Naturally occurring peptidoglycan variants of Streptococcus pneumoniae

Encapsulation of the septal cell wall protects Streptococcus pneumoniae from its major peptidoglycan hydrolase and host defenses

Synthesis of the capsular polysaccharide, a major virulence factor for many pathogenic bacteria, is required for bacterial survival within the infected host. In Streptococcus pneumoniae, Wze, an autophosphorylating tyrosine kinase, and Wzd, a membrane protein required for Wze autophosphorylation, co-localize at the division septum and guarantee the presence of capsule at this subcellular location. To determine how bacteria regulate capsule synthesis, we studied pneumococcal proteins that interact with Wzd and Wze using bacterial two hybrid assays and fluorescence microscopy. We found that Wzd interacts with Wzg, the putative ligase that attaches capsule to the bacterial cell wall, and recruits it to the septal area. This interaction required residue V56 of Wzd and both the transmembrane regions and DNA-PPF domain of Wzg. When compared to the wild type, Wzd null pneumococci lack capsule at midcell, bind the peptidoglycan hydrolase LytA better and are more susceptible to LytA-induced lysis, and are less virulent in a zebrafish embryo infection model. In this manuscript, we propose that the Wzd/Wze pair guarantees full encapsulation of pneumococcal bacteria by recruiting Wzg to the division septum, ensuring that capsule attachment is coordinated with peptidoglycan synthesis. Impairing the encapsulation process, at localized subcellular sites, may facilitate elimination of bacteria by strategies that target the pneumococcal peptidoglycan.

Structure-based modeling and dynamics of MurM, a Streptococcus pneumoniae penicillin resistance determinant present at the cytoplasmic membrane

Branched Lipid II, required for the formation of indirectly crosslinked peptidoglycan, is generated by MurM, a protein essential for high-level penicillin resistance in the human pathogen Streptococcus pneumoniae. We have solved the X-ray crystal structure of Staphylococcus aureus FemX, an isofunctional homolog, and have used this as a template to generate a MurM homology model. Using this model, we perform molecular docking and molecular dynamics to examine the interaction of MurM with the phospholipid bilayer and the membrane-embedded Lipid II substrate. Our model suggests that MurM is associated with the major membrane phospholipid cardiolipin, and experimental evidence confirms that the activity of MurM is enhanced by this phospholipid and inhibited by its direct precursor phosphatidylglycerol. The spatial association of pneumococcal membrane phospholipids and their impact on MurM activity may therefore be critical to the final architecture of peptidoglycan and the expression of clinically relevant penicillin resistance in this pathogen.

Antibacterial mechanism of gold nanoparticles on Streptococcus pneumoniae

Streptococcus pneumoniae is a causal agent of otitis media, pneumonia, meningitis and severe cases of septicemia. This human pathogen infects elderly people and children with a high mortality rate of approximately one million deaths per year worldwide. Antibiotic-resistance of S. pneumoniae strains is an increasingly serious health problem; therefore, new therapies capable of combating pneumococcal infections are indispensable. The application of gold nanoparticles has emerged as an option in the control of bacterial infections; however, the mechanism responsible for bacterial cell lysis remains unclear. Specifically, it has been observed that gold nanoparticles are capable of crossing different structures of the S. pneumoniae cells, reaching the cytosol where inclusion bodies of gold nanoparticles are noticed. In this work, a novel process for the separation of such inclusion bodies that allowed the analysis of the biomolecules such as carbohydrates, lipids and proteins associated with the gold nanoparticles was developed. Then, it was possible to separate and identify proteins associated with the gold nanoparticles, which were suggested as possible candidates that facilitate the interaction and entry of gold nanoparticles into S. pneumoniae cells.

The Cell Wall of Streptococcus pneumoniae

Streptococcus pneumoniae has a complex cell wall that plays key roles in cell shape maintenance, growth and cell division, and interactions with components of the human host. The peptidoglycan has a heterogeneous composition with more than 50 subunits (muropeptides)-products of several peptidoglycan-modifying enzymes. The amidation of glutamate residues in the stem peptide is needed for efficient peptide cross-linking, and peptides with a dipeptide branch prevail in some beta-lactam-resistant strains. The glycan strands are modified by deacetylation of N-acetylglucosamine residues and O-acetylation of N-acetylmuramic acid residues, and both modifications contribute to pneumococcal resistance to lysozyme. The glycan strands carry covalently attached wall teichoic acid and capsular polysaccharide. Pneumococci are unique in that the wall teichoic acid and lipoteichoic acid contain the same unusually complex repeating units decorated with phosphoryl choline residues, which anchor the choline-binding proteins. The structures of lipoteichoic acid and the attachment site of wall teichoic acid to peptidoglycan have recently been revised. During growth, pneumococci assemble their cell walls at midcell in coordinated rounds of cell elongation and division, leading to the typical ovococcal cell shape. Cell wall growth depends on the cytoskeletal FtsA and FtsZ proteins and is regulated by several morphogenesis proteins that also show patterns of dynamic localization at midcell. Some of the key regulators are phosphorylated by StkP and dephosphorylated by PhpP to facilitate robust selection of the division site and plane and to maintain cell shape.

Studying intact bacterial peptidoglycan by proton-detected NMR spectroscopy at 100 kHz MAS frequency

The bacterial cell wall is composed of the peptidoglycan (PG), a large polymer that maintains the integrity of the bacterial cell. Due to its multi-gigadalton size, heterogeneity, and dynamics, atomic-resolution studies are inherently complex. Solid-state NMR is an important technique to gain insight into its structure, dynamics and interactions. Here, we explore the possibilities to study the PG with ultra-fast (100 kHz) magic-angle spinning NMR. We demonstrate that highly resolved spectra can be obtained, and show strategies to obtain site-specific resonance assignments and distance information. We also explore the use of proton-proton correlation experiments, thus opening the way for NMR studies of intact cell walls without the need for isotope labeling.

