''Clostridioides difficile'' toxin A (TcdA) is a

toxin A toxin is a naturally occurring poison produced by metabolic activities of living cells or organisms. They occur especially as proteins, often conjugated. The term was first used by organic chemist Ludwig Brieger (1849–1919), derived ...

produced by the bacteria ''

Clostridioides difficile ''Clostridioides difficile'' ( syn. ''Clostridium difficile'') is a bacterium known for causing serious diarrheal infections, and may also cause colon cancer. It is known also as ''C. difficile'', or ''C. diff'' (), and is a Gram-positive spec ...

'', formerly known as ''Clostridium difficile''. It is similar to Clostridioides difficile Toxin B. The toxins are the main

virulence factors Virulence factors (preferably known as pathogenicity factors or effectors in botany) are cellular structures, molecules and regulatory systems that enable microbial pathogens (bacteria, viruses, fungi, and protozoa) to achieve the following: * co ...

produced by the

gram positive 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. The Gram stain i ...

, anaerobic, ''Clostridioides difficile'' bacteria. The toxins function by damaging the

intestinal mucosa The gastrointestinal wall of the gastrointestinal tract is made up of four layers of specialised tissue. From the inner cavity of the gut (the lumen) outwards, these are the mucosa, the submucosa, the muscular layer and the serosa or adventitia. ...

and cause the symptoms of ''C. difficile'' infection, including

pseudomembranous colitis Colitis is swelling or inflammation of the large intestine ( colon). Colitis may be acute and self-limited or long-term. It broadly fits into the category of digestive diseases. In a medical context, the label ''colitis'' (without qualificati ...

. TcdA is one of the largest bacterial toxins known. With a molecular mass of 308 kDa, it is usually described as a potent

enterotoxin An enterotoxin is a protein exotoxin released by a microorganism that targets the intestines. They can be chromosomally or plasmid encoded. They are heat labile (> 60 °C), of low molecular weight and water-soluble. Enterotoxins are frequently cy ...

, but it also has some activity as a

cytotoxin Cytotoxicity is the quality of being toxic Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium ...

. The toxin acts by modifying host cell GTPase proteins by glucosylation, leading to changes in cellular activities.

Risk factors In epidemiology, a risk factor or determinant is a variable associated with an increased risk of disease or infection. Due to a lack of harmonization across disciplines, determinant, in its more widely accepted scientific meaning, is often ...

for ''C. difficile'' infection include antibiotic treatment, which can disrupt normal intestinal microbiota and lead to colonization of ''C. difficile'' bacteria.

''tcdA'' gene

The

gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

contains an

open reading frame In molecular biology, reading frames are defined as spans of DNA sequence between the start and stop codons. Usually, this is considered within a studied region of a prokaryotic DNA sequence, where only one of the six possible reading frames ...

(ORF) of 8,133

nucleotide Nucleotides are Organic compound, organic molecules composed of a nitrogenous base, a pentose sugar and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both o ...

s, coding for 2,710

amino acids Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the Proteinogenic amino acid, 22 α-amino acids incorporated into p ...

. TcdA and TcdB share 63% homology in their amino acid sequences. These genes are expressed during late log phase and stationary phase in response to environmental factors. Environmental stresses such as

antibiotics An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting pathogenic bacteria, bacterial infections, and antibiotic medications are widely used in the therapy ...

and

catabolite repression Carbon catabolite repression, or simply catabolite repression, is an important part of global control system of various bacteria and other microorganisms. Catabolite repression allows microorganisms to adapt quickly to a preferred (rapidly metabo ...

can influence toxin expression.

