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In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall.
Peptidoglycan or murein is a unique large macromolecule, a polysaccharide, consisting of sugars and amino acids that forms a mesh-like peptidoglycan layer outside the plasma membrane, the rigid cell wall characteristic of most bacteria. The sugar component consists of alternating residues of β-(1,4) linked N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). Attached to the N-acetylmuramic acid is an oligopeptide chain made of three to five amino acids. The peptide chain can be cross-linked to the peptide chain of another strand forming the 3D mesh-like layer. Peptidoglycan serves a structural role in the bacterial cell wall, giving structural strength, as well as counteracting the osmotic pressure of the cytoplasm. This repetitive linking results in a dense peptidoglycan layer which is critical for maintaining cell form and withstanding high osmotic pressures, and it is regularly replaced by peptidoglycan production. Peptidoglycan hydrolysis and synthesis are two processes that must occur in order for cells to grow and multiply, a technique carried out in three stages: clipping of current material, insertion of new material, and re-crosslinking of existing material to new material.
Teichoic acids are bacterial copolymers of glycerol phosphate or ribitol phosphate and carbohydrates linked via phosphodiester bonds.
The periplasm is a concentrated gel-like matrix in the space between the inner cytoplasmic membrane and the bacterial outer membrane called the periplasmic space in gram-negative bacteria. Using cryo-electron microscopy it has been found that a much smaller periplasmic space is also present in gram-positive bacteria, between cell wall and the plasma membrane.
Lipoteichoic acid (LTA) is a major constituent of the cell wall of gram-positive bacteria. These organisms have an inner membrane and, external to it, a thick peptidoglycan layer. The structure of LTA varies between the different species of Gram-positive bacteria and may contain long chains of ribitol or glycerol phosphate. LTA is anchored to the cell membrane via a diacylglycerol. It acts as regulator of autolytic wall enzymes (muramidases). It has antigenic properties being able to stimulate specific immune response.
The bacterium, despite its simplicity, contains a well-developed cell structure which is responsible for some of its unique biological structures and pathogenicity. Many structural features are unique to bacteria and are not found among archaea or eukaryotes. Because of the simplicity of bacteria relative to larger organisms and the ease with which they can be manipulated experimentally, the cell structure of bacteria has been well studied, revealing many biochemical principles that have been subsequently applied to other organisms.
Pilin refers to a class of fibrous proteins that are found in pilus structures in bacteria. These structures can be used for the exchange of genetic material, or as a cell adhesion mechanism. Although not all bacteria have pili or fimbriae, bacterial pathogens often use their fimbriae to attach to host cells. In Gram-negative bacteria, where pili are more common, individual pilin molecules are linked by noncovalent protein-protein interactions, while Gram-positive bacteria often have polymerized LPXTG pilin.
An isopeptide bond is a type of amide bond formed between a carboxyl group of one amino acid and an amino group of another. An isopeptide bond is the linkage between the side chain amino or carboxyl group of one amino acid to the α-carboxyl, α-amino group, or the side chain of another amino acid. In a typical peptide bond, also known as eupeptide bond, the amide bond always forms between the α-carboxyl group of one amino acid and the α-amino group of the second amino acid. Isopeptide bonds are rarer than regular peptide bonds. Isopeptide bonds lead to branching in the primary sequence of a protein. Proteins formed from normal peptide bonds typically have a linear primary sequence.
Lysins, also known as endolysins or murein hydrolases, are hydrolytic enzymes produced by bacteriophages in order to cleave the host's cell wall during the final stage of the lytic cycle. Lysins are highly evolved enzymes that are able to target one of the five bonds in peptidoglycan (murein), the main component of bacterial cell walls, which allows the release of progeny virions from the lysed cell. Cell-wall-containing Archaea are also lysed by specialized pseudomurein-cleaving lysins, while most archaeal viruses employ alternative mechanisms. Similarly, not all bacteriophages synthesize lysins: some small single-stranded DNA and RNA phages produce membrane proteins that activate the host's autolytic mechanisms such as autolysins.
M protein is a virulence factor that can be produced by certain species of Streptococcus.
Sortase refers to a group of prokaryotic enzymes that modify surface proteins by recognizing and cleaving a carboxyl-terminal sorting signal. For most substrates of sortase enzymes, the recognition signal consists of the motif LPXTG (Leu-Pro-any-Thr-Gly), then a highly hydrophobic transmembrane sequence, followed by a cluster of basic residues such as arginine. Cleavage occurs between the Thr and Gly, with transient attachment through the Thr residue to the active site Cys residue, followed by transpeptidation that attaches the protein covalently to cell wall components. Sortases occur in almost all Gram-positive bacteria and the occasional Gram-negative bacterium or Archaea, where cell wall LPXTG-mediated decoration has not been reported. Although sortase A, the "housekeeping" sortase, typically acts on many protein targets, other forms of sortase recognize variant forms of the cleavage motif, or catalyze the assembly of pilins into pili.