New Aspects of the Interplay between Penicillin Binding Proteins, murM, and the Two-Component System CiaRH of Penicillin-Resistant Streptococcus pneumoniae Serotype 19A Isolates from Hungary

The Streptococcus pneumoniae clone Hungary^19A-6 expresses unusually high levels of β-lactam resistance, which is in part due to mutations in the MurM gene, encoding a transferase involved in the synthesis of branched peptidoglycan. Moreover, it contains the allele ciaH232, encoding the histidine kinase CiaH (M. Müller, P. Marx, R. Hakenbeck, and R. Brückner, Microbiology 157:3104–3112, 2011, https://doi.org/10.1099/mic.0.053157-0). High-level penicillin resistance primarily requires the presence of low-affinity (mosaic) penicillin binding protein (PBP) genes, as, for example, in strain Hu17, a closely related member of the Hungary^19A-6 lineage. Interestingly, strain Hu15 is β-lactam sensitive due to the absence of mosaic PBPs. This unique situation prompted us to investigate the development of cefotaxime resistance in transformation experiments with genes known to play a role in this phenotype, pbp2x, pbp1a, murM, and ciaH, and penicillin-sensitive recipient strains R6 and Hu15. Characterization of phenotypes, peptidoglycan composition, and CiaR-mediated gene expression revealed several novel aspects of penicillin resistance. The murM gene of strain Hu17 (murM_Hu17), which is highly similar to murM of Streptococcus mitis, induced morphological changes which were partly reversed by ciaH232. murM_Hu17 conferred cefotaxime resistance only in the presence of the pbp2x of strain Hu17 (pbp2x_Hu17). The ciaH232 allele contributed to a remarkable increase in cefotaxime resistance in combination with pbp2x_Hu17 and pbp1a of strain Hu17 (pbp1a_Hu17), accompanied by higher levels of expression of CiaR-regulated genes, documenting that ciaH232 responds to PBP1a_Hu17-mediated changes in cell wall synthesis. Most importantly, the proportion of branched peptides relative to the proportion of linear muropeptides increased in cells containing mosaic PBPs, suggesting an altered enzymatic activity of these proteins.

Dissecting Bacterial Cell Wall Entry and Signaling in Eukaryotic Cells: an Actin-Dependent Pathway Parallels Platelet-Activating Factor Receptor-Mediated Endocytosis

The Gram-positive bacterial cell wall (CW) peptidoglycan-teichoic acid complex is released into the host environment during bacterial metabolism or death. It is a highly inflammatory Toll-like receptor 2 (TLR2) ligand, and previous in vivo studies have demonstrated its ability to recapitulate pathological features of pneumonia and meningitis. We report that an actin-dependent pathway is involved in the internalization of the CW by epithelial and endothelial cells, in addition to the previously described platelet-activating factor receptor (PAFr)-dependent uptake pathway. Unlike the PAFr-dependent pathway, which is mediated by clathrin and dynamin and does not lead to signaling, the alternative pathway is sensitive to 5-(N-ethyl-N-isopropyl) amiloride (EIPA) and engenders Rac1, Cdc42, and phosphatidylinositol 3-kinase (PI3K) signaling. Upon internalization by this macropinocytosis-like pathway, CW is trafficked to lysosomes. Intracellular CW trafficking is more complex than previously recognized and suggests multiple points of interaction with and without innate immune signaling.

Streptococcus pneumoniae is a major human pathogen infecting the respiratory tract and brain. It is an established model organism for understanding how infection injures the host. During infection or bacterial growth, bacteria shed their cell wall (CW) into the host environment and trigger inflammation. A previous study has shown that CW enters and crosses cell barriers by interacting with a receptor on the surfaces of host cells, termed platelet-activating factor receptor (PAFr). In the present study, by using cells that are depleted of PAFr, we identified a second pathway with features of macropinocytosis, which is a receptor-independent fluid uptake mechanism by cells. Each pathway contributes approximately the same amount of cell wall trafficking, but the PAFr pathway is silent, while the new pathway appears to contribute to the host inflammatory response to CW insult.

Toll-like receptor-dependent discrimination of streptococci

Streptococcus pneumoniae and Streptococcus agalactiae cause distinct infectious diseases in small children. Similarly, these bacteria elicit very different host-cell responses in vitro. Inactivated S. agalactiae by far exceeds S. pneumoniae in the activation of inflammatory cytokines and upstream signaling intermediates such as the MAP kinase JNK. The inflammatory response to both Streptococcus spp. is mediated by MyD88, an essential adapter protein of Toll-like receptors (TLRs), although the specific TLRs that are involved have not been fully resolved. Furthermore, during logarithmic growth, S. pneumoniae releases pneumolysin that interacts with TLR4 whereas S. agalactiae releases diacylated molecules that interact with TLR2/6. Interaction of these soluble bacterial products with their cognate TLRs is critical for limiting bacterial dissemination and and systemic inflammation in mice. This might be due, in part, to TLR-mediated apoptosis induced by these factors. In conclusion related streptococcal species induce specific events in TLR-mediated signal transduction. Comparative analysis of the host-cell response to these bacteria reveals molecules such as JNK as valuable targets for adjunctive sepsis therapy.

Transfer of penicillin resistance from Streptococcus oralis to Streptococcus pneumoniae identifies murE as resistance determinant

Beta-lactam resistant clinical isolates of Streptococcus pneumoniae contain altered penicillin-binding protein (PBP) genes and occasionally an altered murM, presumably products of interspecies gene transfer. MurM and MurN are responsible for the synthesis of branched lipid II, substrate for the PBP catalyzed transpeptidation reaction. Here we used the high-level beta-lactam resistant S. oralis Uo5 as donor in transformation experiments with the sensitive laboratory strain S. pneumoniae R6 as recipient. Surprisingly, piperacillin-resistant transformants contained no alterations in PBP genes but carried murEUo5 encoding the UDP-N-acetylmuramyl tripeptide synthetase. Codons 83-183 of murEUo5 were sufficient to confer the resistance phenotype. Moreover, the promoter of murEUo5 , which drives a twofold higher expression compared to that of S. pneumoniae R6, could also confer increased resistance. Multiple independent transformations produced S. pneumoniae R6 derivatives containing murEUo5 , pbp2xUo5 , pbp1aUo5 and pbp2bUo5 , but not murMUo5 sequences; however, the resistance level of the donor strain could not be reached. S. oralis Uo5 harbors an unusual murM, and murN is absent. Accordingly, the peptidoglycan of S. oralis Uo5 contained interpeptide bridges with one L-Ala residue only. The data suggest that resistance in S. oralis Uo5 is based on a complex interplay of distinct PBPs and other enzymes involved in peptidoglycan biosynthesis.

Envelope Structures of Gram-Positive Bacteria

Gram-positive organisms, including the pathogens Staphylococcus aureus, Streptococcus pneumoniae, and Enterococcus faecalis, have dynamic cell envelopes that mediate interactions with the environment and serve as the first line of defense against toxic molecules. Major components of the cell envelope include peptidoglycan (PG), which is a well-established target for antibiotics, teichoic acids (TAs), capsular polysaccharides (CPS), surface proteins, and phospholipids. These components can undergo modification to promote pathogenesis, decrease susceptibility to antibiotics and host immune defenses, and enhance survival in hostile environments. This chapter will cover the structure, biosynthesis, and important functions of major cell envelope components in gram-positive bacteria. Possible targets for new antimicrobials will be noted.