Pathogenicity locus

The ''tcdA'' and ''tcdB'' genes are situated on the ''Clostridioides difficile'' chromosome in a 19.6-kb

pathogenicity In biology, a pathogen (, "suffering", "passion" and , "producer of"), in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ. The term ...

locus (PaLoc) found only in toxigenic strains of ''C. difficile''. Non toxigenic strains contain a 127 base pair fragment replacing the PaLoc. This locus also contains three other accessory genes ''tcdC'', ''tcdR'', and ''tcdE''. ''TcdC'' expression is high during early exponential phase and declines as growth moves into stationary phase, consistent with increases in ''tcdA'' and ''tcdB'' expression. Accordingly, expression patterns have indicated ''tcdC'' as a possible negative regulator of toxin production. ''tcdR'' may serve as a positive regulator of toxin production. ''tcdE'' has been speculated to facilitate release of TcdA and TcdB through lytic activity on the bacterial cell membrane. Due to its homology with other proteins of similar function, as well as the location of the gene between ''tcdA'' and ''tcdB'', ''tcdE'' is predicted to function as the lytic protein that facilitates release since TcdA and TcdB lack a signal peptide for

secretion 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 mec ...

Structure

The protein contains three domains. The amino

N-terminal The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the amin ...

domain contains the

active site In biology and biochemistry, the active site is the region of an enzyme where substrate molecules bind and undergo a chemical reaction. The active site consists of amino acid residues that form temporary bonds with the substrate, the ''binding s ...

, responsible for the glucosylating activity of the toxin. Both TcdA and TcdB use this highly conserved N-terminal region (74% homology between both toxins) to alter identical

substrate Substrate may refer to: Physical layers *Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached ** Substrate (aquatic environment), the earthy material that exi ...

s. The carboxy

C-terminal The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, carboxy tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain (protein or polypeptide), terminated by a free carboxyl group (-COOH). When t ...

domain contains repeating units that are responsible for receptor binding on target cell surfaces. These short homologous repeating units have been termed combined repetitive

oligopeptide An oligopeptide ('' oligo-'', "a few"), is a peptide consisting of two to twenty amino acids, including dipeptides, tripeptides, tetrapeptides, and other polypeptides. Some of the major classes of naturally occurring oligopeptides include aerugi ...

(CROPs). A recent study demonstrates that the CROPs determine the potency of TcdA through interactions with structures on the cell surface. These CROP regions range from 21-50 residues and play a role in receptor binding. This C-terminal repetitive region is designated as the immuno-dominant region since

ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...

binding can be blocked by

monoclonal antibodies A monoclonal antibody (mAb, more rarely called moAb) is an antibody produced from a Lineage (evolution), cell lineage made by cloning a unique white blood cell. All subsequent antibodies derived this way trace back to a unique parent cell. Mon ...

specific to this region. This region contains the most

hydrophilic A hydrophile is a molecule or other molecular entity that is attracted to water molecules and tends to be dissolved by water.Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon'' Oxford: Clarendon Press. In contrast, hydrophobes are n ...

portion of the molecule. A centrally located

hydrophobic In chemistry, hydrophobicity is the chemical property of a molecule (called a hydrophobe) that is seemingly repelled from a mass of water. In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, thu ...

domain containing a cluster of 172 highly conserved

amino acids is thought to be important for translocation of the enzymatic portion of the protein.

Mechanism of action

TcdA must be internalized into the host cell via

endocytosis Endocytosis is a cellular process in which Chemical substance, substances are brought into the cell. The material to be internalized is surrounded by an area of cell membrane, which then buds off inside the cell to form a Vesicle (biology and chem ...

in order to access the

cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells ( intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...

. Receptor binding is the first step required for entry into the cell via endocytosis in an acidic

endosome Endosomes are a collection of intracellular sorting organelles in eukaryotic cells. They are parts of the endocytic membrane transport pathway originating from the trans Golgi network. Molecules or ligands internalized from the plasma membra ...

