Toxin A (TcdA) and Toxin B (TcdB) are the two major disease-causing proteins produced by Clostridioides difficile, and while they share a broadly similar structure and mechanism, they differ in the cells they target, the range of host proteins they attack, and their relative importance in driving illness. For years, Toxin A was considered the primary villain, but research over the past two decades has firmly established that Toxin B is the more critical driver of disease, to the point where strains that produce only Toxin B and no Toxin A still cause severe infections. That shift in understanding has reshaped how we diagnose, treat, and attempt to vaccinate against C. diff.
A Shared Blueprint With Different Details
Both toxins belong to the same family of large clostridial glucosyltransferases. They share roughly the same architecture when viewed under electron microscopy, with a pincher-like head region involved in delivering the toxic payload and two tail-like extensions corresponding to their receptor-binding and enzyme domains.1PubMed Central. Structural organization of the functional domains of Clostridium difficile toxins A and B TcdA is the larger of the two, weighing in at about 308 kilodaltons compared to TcdB’s roughly 270 kilodaltons. Both carry the same core functional toolkit: a glucosyltransferase enzyme at one end that does the actual damage inside host cells, a cysteine protease that clips the enzyme free once it arrives in the cell’s interior, a translocation domain that punches through membranes to deliver the enzyme, and a receptor-binding domain at the other end that latches onto cell surfaces.
Despite these shared blueprints, the two toxins diverge in important ways at every step of their attack. They bind different receptors on host cells, they have different preferences for which host proteins they modify, and TcdB is far more potent on a molecule-for-molecule basis in cell culture experiments. Those differences have real consequences for how disease plays out and how medicine targets these toxins.
How Both Toxins Get Inside Cells
The entry route is similar for both toxins. After binding to a receptor on the surface of an intestinal cell, the toxin gets pulled inside through normal cellular recycling processes. Once trapped in an acidic compartment inside the cell, the toxin undergoes a shape change that allows its translocation domain to punch a pore through the membrane. The enzyme portion then threads through that pore into the cell’s interior, where it is clipped free by the built-in protease and released to do its work.2PubMed Central. The C. difficile toxin B membrane translocation machinery is an evolutionarily conserved protein delivery apparatus
The pore-forming step is pH-dependent. For Toxin A, pore formation peaks at a pH around 5.0, which corresponds to the acidity found in early endosomes. At pH 5.5, pore activity drops to roughly a third of its maximum, and above pH 5.6 no pores form at all.3Journal of Biological Chemistry. Cholesterol-dependent Pore Formation of Clostridium difficile Toxin A Toxin B has its own minimal pore-forming region mapped to a stretch of amino acids between positions 830 and 990, with two specific glutamate residues playing a critical role.4PubMed. Structural determinants for membrane insertion, pore formation and translocation of Clostridium difficile toxin B Although the overall entry strategy is conserved, the structural details of how each toxin forms its pore differ, which could eventually matter for developing drugs that block cell entry.
Once Inside, They Attack the Same Targets Differently
Both toxins disable the cell by attaching a sugar molecule (glucosylation) to small signaling proteins called Rho GTPases, which act as molecular switches controlling the cell’s internal skeleton, its ability to maintain barriers between cells, and inflammatory signaling.5PubMed Central. The Role of Rho GTPases in Toxicity of Clostridium difficile Toxins When these switches get locked in the “off” position by the toxin-added sugar, the cell’s structural scaffolding collapses, tight junctions between cells break down, and inflammatory alarm signals go haywire.
The key difference is selectivity. TcdA primarily targets the classic Rho family members: RhoA, Rac1, and Cdc42. TcdB hits those same targets but also modifies members of the Ras family of GTPases, giving it a broader reach into cell signaling pathways.6PubMed Central. Structural basis for selective modification of Rho and Ras GTPases by Clostridioides difficile toxin B This expanded substrate range helps explain why TcdB tends to be more cytotoxic in laboratory assays. TcdB also shows differential selectivity depending on its subtype, with some variants preferentially modifying Ras-family proteins while others lean more toward the Rho family.7PubMed. MuGger Toxins: Exploring the Selective Binding Mechanism of Clostridial Glucosyltransferase Toxin B and Host GTPases
They Bind to Different Receptors on the Cell Surface
This is where the two toxins diverge most sharply. TcdA was originally identified as an “enterotoxin” because it binds to carbohydrate structures on the surface of intestinal epithelial cells, causing direct fluid secretion and tissue damage in animal intestinal loop models. Its receptor-binding domain is large and made up of repeating units called CROPs (combined repetitive oligopeptides), which latch onto specific sugar residues on the gut lining.