Bacteriocin protein BacL1 of Enterococcus faecalis targets cell division loci and specifically recognizes L-Ala2-cross-bridged peptidoglycan

Bacteriocin 41 (Bac41) is produced from clinical isolates of Enterococcus faecalis and consists of two extracellular proteins, BacL1 and BacA. We previously reported that BacL1 protein (595 amino acids, 64.5 kDa) is a bacteriolytic peptidoglycan D-isoglutamyl-L-lysine endopeptidase that induces cell lysis of E. faecalis when an accessory factor, BacA, is copresent. However, the target of BacL1 remains unknown. In this study, we investigated the targeting specificity of BacL1. Fluorescence microscopy analysis using fluorescent dye-conjugated recombinant protein demonstrated that BacL1 specifically localized at the cell division-associated site, including the equatorial ring, division septum, and nascent cell wall, on the cell surface of target E. faecalis cells. This specific targeting was dependent on the triple repeat of the SH3 domain located in the region from amino acid 329 to 590 of BacL1. Repression of cell growth due to the stationary state of the growth phase or to treatment with bacteriostatic antibiotics rescued bacteria from the bacteriolytic activity of BacL1 and BacA. The static growth state also abolished the binding and targeting of BacL1 to the cell division-associated site. Furthermore, the targeting of BacL1 was detectable among Gram-positive bacteria with an L-Ala-L-Ala-cross-bridging peptidoglycan, including E. faecalis, Streptococcus pyogenes, or Streptococcus pneumoniae, but not among bacteria with alternate peptidoglycan structures, such as Enterococcus faecium, Enterococcus hirae, Staphylococcus aureus, or Listeria monocytogenes. These data suggest that BacL1 specifically targets the L-Ala-L-Ala-cross-bridged peptidoglycan and potentially lyses the E. faecalis cells during cell division.

Structure of the pneumococcal l,d-carboxypeptidase DacB and pathophysiological effects of disabled cell wall hydrolases DacA and DacB

Bacterial cell wall hydrolases are essential for peptidoglycan turnover and crucial to preserve cell shape. The d,d-carboxypeptidase DacA and l,d-carboxypeptidase DacB of Streptococcus pneumoniae function in a sequential manner. Here, we determined the structure of the surface-exposed lipoprotein DacB. The crystal structure of DacB, radically different to that of DacA, contains a mononuclear Zn(2+) catalytic centre located in the middle of a large and fully exposed groove. Two different conformations were found presenting a different arrangement of the active site topology. The critical residues for catalysis and substrate specificity were identified. Loss-of-function of DacA and DacB altered the cell shape and this was consistent with a modified peptidoglycan peptide composition in dac mutants. Contrary, an lgt mutant lacking lipoprotein diacylglyceryl transferase activity required for proper lipoprotein maturation retained l,d-carboxypeptidase activity and showed an intact murein sacculus. In addition we demonstrated pathophysiological effects of disabled DacA or DacB activities. Real-time bioimaging of intranasal infected mice indicated a substantial attenuation of ΔdacB and ΔdacAΔdacB pneumococci, while ΔdacA had no significant effect. In addition, uptake of these mutants by professional phagocytes was enhanced, while the adherence to lung epithelial cells was decreased. Thus, structural and functional studies suggest DacA and DacB as optimal drug targets.

A highly active and negatively charged Streptococcus pyogenes lysin with a rare D-alanyl-L-alanine endopeptidase activity protects mice against streptococcal bacteremia

Bacteriophage endolysins have shown great efficacy in killing Gram-positive bacteria. PlyC, a group C streptococcal phage lysin, represents the most efficient lysin characterized to date, with a remarkably high specificity against different streptococcal species, including the important pathogen Streptococcus pyogenes. However, PlyC is a unique lysin, in terms of both its high activity and structure (two distinct subunits). We sought to discover and characterize a phage lysin active against S. pyogenes with an endolysin architecture distinct from that of PlyC to determine if it relies on the same mechanism of action as PlyC. In this study, we identified and characterized an endolysin, termed PlyPy (phage lysin from S. pyogenes), from a prophage infecting S. pyogenes. By in silico analysis, PlyPy was found to have a molecular mass of 27.8 kDa and a pI of 4.16. It was active against a majority of group A streptococci and displayed high levels of activity as well as binding specificity against group B and C streptococci, while it was less efficient against other streptococcal species. PlyPy showed the highest activity at neutral pH in the presence of calcium and NaCl. Surprisingly, its activity was not affected by the presence of the group A-specific carbohydrate, while the activity of PlyC was partly inhibited. Additionally, PlyPy was active in vivo and could rescue mice from systemic bacteremia. Finally, we developed a novel method to determine the peptidoglycan bond cleaved by lysins and concluded that PlyPy exhibits a rare d-alanyl-l-alanine endopeptidase activity. PlyPy thus represents the first lysin characterized from Streptococcus pyogenes and has a mechanism of action distinct from that of PlyC.

Streptococcus pneumoniae R6 interspecies transformation: genetic analysis of penicillin resistance determinants and genome-wide recombination events

Interspecies gene transfer has been implicated as the major driving force for the evolution of penicillin resistance in Streptococcus pneumoniae. Genomic alterations of S. pneumoniae R6 introduced during four successive transformations with DNA of the high-level penicillin-resistant Streptococcus mitis B6 with beta-lactam selection have now been determined and the contribution of genes to high resistance levels was analysed genetically. Essential for high level resistance to penicillins of the transformant CCCB was the combination of murM(B) (6) and the 3' region of pbp2b(B) (6) . Sequences of both genes were detected in clinical isolates of S. pneumoniae, confirming the participation of S. mitis in the global gene pool of beta-lactam resistance determinants. The S. mitis PBP1b gene which contains an authentic stop codon within the transpeptidase domain is now shown to contribute only marginal to resistance, but it is possible that the presence of its transglycosylase domain is important in the context of cognate PBPs. The genome sequence of CCCB revealed 36 recombination events, including deletion and acquisition of genes and repeat elements. A total of 78 genes were affected representing 67 kb or 3.3% of the genome, documenting extensive alterations scattered throughout the genome.

Structure of a peptidoglycan amidase effector targeted to Gram-negative bacteria by the type VI secretion system

The target range of a bacterial secretion system can be defined by effector substrate specificity or by the efficacy of effector delivery. Here, we report the crystal structure of Tse1, a type VI secretion (T6S) bacteriolytic amidase effector from Pseudomonas aeruginosa. Consistent with its role as a toxin, Tse1 has a more accessible active site than related housekeeping enzymes. The activity of Tse1 against isolated peptidoglycan shows its capacity to act broadly against Gram-negative bacteria and even certain Gram-positive species. Studies with intact cells indicate that Gram-positive bacteria can remain vulnerable to Tse1 despite cell wall modifications. However, interbacterial competition studies demonstrate that Tse1-dependent lysis is restricted to Gram-negative targets. We propose that the previously observed specificity for T6S against Gram-negative bacteria is a consequence of high local effector concentration achieved by T6S-dependent targeting to its site of action rather than inherent effector substrate specificity.

