Gram-negative bacteria are bacteria that unlike gram-positive bacteria do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is their cell envelope, which consists of a thin peptidoglycan cell wall sandwiched between an inner (cytoplasmic) membrane and an outer membrane. These bacteria are found in all environments that support life on Earth.
A transmembrane protein is a type of integral membrane protein that spans the entirety of the cell membrane. Many transmembrane proteins function as gateways to permit the transport of specific substances across the membrane. They frequently undergo significant conformational changes to move a substance through the membrane. They are usually highly hydrophobic and aggregate and precipitate in water. They require detergents or nonpolar solvents for extraction, although some of them (beta-barrels) can be also extracted using denaturing agents.
Lipopolysaccharide, now more commonly known as Endotoxin, is a collective term for components of the outermost membrane of cell envelope of Gram-negative bacteria, such as E. coli and Salmonella. with a common structural architecture. Lipopolysaccharides (LPS) are large molecules consisting of 3 parts: an outer core polysaccharide termed the O-antigen, an inner core oligosaccharide and Lipid A, all covalently linked. In current terminology, the term endotoxin is often used synonymously with LPS, although there are a few endotoxins that are not related to LPS, such as the so-called delta endotoxin proteins produced by Bacillus thuringiensis.
Secretion is the movement of material from one point to another, such as a secreted chemical substance from a cell or gland. In contrast, excretion is the removal of certain substances or waste products from a cell or organism. The classical mechanism of cell secretion is via secretory portals at the plasma membrane called porosomes. Porosomes are permanent cup-shaped lipoprotein structures embedded in the cell membrane, where secretory vesicles transiently dock and fuse to release intra-vesicular contents from the cell.
Polymyxins are antibiotics. Polymyxins B and E are used in the treatment of Gram-negative bacterial infections. They work mostly by breaking up the bacterial cell membrane. They are part of a broader class of molecules called nonribosomal peptides.
Porins are beta barrel proteins that cross a cellular membrane and act as a pore, through which molecules can diffuse. Unlike other membrane transport proteins, porins are large enough to allow passive diffusion, i.e., they act as channels that are specific to different types of molecules. They are present in the outer membrane of gram-negative bacteria and some gram-positive mycobacteria, the outer membrane of mitochondria, and the outer chloroplast membrane.
The ABC transporters, ATP synthase (ATP)-binding cassette transporters are a transport system superfamily that is one of the largest and possibly one of the oldest gene families. It is represented in all extant phyla, from prokaryotes to humans. ABC transporters belong to translocases.
Lipid A is a lipid component of an endotoxin held responsible for the toxicity of gram-negative bacteria. It is the innermost of the three regions of the lipopolysaccharide (LPS), also called endotoxin molecule, and its hydrophobic nature allows it to anchor the LPS to the outer membrane. While its toxic effects can be damaging, the sensing of lipid A by the immune system may also be critical for the onset of immune responses to gram-negative infection, and for the subsequent successful fight against the infection.
A 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.
General bacterial porins are a family of porin proteins from the outer membranes of Gram-negative bacteria. The porins act as molecular filters for hydrophilic compounds. They are responsible for the 'molecular sieve' properties of the outer membrane. Porins form large water-filled channels which allow the diffusion of hydrophilic molecules into the periplasmic space. Some porins form general diffusion channels that allow any solute up to a certain size to cross the membrane, while other porins are specific for one particular solute and contain a binding site for that solute inside the pores. As porins are the major outer membrane proteins, they also serve as receptor sites for the binding of phages and bacteriocins.
Outer membrane receptors, also known as TonB-dependent receptors, are a family of beta barrel proteins named for their localization in the outer membrane of gram-negative bacteria. TonB complexes sense signals from the outside of bacterial cells and transmit them into the cytoplasm, leading to transcriptional activation of target genes. TonB-dependent receptors in gram-negative bacteria are associated with the uptake and transport of large substrates such as iron siderophore complexes and vitamin B12.
OmpA-like transmembrane domain is an evolutionarily conserved domain of bacterial outer membrane proteins. This domain consists of an eight-stranded beta barrel. OmpA is the predominant cell surface antigen in enterobacteria found in about 100,000 copies per cell. The expression of OmpA is tightly regulated by a variety of mechanisms. One mechanism by which OmpA expression is regulated in Vibrio species is by an antisense non-coding RNA called VrrA.