TcdB’s receptor story is more complex and has only come into focus recently. Rather than relying primarily on sugar-based binding, TcdB uses protein receptors, and the specific receptor depends on the toxin subtype. TcdB1, the variant carried by many common clinical strains, binds to Frizzled receptors (FZD1, FZD2, and FZD7) on the surface of colonocytes. TcdB2, the variant carried by the epidemic RT027 lineage, binds instead to tissue factor pathway inhibitor (TFPI) and to chondroitin sulfate proteoglycan 4 (CSPG4).8The Innovation. Another surprise in receptor binding of C. difficile toxins These receptor differences mean that different TcdB subtypes may target different cell populations within the colon, potentially explaining why certain strains cause different patterns of disease.
The diversity of TcdB subtypes goes well beyond two variants. Comparative genomics has identified at least twelve TcdB subtypes (B1 through B12), compared to only seven known TcdA variants, which mainly differ in the number and length of their CROPS repeats. The gene encoding TcdB shows mosaic regions across its full length, likely generated by recombination events during the pathogen’s evolution, making it far more variable than its partner.9Cell Reports. From colonization to infection: Genomic evolution of Clostridioides difficile pathogenesis
TcdA’s Direct Effect on the Gut Barrier
One area where Toxin A has been studied in fine detail is its direct impact on the intestinal barrier. In cultured human intestinal epithelial cells, even low concentrations of TcdA can nearly abolish the electrical resistance across a cell monolayer within six to eight hours, a measure of how well the barrier is sealing. This barrier collapse happens while the cells are still alive and intact, localizing the damage specifically to the tight junctions between cells rather than to the cells themselves. Fluorescent staining at the same time point shows a dramatic loss of the structural protein F-actin, which normally holds the cell’s skeleton together and regulates how tight those junctions are.10PubMed Central. Clostridium difficile toxin A perturbs cytoskeletal structure and tight junction permeability of cultured human intestinal epithelial monolayers
TcdB causes similar downstream effects on the cytoskeleton through the same Rho GTPase inactivation, but the early experimental literature focused more heavily on TcdA for barrier studies because it was initially easier to purify and because animal models responded more dramatically to it. That experimental bias contributed to the long-standing view that TcdA was the “enterotoxin” responsible for diarrhea and that TcdB was merely a “cytotoxin” important for cell rounding in laboratory assays. The clinical evidence has since shown that distinction to be misleading.
Strains That Produce Only Toxin B
The discovery that some C. diff strains produce Toxin B but not Toxin A was one of the most important developments in understanding this pathogen. These TcdA-negative, TcdB-positive strains have been isolated from patients with the full spectrum of C. diff disease, including severe pseudomembranous colitis.11PubMed Central. The role of toxin A and toxin B in Clostridium difficile-associated disease: Past and present perspectives In one survey of over 400 isolates, 48 strains were identified that lacked functional TcdA but still produced TcdB, and the cytotoxic effects of their TcdB on cell cultures were clearly neutralized by anti-TcdB antibodies.12PubMed Central. Identification of toxin A-negative, toxin B-positive Clostridium difficile by PCR
These strains are not confined to a single genetic background. Multiple distinct toxinotypes producing only TcdB have been identified from different geographic regions, including variants from Asia that represent entirely new genetic groups.13PubMed Central. New types of toxin A-negative, toxin B-positive strains among Clostridium difficile isolates from Asia The existence and clinical severity of these TcdA-minus strains effectively settled the debate: TcdB alone is sufficient to cause the full range of C. diff disease. The reverse, strains that produce only TcdA without TcdB, have not been found in clinical settings, underscoring TcdB’s central role.
