein during amino acid starvation

We have studied the mechanisms by which amino acid starvation of Escherichia coli induces resistance against the lytic and bactericidal effects of penicillin. Starvation of E. coli strain W7 of the amino acids lysine or methionine resulted in the rapid development of resistance to autolytic cell wall degradation, which may be effectively triggered in growing bacteria by a number of chemical or physical treatments. The mechanism of this effect in the amino acid-starved cells involved the production of a murein relatively resistant to the hydrolytic action of crude murein hydrolase extracts prepared from normally growing E. coli. Resistance to the autolysins was not due to the covalently linked lipoprotein. Resistance to murein hydrolase developed most rapidly and most extensively in the portion of cell wall synthesized after the onset of amino acid starvation. Lysozymes digests of the autolysin-resistant murein synthesized during the first 10 min of lysine starvation yielded (in addition to the characteristic degradation products) a high-molecular-weight material that was absent from the lysozyme-digests of control cell wall preparations. It is proposed that inhibition of protein synthesis causes a rapid modification of murein structure at the cell wall growth zone in such a manner that attachment of murein hydrolase molecules is inhibited. The mechanism may involve some aspects of the relaxed control system since protection against penicillin-induced lysis developed much slower in amino acid-starved relaxed controlled (relA) cells than in isogenic stringently controlled (relA+) bacteria.

Membranes of the protoplast L-form of Proteus mirabilis

Isolated membranes of the cell wall-less stable protoplast L-form of Proteus mirabilis were characterized by density gradient centrifugation and by assay for their major chemical constituents, proteins, phospholipids and lipopolysacchartide, and for some specific marker enzymes of the cytoplasmic membrane. In most of the analyzed properties the L-form protoplast membrane resembled the bacterial cytoplasmic membrane, with some notable modifications. Considerable amounts of lipopolysaccharide, normally an exclusive constituent of the outer membrane, were found. Furthermore, the L-form membranes contained the functions of the reduced nicotinamide adenine dinucleotide oxidase system, of D-lactate dehydrogenase (EC 1.1.1.28) and of succinate dehydrogenase (EC 1.3.99.1) at specific activities comparable to, or in some cases considerably higher than, those present in cytoplasmic membranes of the bacterial form. Of two peptidoglycan DD-carboxypeptidase/transpeptidases (EC 3.4.17.8 and EC 2.3.2.10). which are normally present in the cytoplasmic membrane of the bacterial form of P. mirabilis, the membrane of the protoplast L-form contained only one. Electron microscopy of thin sectioned L-form protoplasts showed extensive heterogeneity of membraneous structures. In addition to the single membraneous integument, internal membrane-bounded vesicles and multiple stacks of membranes were present, as the result of unbalanced growth and membrane synthesis in the L-form state.

Identification of peptide-cross-linked trisdisaccharide peptide trimers in murein of Escherichia coli

Purified murein from Escherichia coli K-12 was degraded into disaccharide peptide fragments by endo-N-acetylmuramidase from Chalaropsis. About 5% of the total murein fragments were recovered as peptide-cross-linked trisdisaccharide peptide trimers.