. Low pH in the endosome induces structural changes such as exposure of the hydrophobic domains that are crucial for TcdA function. The N-terminal domain of TcdA functions to catalyze a glucotransferase reaction, which transfers a

glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...

molecule from

UDP-Glucose Uridine diphosphate glucose (uracil-diphosphate glucose, UDP-glucose) is a nucleotide sugar. It is involved in glycosyltransferase reactions in metabolism. Functions UDP-glucose is used in nucleotide sugar metabolism as an activated form of g ...

and covalently attaches it to conserved amino acids in target molecules. Therefore, TcdA catalyzes glucosylation and the subsequent irreversible inactivation of target molecules in the

Ras family Small GTPases (), also known as small G-proteins, are a family of hydrolase enzymes that can bind and hydrolyze guanosine triphosphate (GTP). They are a type of G-protein found in the cytosol that are homologous to the alpha subunit of heterotrim ...

of small GTPases. These target molecules include

RhoA Transforming protein RhoA, also known as Ras homolog family member A (RhoA), is a small GTPase protein in the Rho family of GTPases that in humans is encoded by the ''RHOA'' gene. While the effects of RhoA activity are not all well known, it is ...

Rac RAC or Rac may refer to: Organizations * Radio Amateurs of Canada * RATCH-Australia Corporation, electricity generator * Refugee Action Collective (Victoria), Melbourne, Australia * Religious Action Center of Reform Judaism, US * Rent-A-Cent ...

, and

Cdc42 Cell division control protein 42 homolog (Cdc42 or CDC42) is a protein that in humans is encoded by the ''CDC42'' gene. Cdc42 is involved in regulation of the cell cycle. It was originally identified in ''S. cerevisiae'' (yeast) as a mediator of ...

, which are regulatory proteins of the eukaryotic

actin Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of ...

cytoskeleton The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is compos ...

and modulators of many various cell signaling pathways.

Intracellular targets

TcdA primarily targets

Rho Rho (; uppercase Ρ, lowercase ρ or ; or ) is the seventeenth letter of the Greek alphabet. In the system of Greek numerals it has a value of 100. It is derived from Phoenician alphabet, Phoenician letter resh . Its uppercase form uses the same ...

, and

. These molecules are important regulators of cell signaling. Small GTPases such as Rho, Rac, and Cdc42 regulate their activity by alternating between an active GTP-bound state, and an inactive

GDP Gross domestic product (GDP) is a monetary measure of the total market value of all the final goods and services produced and rendered in a specific time period by a country or countries. GDP is often used to measure the economic performance o ...

-bound state. Guanine exchange factors (GEFs) regulate the exchange of GTP and

. TcdA glucosylates

by transferring a glucose molecule from

UDP-glucose Uridine diphosphate glucose (uracil-diphosphate glucose, UDP-glucose) is a nucleotide sugar. It is involved in glycosyltransferase reactions in metabolism. Functions UDP-glucose is used in nucleotide sugar metabolism as an activated form of g ...

, a nucleotide sugar, to Thr-37 of the RhoA GTPase. In

and

, the sugar moiety is transferred to the Thr-35. The glucosylation prevents proper binding of GTP and blocks activation. TcdA acts preferentially on the GDP-bound form of the GTPase proteins since this configuration exposes the

threonine Threonine (symbol Thr or T) is an amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form when dissolved in water), a carboxyl group (which is in the deprotonated −COO− ...

residue that is glucosylated by the toxin.

regulates the actin cytoskeleton and forms

stress fiber Stress fibers are contractile actin bundles found in non-muscle cells. They are composed of actin (microfilaments) and non-muscle myosin II (NMMII), and also contain various crosslinking proteins, such as α-actinin, to form a highly regulated ...

s and

focal adhesions In cell biology, focal adhesions (also cell–matrix adhesions or FAs) are large macromolecular assemblies through which mechanical force and regulatory signals are transmitted between the extracellular matrix (ECM) and an interacting cell. Mor ...

. When

is inactivated via TcdA, its interaction with downstream

effector Effector may refer to: *Effector (biology), a molecule that binds to a protein and thereby alters the activity of that protein * ''Effector'' (album), a music album by the Experimental Techno group Download * ''EFFector'', a publication of the El ...

s is inhibited. This leads to changes in the actin cytoskeleton that increase permeability of the

intestinal epithelium The intestinal epithelium is the single cell layer that forms the luminal surface (lining) of both the small and large intestine (colon) of the gastrointestinal tract. Composed of simple columnar epithelium its main functions are absorptio ...

and

are involved in filopodium formation crucial for movement and cell migration. Overall,

, and

all regulate processes in cells that are dependent on actin polymerization. Many of the physiologic effects that cells experience after exposure to TcdA can be linked to disregulation of actin polymerization and cellular pathways controlled by TcdA targets.