Core oligosaccharide is a short chain of sugar residues within Gram-negative lipopolysaccharide (LPS). Core-OS are highly diverse among bacterial species and even within strains of species
Omptins are a family of bacterial proteases. They are aspartate proteases, which cleave peptides with the use of a water molecule. Found in the outer membrane of gram-negative enterobacteria such as Shigella flexneri, Yersinia pestis, Escherichia coli, and Salmonella enterica. Omptins consist of a widely conserved beta barrel spanning the membrane with 5 extracellular loops. These loops are responsible for the various substrate specificities. These proteases rely upon binding of lipopolysaccharide for activity.
In molecular biology, the lipopolysaccharide kinase (Kdo/WaaP) family is a family of lipopolysaccharide kinases that includes lipopolysaccharide core heptose(I) kinase rfaP. Lipopolysaccharide core heptose(I) kinase rfaP is required for the addition of phosphate to O-4 of the first heptose residue of the lipopolysaccharide (LPS) inner core region. It has previously been shown that it is necessary for resistance to hydrophobic and polycationic antimicrobials in E. coli and that it is required for virulence in invasive strains of Salmonella enterica. The family also includes 3-deoxy-D-manno-octulosonic acid kinase from Haemophilus influenzae, which phosphorylates Kdo-lipid IV(A), a lipopolysaccharide precursor, and is involved in virulence.
In molecular biology, the OmpA domain is a conserved protein domain with a beta/alpha/beta/alpha-beta(2) structure found in the C-terminal region of many Gram-negative bacterial outer membrane proteins, such as porin-like integral membrane proteins, small lipid-anchored proteins, and MotB proton channels. The N-terminal half of these proteins is variable although some of the proteins in this group have the OmpA-like transmembrane domain at the N terminus. OmpA from Escherichia coli is required for pathogenesis, and can interact with host receptor molecules. MotB serve two functions in E. coli, the MotA(4)-MotB(2) complex attaches to the cell wall via MotB to form the stator of the flagellar motor, and the MotA-MotB complex couples the flow of ions across the cell membrane to movement of the rotor.
OmpT is an aspartyl protease found on the outer membrane of Escherichia coli. OmpT is a subtype of the family of omptin proteases, which are found on some gram-negative species of bacteria.
The multidrug/oligosaccharidyl-lipid/polysaccharide (MOP) flippase superfamily is a group of integral membrane protein families. The MOP flippase superfamily includes twelve distantly related families, six for which functional data are available:
- One ubiquitous family (MATE) specific for drugs - (TC# 2.A.66.1) The Multi Antimicrobial Extrusion (MATE) Family
- One (PST) specific for polysaccharides and/or their lipid-linked precursors in prokaryotes - (TC# 2.A.66.2) The Polysaccharide Transport (PST) Family
- One (OLF) specific for lipid-linked oligosaccharide precursors of glycoproteins in eukaryotes - (TC# 2.A.66.3) The Oligosaccharidyl-lipid Flippase (OLF) Family
- One (MVF) lipid-peptidoglycan precursor flippase involved in cell wall biosynthesis - (TC# 2.A.66.4) The Mouse Virulence Factor (MVF) Family
- One (AgnG) which includes a single functionally characterized member that extrudes the antibiotic, Agrocin 84 - (TC# 2.A.66.5) The Agrocin 84 Antibiotic Exporter (AgnG) Family
- And finally, one (Ank) that shuttles inorganic pyrophosphate (PPi) - (TC# 2.A.66.9) The Progressive Ankylosis (Ank) Family
BamA is a β-barrel, outer membrane protein found in Gram-negative bacteria and it is the main and vital component of the β-barrel assembly machinery (BAM) complex in those bacteria. BAM Complex consists of five components; BamB, BamC, BamD, BamE and BamA. This complex is responsible in catalyzing folding and insertion of β-barrel proteins into the outer membrane of Gram-negative bacteria.
Bacterial secretion systems are protein complexes present on the cell membranes of bacteria for secretion of substances. Specifically, they are the cellular devices used by pathogenic bacteria to secrete their virulence factors to invade the host cells. They can be classified into different types based on their specific structure, composition and activity. Generally, proteins can be secreted through two different processes. One process is a one-step mechanism in which proteins from the cytoplasm of bacteria are transported and delivered directly through the cell membrane into the host cell. Another involves a two-step activity in which the proteins are first transported out of the inner cell membrane, then deposited in the periplasm, and finally through the outer cell membrane into the host cell.