Substrate specificity of low-molecular mass bacterial DD-peptidases

The bacterial DD-peptidases or penicillin-binding proteins (PBPs) catalyze the formation and regulation of cross-links in peptidoglycan biosynthesis. They are classified into two groups, the high-molecular mass (HMM) and low-molecular mass (LMM) enzymes. The latter group, which is subdivided into classes A-C (LMMA, -B, and -C, respectively), is believed to catalyze DD-carboxypeptidase and endopeptidase reactions in vivo. To date, the specificity of their reactions with particular elements of peptidoglycan structure has not, in general, been defined. This paper describes the steady-state kinetics of hydrolysis of a series of specific peptidoglycan-mimetic peptides, representing various elements of stem peptide structure, catalyzed by a range of LMM PBPs (the LMMA enzymes, Escherichia coli PBP5, Neisseria gonorrhoeae PBP4, and Streptococcus pneumoniae PBP3, and the LMMC enzymes, the Actinomadura R39 dd-peptidase, Bacillus subtilis PBP4a, and N. gonorrhoeae PBP3). The R39 enzyme (LMMC), like the previously studied Streptomyces R61 DD-peptidase (LMMB), specifically and rapidly hydrolyzes stem peptide fragments with a free N-terminus. In accord with this result, the crystal structures of the R61 and R39 enzymes display a binding site specific to the stem peptide N-terminus. These are water-soluble enzymes, however, with no known specific function in vivo. On the other hand, soluble versions of the remaining enzymes of those noted above, all of which are likely to be membrane-bound and/or associated in vivo and have been assigned particular roles in cell wall biosynthesis and maintenance, show little or no specificity for peptides containing elements of peptidoglycan structure. Peptidoglycan-mimetic boronate transition-state analogues do inhibit these enzymes but display notable specificity only for the LMMC enzymes, where, unlike peptide substrates, they may be able to effectively induce a specific active site structure. The manner in which LMMA (and HMM) DD-peptidases achieve substrate specificity, both in vitro and in vivo, remains unknown.

A computational evaluation of the mechanism of penicillin-binding protein-catalyzed cross-linking of the bacterial cell wall

Penicillin-binding protein 1b (PBP 1b) of the gram-positive bacterium Streptococcus pneumoniae catalyzes the cross-linking of adjacent peptidoglycan strands, as a critical event in the biosynthesis of its cell wall. This enzyme is representative of the biosynthetic PBP structures of the β-lactam-recognizing enzyme superfamily and is the target of the β-lactam antibiotics. In the cross-linking reaction, the amide between the -D-Ala-D-Ala dipeptide at the terminus of a peptide stem acts as an acyl donor toward the ε-amino group of a lysine found on an adjacent stem. The mechanism of this transpeptidation was evaluated using explicit-solvent molecular dynamics simulations and ONIOM quantum mechanics/molecular mechanics calculations. Sequential acyl transfer occurs to, and then from, the active site serine. The resulting cross-link is predicted to have a cis-amide configuration. The ensuing and energetically favorable cis- to trans-amide isomerization, within the active site, may represent the key event driving product release to complete enzymatic turnover.

Insights into pneumococcal fratricide from the crystal structures of the modular killing factor LytC

The first structure of a pneumococcal autolysin, that of the LytC lysozyme, has been solved in ternary complex with choline and a pneumococcal peptidoglycan (PG) fragment. The active site of the hydrolase module is not fully exposed but is oriented toward the choline-binding module, which accounts for its unique in vivo features in PG hydrolysis, its activation and its regulatory mechanisms. Because of the unusual hook-shaped conformation of the multimodular protein, it is only able to hydrolyze non-cross-linked PG chains, an assertion validated by additional experiments. These results explain the activation of LytC by choline-binding protein D (CbpD) in fratricide, a competence-programmed mechanism of predation of noncompetent sister cells. The results provide the first structural insights to our knowledge into the critical and central function that LytC plays in pneumococcal virulence and explain a long-standing puzzle of how murein hydrolases can be controlled to avoid self-lysis during bacterial growth and division.

Influences of capsule on cell shape and chain formation of wild-type and pcsB mutants of serotype 2 Streptococcus pneumoniae

PcsB is a protein of unknown function that plays a critical role in cell division in Streptococcus pneumoniae and other ovococcus species of Streptococcus. We constructed isogenic sets of mutants expressing different amounts of PcsB in laboratory strain R6 and virulent serotype 2 strain D39 to evaluate its cellular roles. Insertion mutagenesis in parent and pcsB(+) merodiploid strains indicated that pcsB is essential in serotype 2 S. pneumoniae. Quantitative Western blotting of wild-type and epitope-tagged PcsB showed that all PcsB was processed into cell-associated and secreted forms of the same molecular mass and that cell-associated PcsB was moderately abundant and present at approximately 4,900 monomers per cell. Controlled expression and complementation experiments indicated that there was a causative relationship between the severity of defects in cell division and decreasing PcsB amount. These experiments also showed that perturbations of expression of the upstream mreCD genes did not contribute to the cell division defects of pcsB mutants and that mreCD could be deleted. Unexpectedly, capsule influenced the cell shape and chain formation phenotypes of the wild-type D39 strain and mutants underexpressing PcsB or deleted for other genes involved in peptidoglycan biosynthesis, such as dacA. Underexpression of PcsB did not result in changes in the amounts or composition of lactoyl-peptides, which were markedly different in the R6 and D39 strains, and there was no correlation between decreased PcsB amount and sensitivity to penicillin. Finally, microarray analyses indicated that underexpression of PcsB may generate a signal that increases expression of the VicRK regulon, which includes pcsB.

PBP active site flexibility as the key mechanism for beta-lactam resistance in pneumococci

Penicillin-binding proteins (PBPs), the main targets of beta-lactam antibiotics, are membrane-associated enzymes that catalyze the two last steps in the biosynthesis of peptidoglycan. In Streptococcus pneumoniae, a major human pathogen, the surge in resistance to such antibiotics is a direct consequence of the proliferation of mosaic PBP-encoding genes, which give rise to proteins containing tens of mutations. PBP2b is a major drug resistance target, and its modification is essential for the development of high levels of resistance to piperacillin. In this work, we have solved the crystal structures of PBP2b from a wild-type pneumococcal strain, as well as from a highly drug-resistant clinical isolate displaying 58 mutations. Although mutations are present throughout the entire PBP structure, those surrounding the active site influence the total charge and the polar character of the region, while those in close proximity to the catalytic nucleophile impart flexibility onto the beta3/beta4 loop area, which encapsulates the cleft. The wealth of structural data on pneumococcal PBPs now underlines the importance of high malleability in active site regions of drug-resistant strains, suggesting that active site "breathing" could be a common mechanism employed by this pathogen to prevent targeting by beta-lactams.