Physiologic effects

Cell morphology

Exposure to TcdA leads to immediate changes in cell morphology, including loss of structural integrity due to a decrease in filamentous actin (

F-actin Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of ...

), and an increase in globular

. Disorganization of

actin filaments Microfilaments, also called actin filaments, are protein filaments in the cytoplasm of eukaryotic cells that form part of the cytoskeleton. They are primarily composed of polymers of actin, but are modified by and interact with numerous other p ...

and the cytoskeleton leads to increased permeability of

tight junctions Tight junctions, also known as occluding junctions or ''zonulae occludentes'' (singular, ''zonula occludens''), are multiprotein junctional complexes between epithelial cells, sealing and preventing leakage of solutes and water. They also play a ...

resulting in severe

epithelial cell Epithelium or epithelial tissue is a thin, continuous, protective layer of Cell (biology), cells with little extracellular matrix. An example is the epidermis, the outermost layer of the skin. Epithelial (Mesothelium, mesothelial) tissues line ...

damage and fluid secretion. Fluid accumulation and secretion are secondary to mucosal damage that occurs after exposure to TcdA. Distinct changes in the

microfilament Microfilaments, also called actin filaments, are protein filaments in the cytoplasm of eukaryotic cells that form part of the cytoskeleton. They are primarily composed of polymers of actin, but are modified by and interact with numerous other ...

system lead to cell rounding and cell death. These changes result from the inactivation of

proteins, which play an important role in regulating

Apoptosis

Apoptosis Apoptosis (from ) is a form of programmed cell death that occurs in multicellular organisms and in some eukaryotic, single-celled microorganisms such as yeast. Biochemistry, Biochemical events lead to characteristic cell changes (Morphology (biol ...

is the most likely mechanism accounting for death of cells exposed to TcdA.

inactivation can activate

caspase-3 Caspase-3 is a caspase protein that interacts with caspase-8 and caspase-9. It is encoded by the ''CASP3'' gene. ''CASP3'' orthologs have been identified in numerous mammals for which complete genome data are available. Unique orthologs are also ...

and

caspase-9 Caspase-9 is an enzyme that in humans is encoded by the ''CASP9'' gene. It is an initiator caspase, critical to the apoptotic pathway found in many tissues. Caspase-9 homologs have been identified in all mammals for which they are known to exist, ...

; two key components of the apoptotic pathway. TcdA has been linked to

mitochondrial membrane A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is used ...

disruption and release of cytochrome C through

caspase Caspases (cysteine-aspartic proteases, cysteine aspartases or cysteine-dependent aspartate-directed proteases) are a family of protease enzymes playing essential roles in programmed cell death. They are named caspases due to their specific cyste ...

activation and

inactivation, further suggesting that TcdA is capable of inducing apoptosis.

Clinical significance

''Clostridioides difficile'' associated diarrhea (CDAD)

Animal models have shown TcdA includes diarrhea,

neutrophil Neutrophils are a type of phagocytic white blood cell and part of innate immunity. More specifically, they form the most abundant type of granulocytes and make up 40% to 70% of all white blood cells in humans. Their functions vary in differe ...

infiltration,

inflammation Inflammation (from ) is part of the biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. The five cardinal signs are heat, pain, redness, swelling, and loss of function (Latin ''calor'', '' ...

, and

necrosis Necrosis () is a form of cell injury which results in the premature death of cells in living tissue by autolysis. The term "necrosis" came about in the mid-19th century and is commonly attributed to German pathologist Rudolf Virchow, who i ...

epithelial cells Epithelium or epithelial tissue is a thin, continuous, protective layer of cells with little extracellular matrix. An example is the epidermis, the outermost layer of the skin. Epithelial ( mesothelial) tissues line the outer surfaces of man ...