Sortase A is an enzyme. This enzyme catalyses a cell wall sorting reaction, in which a surface protein with a sorting signal containing a LPXTG motif, is cleaved between the Thr and Gly residue.
Sortases are membrane anchored enzyme that sort these surface proteins onto the bacterial cell surface and anchor them to the peptidoglycan. There are different types of sortases and each catalyse the anchoring of different proteins to cell walls.
An archaeosortase is a protein that occurs in the cell membranes of some archaea. Archaeosortases recognize and remove carboxyl-terminal protein sorting signals about 25 amino acids long from secreted proteins. A genome that encodes one archaeosortase may encode over fifty target proteins. The best characterized archaeosortase target is the Haloferax volcanii S-layer glycoprotein, an extensively modified protein with O-linked glycosylations, N-linked glycosylations, and a large prenyl-derived lipid modification toward the C-terminus. Knockout of the archaeosortase A (artA) gene, or permutation of the motif Pro-Gly-Phe (PGF) to Pro-Phe-Gly in the S-layer glycoprotein, blocks attachment of the lipid moiety as well as blocking removal of the PGF-CTERM protein-sorting domain. Thus archaeosortase appears to be a transpeptidase, like sortase, rather than a simple protease.
LPXTGase refers to an endopeptidase enzyme from Streptococci and Staphylococci with the capacity to cleave the carboxy-terminal LPXTG anchor motif of surface proteins similar to Sortase. However, LPXTGase differs significantly from Sortase in several ways: a) it is glycosylated, b) it contains unconventional amino acids, and c) it contains D-amino acids. The latter two characteristics indicate that ribosomes are not involve in the synthesis of LPXTGase. Data suggest that the enzymes responsible for cell wall assembly also assemble LPXTGase.
Antivirulence is the concept of blocking virulence factors. In regards to bacteria, the idea is to design agents that block virulence rather than kill bacteria en masse, as the current regime results in much more selective pressure.
The LCP family or TagU family of proteins is a conserved family of phosphotransferases that are involved in the attachment of teichoic acid (TA) molecules to gram-positive cell wall or cell membrane. It was initially thought as the LytR component of a LytABC operon encoding autolysins, but the mechanism of regulation was later realized to be the production of TA molecules. It was accordingly renamed TagU.
A protein-sorting transpeptidase is an enzyme, such as the sortase SrtA of Staphylococcus aureus, that cleaves one or more target proteins produced by the same cell, as part of a specialized pathway of protein targeting. The typical prokaryotic protein-sorting transpeptidase is characterized as a protease, but does not simply hydrolyze a peptide bond. Instead, the larger, N-terminal portion of the cleaved polypeptide is transferred onto another molecule, such as a precursor of the peptidoglycan cell wall in Gram-positive bacteria.
Luciano Marraffini is an Argentinian-American microbiologist. He is currently professor and head of the laboratory of bacteriology at The Rockefeller University. He is recognized for his work on CRISPR-Cas systems, being one of the first scientists to elucidate how these systems work at the molecular level.
Vincent A. Fischetti is a world renowned American microbiologist and immunologist. He is Professor of and Head of the Laboratory of Bacterial Pathogenesis and Immunology at Rockefeller University in New York City. His Laboratory is the oldest continuous laboratory at Rockefeller that started in 1926 and headed by 4 leading scientists over its near 100 year history: Homer Swift, Maclyn McCarty, Emil Gotschlich and now Vincent Fischetti. Keeping with the historical theme of infectious diseases, Fischetti's primary areas of research are bacterial pathogenesis, bacterial genomics, immunology, virology, microbiology, and therapeutics. He was the first scientist to clone and sequence a surface protein on gram-positive bacteria, the M protein from S. pyogenes, and determine its unique coiled-coil structure. He also was the first use phage lysins as a therapeutic and an effective alternative to conventional antibiotics.
References
- 1 2 3 "Olaf Schneewind, world-renowned authority on infectious diseases, 1961-2019 | University of Chicago News". Republished as: "Department of Microbiology mourns the loss of Louis Block Professor and Chair Olaf Schneewind". University of Chicago Department of Microbiology. 28 May 2019. Retrieved 10 October 2021.
- 1 2 "Olaf Schneewind". www.nasonline.org. Retrieved 5 January 2022.
- ↑ Navarre WW, Schneewind O (1994). "Proteolytic cleavage and cell wall anchoring at the LPXTG motif of surface proteins in gram-positive bacteria". Mol Microbiol. 14 (1): 115–21. doi:10.1111/j.1365-2958.1994.tb01271.x. PMID 7830549. S2CID 9501258.
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