Use of 4-sulfophenyl isothiocyanate labeling and mass spectrometry to determine the site of action of the streptococcolytic peptidoglycan hydrolase zoocin A

Zoocin A is a streptococcolytic peptidoglycan hydrolase with an unknown site of action that is produced by Streptococcus equi subsp. zooepidemicus 4881. Zoocin A has now been determined to be a d-alanyl-l-alanine endopeptidase by digesting susceptible peptidoglycan with a combination of mutanolysin and zoocin A, separating the resulting muropeptides by reverse-phase high-pressure liquid chromatography, and analyzing them by mass spectrometry (MS) in both the positive- and negative-ion modes to determine their compositions. In order to distinguish among possible structures for these muropeptides, they were N-terminally labeled with 4-sulfophenyl isothiocyanate (SPITC) and analyzed by tandem MS in the negative-ion mode. This novel application of SPITC labeling and MS/MS analysis can be used to analyze the structure of peptidoglycans and to determine the sites of action of other peptidoglycan hydrolases.

Molecular control of bacterial death and lysis

Although the phenomenon of bacterial cell death and lysis has been studied for over 100 years, the contribution of these important processes to bacterial physiology and development has only recently been recognized. Contemporary study of cell death and lysis in a number of different bacteria has revealed that these processes, once thought of as being passive and unregulated, are actually governed by highly complex regulatory systems. An emerging paradigm in this field suggests that, analogous to programmed cell death in eukaryotes, regulated cell death and lysis in bacteria play an important role in both developmental processes, such as competence and biofilm development, and the elimination of damaged cells, such as those irreversibly injured by environmental or antibiotic stress. Further study in this exciting field of bacterial research may provide new insight into the potential evolutionary link between control of cell death in bacteria and programmed cell death (apoptosis) in eukaryotes.

Characterization of tRNA-dependent peptide bond formation by MurM in the synthesis of Streptococcus pneumoniae peptidoglycan

MurM is an aminoacyl ligase that adds l-serine or l-alanine as the first amino acid of a dipeptide branch to the stem peptide lysine of the pneumococcal peptidoglycan. MurM activity is essential for clinical pneumococcal penicillin resistance. Analysis of peptidoglycan from the highly penicillin-resistant Streptococcus pneumoniae strain 159 revealed that in vivo and in vitro, in the presence of the appropriate acyl-tRNA, MurM(159) alanylated the peptidoglycan epsilon-amino group of the stem peptide lysine in preference to its serylation. However, in contrast, identical analyses of the penicillin-susceptible strain Pn16 revealed that MurM(Pn16) activity supported serylation more than alanylation both in vivo and in vitro. Interestingly, both MurM(Pn16) acylation activities were far lower than the alanylation activity of MurM(159). The resulting differing stem peptide structures of 159 and Pn16 were caused by the profoundly greater catalytic efficiency of MurM(159) compared with MurM(Pn16) bought about by sequence variation between these enzymes and, to a lesser extent, differences in the in vivo tRNA(Ala):tRNA(Ser) ratio in 159 and Pn16. Kinetic analysis revealed that MurM(159) acted during the lipid-linked stages of peptidoglycan synthesis, that the d-alanyl-d-alanine of the stem peptide and the lipid II N-acetylglucosaminyl group were not essential for substrate recognition, that epsilon-carboxylation of the lysine of the stem peptide was not tolerated, and that lipid II-alanine was a substrate, suggesting an evolutionary link to staphylococcal homologues of MurM such as FemA. Kinetic analysis also revealed that MurM recognized the acceptor stem and/or the TPsiC loop stem of the tRNA(Ala). It is anticipated that definition of the minimal structural features of MurM substrates will allow development of novel resistance inhibitors that will restore the efficacy of beta-lactams for treatment of pneumococcal infection.

Attenuation of penicillin resistance in a peptidoglycan O-acetyl transferase mutant of Streptococcus pneumoniae

The level of penicillin resistance in clinical isolates of Streptococcus pneumoniae depends not only on the reduced affinity of penicillin binding proteins (PBPs) but also on the functioning of enzymes that modify the stem peptide structure of cell wall precursors. We used mariner mutagenesis in search of additional genetic determinants that may further attenuate the level of penicillin resistance in the bacteria. A mariner mutant of the highly penicillin-resistant S. pneumoniae strain Pen6 showed reduction of the penicillin minimum inhibitory concentration (MIC) from 6 to 0.75 microg ml(-1). Decrease in penicillin MIC was also observed upon introduction of the mutation (named provisionally adr, for attenuator of drug resistance) into representatives of major epidemic clones of penicillin-resistant pneumococci. Attenuation of resistance levels was specific for beta-lactams. The adr mutant has retained unchanged (low affinity) PBPs, unaltered murM gene and unchanged cell wall stem peptide composition, but the mutant became hypersensitive to exogenous lysozyme and complementation experiments showed that both phenotypes--reduced resistance and lysozyme sensitivity--were linked to the defective adr gene. DNA sequence comparison and chemical analysis of the cell wall identified adr as the structural gene of the pneumococcal peptidoglycan O-acetylase.

Nucleotide-binding oligomerization domain proteins are innate immune receptors for internalized Streptococcus pneumoniae

Streptococcus pneumoniae, the major cause of community-acquired pneumonia and bacterial meningitis, has been shown to transiently invade epithelial and endothelial cells. Innate immune receptors including Toll-like receptors recognize various pathogens, such as S. pneumoniae, by identifying conserved pathogen-associated molecular patterns. Recently, two members of a novel class of pattern recognition receptors, the cytosolic proteins nucleotide-binding oligomerization domain 1 (Nod1)/CARD4 and Nod2/CARD15, have been found to detect cell wall peptidoglycans. Here we tested the hypothesis that Nod proteins are involved in the intracellular recognition of pneumococci. Data indicate that pneumococci invade HEK293 cells. Genetic complementation studies in these cells demonstrate that NF-kappaB activation induced by S. pneumoniae depends on Nod2. Moreover, intracellular transfection of inactivated pneumococci yielded similar effects, confirming the Nod2 dependence of NF-kappaB activation by pneumococci in HEK293 cells. By dominant negative overexpression and small interfering RNA experiments, we show for the first time that interleukin-1 receptor-associated kinase participates in Nod2-dependent NF-kappaB activation. Additionally, dominant negative interleukin-1 receptor-associated kinase 2, tumor necrosis factor receptor-associated factor 6, NF-kappaB-inducing kinase, transforming growth factor-beta-activated kinase-binding protein 2, and transforming growth factor-beta-activated kinase 1 also inhibited Nod2-dependent NF-kappaB activation. We finally demonstrate that in C57BL/6 mouse lung tissue in vivo as well as in the bronchial epithelial cell line BEAS-2B, Nod1 and Nod2 mRNA expressions were up-regulated after pneumococcal infection. Data presented suggest that Nod proteins contribute to innate immune recognition of S. pneumoniae. Furthermore, Rip-2 and members of the Toll-like receptor-signaling cascade are involved in the Nod2-dependent activation of NF-kappaB induced by pneumococci.