. This toxin is considered the main cause of CDAD. TcdA damages intestinal villous tips, which disrupts the

brush border A brush border (striated border or brush border membrane) is the microvillus-covered surface of simple cuboidal and simple columnar epithelium found in different parts of the body. Microvilli are approximately 100 nanometers in diameter and th ...

membrane, leading to cell erosion and fluid leakage from the damaged area. This damage and associated fluid response causes the diarrhea associated with ''Clostridioides difficile'' infection.

Pseudomembranous colitis

TcdA can induce the physiological changes that occur in ''C. difficile'' related

(PMC), a severe

ulcer An ulcer is a discontinuity or break in a bodily membrane that impedes normal function of the affected organ. According to Robbins's pathology, "ulcer is the breach of the continuity of skin, epithelium or mucous membrane caused by sloughin ...

ation of the colon. Toxin damage to the colonic mucosa promotes accumulations of

fibrin Fibrin (also called Factor Ia) is a fibrous protein, fibrous, non-globular protein involved in the Coagulation, clotting of blood. It is formed by the action of the protease thrombin on fibrinogen, which causes it to polymerization, polymerize. ...

mucin Mucins () are a family of high molecular weight, heavily glycosylated proteins ( glycoconjugates) produced by epithelial tissues in most animals. Mucins' key characteristic is their ability to form gels; therefore they are a key component in ...

, and dead cells to form a layer of debris in the colon (pseudomembrane), causing an

inflammatory response Inflammation (from ) is part of the biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. The five cardinal signs are heat, pain, redness, swelling, and loss of function (Latin ''calor'', '' ...

. TcdA damage causes increased epithelial permeability,

cytokine Cytokines () are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Cytokines are produced by a broad range of cells, including immune cells like macrophages, B cell, B lymphocytes, T cell, T lymphocytes ...

and

chemokine Chemokines (), or chemotactic cytokines, are a family of small cytokines or signaling proteins secreted by cells that induce directional movement of leukocytes, as well as other cell types, including endothelial and epithelial cells. In addit ...

production, neutrophil infiltration, production of

reactive oxygen species In chemistry and biology, reactive oxygen species (ROS) are highly Reactivity (chemistry), reactive chemicals formed from diatomic oxygen (), water, and hydrogen peroxide. Some prominent ROS are hydroperoxide (H2O2), superoxide (O2−), hydroxyl ...

(ROS),

mast cell A mast cell (also known as a mastocyte or a labrocyte) is a resident cell of connective tissue that contains many granules rich in histamine and heparin. Specifically, it is a type of granulocyte derived from the myeloid stem cell that is a p ...

activation, and direct damage to the intestinal mucosa. All can be attributed to TcdA induced inactivation of Rho GTPase proteins. Loss of

can provide entry for neutrophils into the intestines, leading to neutrophil accumulation; a hallmark of PMC. TcdA induced

production of IL-8 and other inflammatory mediators contributes to the stages of inflammation seen in PMC. Infiltration by neutrophils,

macrophage Macrophages (; abbreviated MPhi, φ, MΦ or MP) are a type of white blood cell of the innate immune system that engulf and digest pathogens, such as cancer cells, microbes, cellular debris and foreign substances, which do not have proteins that ...

s, and

s in response to TcdA damage increases the inflammatory response through production and release of other mediators such as

tumor necrosis factor alpha Tumor necrosis factor (TNF), formerly known as TNF-α, is a chemical messenger produced by the immune system that induces inflammation. TNF is produced primarily by activated macrophages, and induces inflammation by binding to its receptors o ...

, IL-1, IL-6, and other monokines. These mediators cause additional damage to intestinal mucosa and further increase the inflammatory response, influencing PMC persistence. If extensive damage to the intestinal wall occurs, bacteria can enter the bloodstream and cause

septic shock Septic shock is a potentially fatal medical condition that occurs when sepsis, which is organ injury or damage in response to infection, leads to dangerously low blood pressure and abnormalities in cellular metabolism. The Third International C ...

and death.

