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.
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.
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.
Braun's lipoprotein, found in some gram-negative cell walls, is one of the most abundant membrane proteins; its molecular weight is about 7.2 kDa. It is bound at its C-terminal end by a covalent bond to the peptidoglycan layer and is embedded in the outer membrane by its hydrophobic head. BLP tightly links the two layers and provides structural integrity to the outer membrane.
Antimicrobial peptides (AMPs), also called host defence peptides (HDPs) are part of the innate immune response found among all classes of life. Fundamental differences exist between prokaryotic and eukaryotic cells that may represent targets for antimicrobial peptides. These peptides are potent, broad spectrum antimicrobials which demonstrate potential as novel therapeutic agents. Antimicrobial peptides have been demonstrated to kill Gram negative and Gram positive bacteria, enveloped viruses, fungi and even transformed or cancerous cells. Unlike the majority of conventional antibiotics it appears that antimicrobial peptides frequently destabilize biological membranes, can form transmembrane channels, and may also have the ability to enhance immunity by functioning as immunomodulators.
The bacterial outer membrane is found in gram-negative bacteria. Gram-negative bacteria form two lipid bilayers in their cell envelopes - an inner membrane (IM) that encapsulates the cytoplasm, and an outer membrane (OM) that encapsulates the periplasm.
Virulence factors are cellular structures, molecules and regulatory systems that enable microbial pathogens to achieve the following:
The micF RNA is a non-coding RNA stress response gene found in Escherichia coli and related bacteria that post-transcriptionally controls expression of the outer membrane porin gene ompF. The micF gene encodes a non-translated 93 nucleotide antisense RNA that binds its target ompF mRNA and regulates ompF expression by inhibiting translation and inducing degradation of the message. In addition, other factors, such as the RNA chaperone protein StpA also play a role in this regulatory system. The expression of micF is controlled by both environmental and internal stress factors. Four transcriptional regulators are known to bind the micF promoter region and activate micF expression.
RybB is a small non-coding RNA was identified in a large scale screen of Escherichia coli. The function of this short RNA has been studied using a transcriptomic approach and kinetic analyses of target mRNA decay in vivo. RybB was identified as a factor that selectively accelerates the decay of multiple major omp mRNAs upon induction of the envelope stress response. This RNA has been shown to bind to the Hfq protein.
The MicA RNA is a small non-coding RNA that was discovered in E. coli during a large scale screen. Expression of SraD is highly abundant in stationary phase, but low levels could be detected in exponentially growing cells as well.
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.
Virulence-related outer membrane proteins, or outer surface proteins (Osp) in some contexts, are expressed in the outer membrane of gram-negative bacteria and are essential to bacterial survival within macrophages and for eukaryotic cell invasion.
VrrA is a non-coding RNA that is conserved across all Vibrio species of bacteria and acts as a repressor for the synthesis of the outer membrane protein OmpA. This non-coding RNA was initially identified from Tn5 transposon mutant libraries of Vibrio cholerae and its location within the bacterial genome was mapped to the intergenic region between genes VC1741 and VC1743 by RACE analysis.
Yersiniabactin (Ybt) is a siderophore found in the pathogenic bacteria Yersinia pestis, Yersinia pseudotuberculosis, and Yersinia enterocolitica, as well as several strains of enterobacteria including enteropathogenic Escherichia coli and Salmonella enterica. Siderophores, compounds of low molecular mass with high affinities for ferric iron, are important virulence factors in pathogenic bacteria. Iron—an essential element for life used for such cellular processes as respiration and DNA replication—is extensively chelated by host proteins like lactoferrin and ferritin; thus, the pathogen produces molecules with an even higher affinity for Fe3+ than these proteins in order to acquire sufficient iron for growth. As a part of such an iron-uptake system, yersiniabactin plays an important role in pathogenicity of Y. pestis, Y. pseudotuberculosis, and Y. entercolitica.
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.
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.
Plasminogen activator Pla is an enzyme. This enzyme catalyses the following chemical reaction
Bacterial effectors are proteins secreted by pathogenic bacteria into the cells of their host, usually using a type 3 secretion system (TTSS/T3SS), a type 4 secretion system (TFSS/T4SS) or a Type VI secretion system (T6SS). Some bacteria inject only a few effectors into their host’s cells while others may inject dozens or even hundreds. Effector proteins may have many different activities, but usually help the pathogen to invade host tissue, suppress its immune system, or otherwise help the pathogen to survive. Effector proteins are usually critical for virulence. For instance, in the causative agent of plague, the loss of the T3SS is sufficient to render the bacteria completely avirulent, even when they are directly introduced into the bloodstream. Gram negative microbes are also suspected to deploy bacterial outer membrane vesicles to translocate effector proteins and virulence factors via a membrane vesicle trafficking secretory pathway, in order to modify their environment or attack/invade target cells, for example, at the host-pathogen interface.
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.
Virginia L. Miller is a microbiologist known for her work on studying the factors leading to disease caused by bacteria. Miller is an elected fellow of the American Academy of Microbiology (2003) and a former Pew Charitable Trust Biomedical Scholar (1989).