Correlation between alterations of the penicillin-binding protein 2 and modifications of the peptidoglycan structure in Neisseria meningitidis with reduced susceptibility to penicillin G

Reduced susceptibility to penicillin G in Neisseria meningitidis is directly correlated with alterations in the penA gene, which encodes the penicillin-binding protein 2 (PBP2). Using purified PBP2s from different backgrounds, we confirmed that the reduced susceptibility to penicillin G is associated with a decreased affinity of altered PBP2s for penicillin G. Infrared spectroscopy analysis using isogenic penicillin-susceptible strains and strains with reduced susceptibility to penicillin G suggested that the meningococcal cell wall is also modified in a penA-dependent manner. Moreover, reverse-phase high pressure liquid chromatography and mass spectrometry analysis of these meningococcal strains confirmed the modifications of peptidoglycan components and showed an increase in the peaks corresponding to pentapeptide-containing muropeptides. These results suggest that the D,D-transpeptidase and/or D,D-carboxypeptidase activities of PBP2 are modified by the changes in penA gene.

Recognition of pneumococcal peptidoglycan: an expanded, pivotal role for LPS binding protein

Lipopolysaccharide binding protein (LBP) has a well-established role in LPS-induced immune responses. Here, we report that LBP also plays an essential role in the innate immune response to Gram-positive pneumococci, specifically to their major inflammatory component, pneumococcal cell wall (PCW). LBP was present in the CSF of patients with meningitis, and LBP-deficient mice failed to develop meningeal inflammation. LBP enhanced PCW-induced cell signaling and TNF-alpha release. LBP bound specifically to PCW multimers, indicating novel lipid-independent binding capability for LBP. We propose the iterative anionic groups along the glycan backbone of the cell wall are a crucial structure for recognition by LBP. Such a function for LBP expands its role to Gram-positive infections.

Allelic variation in srtAs of Streptococcus suis strains

Streptococcus suis NCTC10234 possesses five srtA homologs: srtA encodes sortase, which anchors surface proteins with an LPXTG motif to the cell wall, while the functions of the other four homologs (the srtBCD cluster and srtE) remain unknown. The genetic organization of the srtA region was found to be conserved in the 59 S. suis strains examined in this study. Although the srtAs in three of these strains showed strong sequence divergence, their functions were verified to be overlapping by genetic complementation, indicating the functional conservation of srtAs during the evolution of these strains. These results indicate the importance of an srtA-mediated cell wall sorting system for displaying proteins on the surface of S. suis.

Antibiotic tolerance in pneumococci

When bacteria such as Staphylococcus aureus and Streptococcus pneumoniae are exposed to lytic antibiotics such as penicillin and vancomycin, a self-induced killing process is initiated in the organism. This killing occurs via both non-lytic and lytic processes. Recent data suggest that the non-lytic killing system, which might affect the cytoplasmic membrane, secondarily activates murein hydrolases that eventually lyse the cell. Disturbances in this suicide pathway can lead to antibiotic tolerance, a process whereby the antibiotic still exerts its bacteriostatic effects but the self-induced killing system is impaired. In mutants obtained in vitro, signaling pathways have been affected that show either increased or decreased antibiotic-induced killing. Among clinical isolates of S. pneumoniae that are tolerant to penicillin and/or vancomycin, we do not yet know whether these signaling pathways are affected. We could, however, demonstrate that the activity of murein hydrolases is negatively controlled by the production of capsular polysaccharides in one vancomycin-tolerant isolate. Hence, type and level of capsular expression might constitute one factor that determines the degree of lysis, once the killing signal has been elicited by the antibiotic.

The role of murMN operon in penicillin resistance and antibiotic tolerance of Streptococcus pneumoniae

The recently identified murMN operon is essential for the production of branched-structured muropeptides in the cell wall and also for the expression of the resistant phenotype in penicillin-resistant strains of Streptococcus pneumoniae. The purpose of studies described in this communication was to understand better the role of murMN in penicillin resistance. Deletion of murM in the penicillin-resistant strain Pen6, which causes reduction in the penicillin MIC from 6.0 to 0.03 microg/ml, was successfully complemented to recover the original high level of penicillin resistance in transformants that received functional murM alleles cloned in plasmid pLS578. Inactivation of penicillin resistance was not accompanied by any detectable change in the low affinity or abnormal molecular size pattern of the penicillin-binding proteins (PBPs) nor in the mosaic sequence of PBP2X typical of resistant strain Pen6. Exposure of strain Pen6 with inactivated murM to 0.05 microg/ml of penicillin (i.e., a concentration more than 100 times below the MIC of the parental strain) initiated a phenotypic response typical of penicillin-susceptible strains of pneumococci: inhibition of growth followed by rapid and extensive loss of viability and lysis. Unexpectedly, inactivation of murMN also caused hypersensitivity to lysis by low concentrations of a variety of cell wall active antibiotics such as fosfomycin, D-cycloserine, and nisin, suggesting that the murMN operon may perform an important regulatory role in the control of the irreversible antimicrobial effects of cell wall inhibitors.

The murMN operon: a functional link between antibiotic resistance and antibiotic tolerance in Streptococcuspneumoniae

Inactivation of the recently identified murMN operon in penicillin-resistant strains of Streptococcus pneumoniae was shown already to cause two major effects: elimination of branched-structured muropeptides from the cell wall and complete loss of penicillin resistance. We now show that cells with inactivated murMN also have a third phenotype: an increased susceptibility to lysis when exposed to low concentrations of fosfomycin, d-cycloserine, vancomycin, and nisin, indicating a wide-spectrum hypersensitivity to inhibitors of both early and late stages of cell wall biosynthesis. Mutants of murMN also lysed faster than the parental strain when treated with the detergent deoxycholate. Several different alleles of murM cloned in plasmid pLS578 and introduced into a murM deletion mutant of the penicillin-resistant strain Pen6 were able to reconstitute each one of the three mutant phenotypes: the highly branched cell wall structure, original high level of penicillin resistance, and normal sensitivity to lysis. In a penicillin-susceptible strain the same experiments caused increased concentration of cell wall branched peptides and suppression of sensitivity to antibiotic induced lysis. The observations suggest that the murMN operon plays a key role in the regulation of a stress-response pathway that can be triggered by perturbation of cell wall biosynthesis in S. pneumoniae.