Toxin detection and diagnosis

TcdA and TcdB are present in

supernatant In an aqueous solution, precipitation is the "sedimentation of a solid material (a precipitate) from a liquid solution". The solid formed is called the precipitate. In case of an inorganic chemical reaction leading to precipitation, the chemic ...

fluids of ''C. difficile'' cultures and can be purified from filtrates. Both toxins are consistently detected in fecal samples from humans and animals and are now used as markers to diagnose ''C. difficile'' infection. Over 90% of patients infected with ''C. difficile'' were found to have

cytotoxic Cytotoxicity is the quality of being toxic to cells. Examples of toxic agents are toxic metals, toxic chemicals, microbe neurotoxins, radiation particles and even specific neurotransmitters when the system is out of balance. Also some types of dr ...

activity in their stool. Glucosylation of Rho GTPases inactivates the GTPase proteins, leading to collapse of the cytoskeleton, resulting in cell rounding. A tissue culture assay has been developed to detect ''C. difficile'' toxins in stool samples. A cell rounding assay (cytotoxicity assay) has been developed to diagnose ''C. difficile'' infection.

Enzyme-linked immunosorbent assay The enzyme-linked immunosorbent assay (ELISA) (, ) is a commonly used analytical biochemistry assay, first described by Eva Engvall and Peter Perlmann in 1971. The assay is a solid-phase type of enzyme immunoassay (EIA) to detect the presence o ...

s (ELISAs) have been used to detect TcdA and TcdB with specific

antibodies An antibody (Ab) or immunoglobulin (Ig) is a large, Y-shaped protein belonging to the immunoglobulin superfamily which is used by the immune system to identify and neutralize antigens such as bacteria and viruses, including those that caus ...

. When used with an ELISA, the cytotoxicity assay is the "gold standard" when used on

Vero cells Vero cells are a lineage of cells used in cell cultures. The 'Vero' lineage was isolated from kidney epithelial cells extracted from an African green monkey (''Chlorocebus'' sp.; formerly called ''Cercopithecus aethiops'', this group of monkey ...

for ''C. difficile'' diagnosis.

Importance of TcdA and TcdB in ''C. difficile'' infection

Since the 1980s and early 1990s, the roles of TcdA and TcdB in ''C. difficile'' infection have been much debated. Previous reports with purified toxins indicated that TcdA alone was enough to cause symptoms of infection and TcdB was unable to do so unless combined with TcdA. A more recent experiment indicated that TcdB was, in fact, essential for

virulence Virulence is a pathogen's or microorganism's ability to cause damage to a host. In most cases, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. The pathogenicity of an organism—its abili ...

. Earlier research established TcdA strictly as an

, and TcdB as a

, but later both toxins were found to have the same mechanism of action. To fully investigate the role of both toxins in pathogenesis of ''C. difficile'' infection, a gene knockout system in a hamster infection model was developed. By permanently knocking out ''tcdA'', ''tcdB'', or both (double knockout), it was shown that ''C. difficile'' producing one or both toxins was capable of cytotoxic activity, and this activity translated directly to virulence

in vivo Studies that are ''in vivo'' (Latin for "within the living"; often not italicized in English) are those in which the effects of various biological entities are tested on whole, living organisms or cells, usually animals, including humans, an ...

. It was also found that a double ''tcdAtcdB'' knockout was completely attenuated in

. Overall, this research has demonstrated the importance of both TcdA and TcdB in ''C. difficile'' infection, showing that either toxin is capable of cytotoxicity.

References

{{Toxins Bacterial toxins

''tcdA'' gene

Pathogenicity locus

Structure

Mechanism of action

Intracellular targets

Physiologic effects

Cell morphology

Apoptosis

Clinical significance

''Clostridioides difficile'' associated diarrhea (CDAD)

Pseudomembranous colitis

Toxin detection and diagnosis

Importance of TcdA and TcdB in ''C. difficile'' infection

See also

References