Crystal structure of PBP2x from a highly penicillin-resistant Streptococcus pneumoniae clinical isolate: a mosaic framework containing 83 mutations

Penicillin-binding proteins (PBPs) are the main targets for beta-lactam antibiotics, such as penicillins and cephalosporins, in a wide range of bacterial species. In some Gram-positive strains, the surge of resistance to treatment with beta-lactams is primarily the result of the proliferation of mosaic PBP-encoding genes, which encode novel proteins by recombination. PBP2x is a primary resistance determinant in Streptococcus pneumoniae, and its modification is an essential step in the development of high level beta-lactam resistance. To understand such a resistance mechanism at an atomic level, we have solved the x-ray crystal structure of PBP2x from a highly penicillin-resistant clinical isolate of S. pneumoniae, Sp328, which harbors 83 mutations in the soluble region. In the proximity of the Sp328 PBP2x* active site, the Thr(338) --> Ala mutation weakens the local hydrogen bonding network, thus abrogating the stabilization of a crucial buried water molecule. In addition, the Ser(389) --> Leu and Asn(514) --> His mutations produce a destabilizing effect that generates an "open" active site. It has been suggested that peptidoglycan substrates for beta-lactam-resistant PBPs contain a large amount of abnormal, branched peptides, whereas sensitive strains tend to catalyze cross-linking of linear forms. Thus, in vivo, an "open" active site could facilitate the recognition of distinct, branched physiological substrates.

Functional analysis of Streptococcus pneumoniae MurM reveals the region responsible for its specificity in the synthesis of branched cell wall peptides

The recently identified murMN operon of Streptococcus pneumoniae encodes enzymes involved in the synthesis of branched structured muropeptides of the pneumococcal cell wall peptidoglycan. Its inactivation was shown to cause production of a peptidoglycan composed exclusively of linear muropeptides and a virtually complete loss of resistance in penicillin-resistant strains. The studies described in this communication follow up these observations in several directions. The substrate of the MurM-catalyzed reaction (addition of alanine or serine) was identified as the lipid-linked N-acetylglucosamine-muramyl pentapeptide. Different murM alleles from several penicillin-resistant S. pneumoniae strains, each with a characteristic branched peptide pattern, were cloned into pLS578, a pneumococcal plasmid capable of replicating in S. pneumoniae, and transformed into the penicillin-susceptible laboratory strain R36A. All transformants remained penicillin-susceptible; however, their cell wall composition changed in directions corresponding to the muropeptide pattern of the strain from which the murM allele was derived. This suggests that the muropeptide composition of the pneumococcal cell walls is determined by the particular murM allele carried by the cells. A 30-amino acid long sequence within the MurM protein was shown to be the main determinant of the specificity of the reaction (addition of alanine versus serine).

Distribution of the mosaic structured murM genes among natural populations of Streptococcus pneumoniae

The presence and sequence variation of the murM gene were studied in a large collection (814 strains) of genetically diverse Streptococcus pneumoniae isolates, which included 27 different serogroups and both penicillin-resistant (423 isolates, 67 pulsed-field gel electrophoretic [PFGE] types) and intermediately penicillin-resistant (165 isolates, 66 PFGE types) and penicillin-susceptible (226 isolates, 135 PFGE types) strains. Diversity of the murM sequences was tested by hybridization with mainly two kinds of probes: one derived from the amplification of the nucleotide sequence between nucleotides 201 and 624 in the penicillin-susceptible laboratory strain R36A (murMA probe) and a second probe that amplified the comparable, highly divergent sequence in the penicillin-resistant strain Pen6 (murMB probe). The great majority of the strains (761 of 814), including both penicillin-susceptible and penicillin-resistant isolates, reacted exclusively with the murMA probe. A smaller group of penicillin-resistant strains (48 of 814 isolates) reacted only with the murMB DNA probe, and an additional 5 isolates reacted with both probes. High-pressure liquid chromatography analysis of the peptidoglycan of strains hybridizing with murMB showed that they invariably contained an increased proportion of branched peptides. Complete sequencing of murM from a group of penicillin-resistant isolates allowed the identification of a number of different murMB alleles that differed in the length and exact position of the divergent (Pen6 type) sequences within the particular murM. The close similarity of these divergent sequences in the various murM alleles suggests a possible common heterologous origin.

Characterization of the murMN operon involved in the synthesis of branched peptidoglycan peptides in Streptococcus pneumoniae

The murMN operon, recently identified in the genome of Streptococcus pneumoniae, encodes for enzymes involved in the synthesis of branched structured muropeptides in the pneumococcal peptidoglycan; inactivation of murMN causes production of a peptidoglycan composed exclusively of linear muropeptides and a virtually complete loss of resistance in penicillin-resistant strains (Filipe, S. R., and Tomasz, A. (2000) Proc. Natl. Acad. Sci. U. S. A. 97, 4891-4896). The experiments described in this paper follow up these observations. Primer extension analysis was used to identify the putative promoter region of the murMN operon in penicillin-susceptible and -resistant strains. Selective inactivation of the murN gene in the penicillin-resistant strain Pen6 caused production of an unusual peptidoglycan that contained only single amino acid residues in the muropeptide branches, indicating that the product of murN was involved with the addition of the second amino acid and the product of murM was involved with the addition of the first amino acid (alanine or serine) to the peptidoglycan cross-bridge. Allelic replacement of the mosaic murM gene of strain Pen6 with murM of the penicillin-susceptible laboratory strain caused enrichment of the peptidoglycan in linear muropeptides. The findings suggest that the genetic determinant primarily controlling the synthesis of branched muropeptides in the pneumococcal peptidoglycan is murM.

The pgdA gene encodes for a peptidoglycan N-acetylglucosamine deacetylase in Streptococcus pneumoniae

Analytical work on the fractionation of the glycan strands of Streptococcus pneumoniae cell wall has led to the observation that an unusually high proportion of hexosamine units (over 80% of the glucosamine and 10% of the muramic acid residues) was not N-acetylated, explaining the resistance of the peptidoglycan to the hydrolytic action of lysozyme, a muramidase that cleaves in the glycan backbone. A gene, pgdA, was identified as encoding for the peptidoglycan N-acetylglucosamine deacetylase A with amino acid sequence similarity to fungal chitin deacetylases and rhizobial NodB chitooligosaccharide deacetylases. Pneumococci in which pgdA was inactivated by insertion duplication mutagenesis produced fully N-acetylated glycan and became hypersensitive to exogenous lysozyme in the stationary phase of growth. The pgdA gene may contribute to pneumococcal virulence by providing protection against host lysozyme, which is known to accumulate in high concentrations at infection sites.

The fib locus in Streptococcus pneumoniae is required for peptidoglycan crosslinking and PBP-mediated beta-lactam resistance

Penicillin resistance in pneumococci is mediated by modified penicillin-binding proteins (PBPs) that have decreased affinity to beta-lactams. In high-level penicillin-resistant transformants of the laboratory strain Streptococcus pneumoniae R6 containing various combinations of low-affinity PBPs, disruption of the fib locus results in a collapse of PBP-mediated resistance. In addition, crosslinked muropeptides are highly reduced. The fib operon consists of two genes, fibA and fibB, homologous to Staphylococcus aureus femA/B which are also required for expression of methicillin resistance in this organism. FibA and FibB belong to a family of proteins of Gram-positive bacteria involved in the formation of interpeptide bridges, thus representing interesting new targets for antimicrobial compounds for this group of pathogens.

Inhibition of the expression of penicillin resistance in Streptococcus pneumoniae by inactivation of cell wall muropeptide branching genes

Penicillin-resistant strains of Streptococcus pneumoniae contain low affinity penicillin-binding proteins and often also produce abnormal indirectly crosslinked cell walls. However the relationship between cell wall abnormality and penicillin resistance has remained obscure. We now show that the genome of S. pneumoniae contains an operon composed of two genes (murM and murN) that encode enzymes involved with the biosynthesis of branched structured cell wall muropeptides. The sequences of murMN were compared in two strains: the penicillin-susceptible strain R36A producing the species-specific pneumococcal cell wall peptidoglycan in which branched stem peptides are rare, and the highly penicillin-resistant transformant strain Pen6, the cell wall of which is enriched for branched-structured stem peptides. The two strains carried different murM alleles: murM of the penicillin-resistant strain Pen6 had a "mosaic" structure encoding a protein that was only 86.5% identical to the product of murM identified in the isogenic penicillin-susceptible strain R36A. Mutants of R36A and Pen6 in which the murMN operon was interrupted by insertion-duplication mutagenesis produced peptidoglycan from which all branched muropeptide components were missing. The insertional mutant of Pen6 carried a pbp2x gene with the same "mosaic" sequence found in Pen6. On the other hand, inactivation of murMN in strain Pen6 and other resistant strains caused a virtually complete loss of penicillin resistance. Our observations indicate that the capacity to produce branched cell wall precursors plays a critical role in the expression of penicillin resistance in S. pneumoniae.

Autolysis and cell wall degradation in a choline-independent strain of Streptococcus pneumoniae

Streptococcus pneumoniae has an auxotrophic requirement for choline, and choline residues that incorporate into the wall and membrane teichoic acids are intimately involved with the control of autolytic phenomena of this bacterium. We report here the re-examination of the role of choline in autolytic cell wall degradation using the choline-independent S. pneumoniae strain R6Cho- recovered from a heterologous cross with DNA from Streptococcus oralis. S pneumoniae Cho- cultured in choline-free medium grew with normal generation time but formed long chains, failed to undergo stationary-phase autolysis, and was also resistant to lysis induced by deoxycholate or penicillin. Cell walls produced under these conditions had reduced phosphorus content, contained no choline residues detectable by nuclear magnetic resonance, and had reduced binding capacity for the pneumococcal autolytic amidase, and complete hydrolysis of such walls by the amidase required prolonged incubation with high concentrations of the enzyme. Addition of choline to the growth medium reversed at these phenomena. High-performance liquid chromatography analysis of amidase digests of cell walls prepared from strain R6Cho- grown with or without choline produced identical stem peptide profiles, which were also similar to that of the parental S. pneumoniae strain R6. Peptidoglycans prepared by hydrofluoric extraction of cell walls from Cho- growth with or without choline or from the parental strain R6 were uniformly susceptible to the autolytic amidase and were fully degraded to the normal family of stem peptides, indicating that, in sharp contrast to the case of cell walls, the amidase degradation of teichoic acid-free peptidoglycan did not require the presence of choline residues in the substrate.

Acquisition of five high-Mr penicillin-binding protein variants during transfer of high-level beta-lactam resistance from Streptococcus mitis to Streptococcus pneumoniae

Penicillin-resistant isolates of Streptococcus pneumoniae generally contain mosaic genes encoding the low-affinity penicillin-binding proteins (PBPs) PBP2x, PBP2b, and PBP1a. We now present evidence that PBP2a and PBP1b also appear to be low-affinity variants and are encoded by distinct alleles in beta-lactam-resistant transformants of S. pneumoniae obtained with chromosomal donor DNA from a Streptococcus mitis isolate. Different lineages of beta-lactam-resistant pneumococcal transformants were analyzed, and transformants with low-affinity variants of all high-molecular-mass PBPs, PBP2x, -2a, -2b, -1a, and -1b, were isolated. The MICs of benzyl-penicillin, oxacillin, and cefotaxime for these transformants were up to 40, 100, and 50 microg/ml, respectively, close to the MICs for the S. mitis donor strain. Recruitment of low-affinity PBPs was accompanied by a decrease in cross-linked muropeptides as revealed by high-performance liquid chromatography of muramidase-digested cell walls, but no qualitative changes in muropeptide chemistry were detected. The growth rates of all transformants were identical to that of the parental S. pneumoniae strain. The results stress the potential for the acquisition by S. pneumoniae of high-level beta-lactam resistance by interspecies gene transfer.

Structural characterization of an abnormally cross-linked muropeptide dimer that is accumulated in the peptidoglycan of methicillin- and cefotaxime-resistant mutants of Staphylococcus aureus

Laboratory mutants of Staphylococcus aureus strain ATCC 8325 (27S) selected for increased minimal inhibitory concentration (MIC) values to methicillin and cefotaxime showed increased rates of cell wall turnover and detergent-induced autolysis in virtual parallel with the increasing MIC for the antibiotic. Also in parallel with the increasing MICs for the particular antibiotic used in the selection was the gradual accumulation of an unusual muropeptide in the peptidoglycan of the mutants, muropeptide 12, which is a minor component of the cell wall of the parental strain. Analysis of muropeptide 12, its peptide derivative, and its lysostaphin degradation products by high pressure liquid chromatography, Edman degradation, and mass spectrometry suggests that muropeptide 12 is a dimer in which the two monomeric components are interlinked by two pentaglycyl cross-bridges, thus generating a 14-member macrocyclic ring structure. This unusual cross-linked structure may be the product of the abnormal activity of penicillin-binding protein 2 which has grossly reduced antibiotic binding capacity in the mutant staphylococci.