The Third Toxin That Complicates Things Further
Some of the most dangerous C. diff strains, including the epidemic NAP1/RT027 lineage, produce not just TcdA and TcdB but a third toxin called CDT, or C. difficile binary toxin. CDT works through an entirely different mechanism than the other two. It is made up of two subunits: CDTb, which binds to cells and creates a channel for entry, and CDTa, which modifies actin filaments inside the cell through ADP-ribosylation. This disrupts the balance between actin and microtubules, triggering the formation of long, finger-like protrusions from the cell surface that increase the bacterium’s ability to stick to intestinal tissue.14PubMed Central. The Binary Toxin of Clostridioides difficile Alters the Proteome and Phosphoproteome of HEp-2 Cells
CDT-producing strains are associated with higher mortality rates, though teasing apart whether CDT independently worsens outcomes or simply co-occurs with more virulent TcdB subtypes remains an active area of research. For diagnostic purposes, the presence of binary toxin in a stool sample can flag a hypervirulent strain, and some molecular tests now report CDT status alongside TcdA and TcdB detection.
What This Means for Diagnosis
The differences between the toxins create practical headaches in the diagnostic lab. For years, laboratory tests for C. diff infection relied on detecting TcdA, either through enzyme immunoassays (EIAs) or through the cytotoxicity of TcdB in cell culture. The existence of TcdA-negative strains means that any test looking only for TcdA will miss some genuine infections.
Modern diagnostic approaches generally use a two-step algorithm. A rapid EIA screening test checks for the presence of both toxins or a C. diff-specific antigen called GDH (glutamate dehydrogenase). If positive, a more sensitive molecular test using nucleic acid amplification (NAAT) confirms the result. The reason for this layered approach is that toxin EIAs have limited sensitivity. In one study, only about half of patients with confirmed mild C. diff infection tested positive by toxin EIA, compared to 98% by molecular testing. Even among patients with severe disease, the toxin EIA only caught about 58%.15PubMed Central. Performance of Clostridium difficile toxin enzyme immunoassay and nucleic acid amplification tests stratified by patient disease severity The combined EIA approach, while convenient, has also shown specificity limitations that make it less reliable as a standalone confirmatory test.16PubMed Central. Detection of clostridium difficile antigen and toxin in stool specimens
The practical takeaway for patients: a negative “rapid test” for C. diff does not reliably rule out infection, particularly if you have symptoms consistent with the disease. Molecular testing picks up cases that the quicker antibody-based tests miss, regardless of whether the strain produces one or both major toxins.
Treatment and the Story of Bezlotoxumab
The relative importance of each toxin has directly shaped therapeutic development. Bezlotoxumab is a monoclonal antibody that targets Toxin B and was approved specifically to reduce the risk of recurrent C. diff infection. In two large randomized trials, patients who received bezlotoxumab alongside standard antibiotics had a sustained cure rate (initial cure without recurrence within twelve weeks) of about 64%, compared to 54% with placebo.17PubMed. Bezlotoxumab for Prevention of Recurrent Clostridium difficile Infection
A companion antibody targeting Toxin A, called actoxumab, was tested alongside bezlotoxumab but turned out to provide no additional benefit. Patients receiving the combination of actoxumab plus bezlotoxumab actually had a slightly lower sustained cure rate than those receiving bezlotoxumab alone.18PubMed Central. Bezlotoxumab for Preventing Recurrent Clostridioides difficile Infection: A Narrative Review from Pathophysiology to Clinical Studies The actoxumab antibody was not pursued for approval. This outcome was perhaps the most striking clinical validation of the idea that Toxin B is the more important therapeutic target, though experimental work suggests that neutralizing both toxins simultaneously could have value in certain prevention contexts.19PubMed. Actoxumab + bezlotoxumab combination: what promise for Clostridium difficile treatment?
Vaccine Efforts Target Both Toxins
Despite TcdB’s dominance in the treatment space, vaccine developers have continued to target both toxins. The rationale is that a preventive immune response, built before infection, operates differently from a therapeutic antibody given during active disease. Generating neutralizing antibodies against both TcdA and TcdB could provide broader protection, particularly against strains that produce high levels of both.
The most advanced candidate, PF-06425090 (developed by Pfizer), uses genetically detoxified versions of both toxins as the vaccine antigen. Phase 1 and 2 studies in adults aged 50 to 85 showed that three doses were safe and produced strong neutralizing antibody responses against both toxins lasting up to four years.20Clinical Infectious Diseases. CLOVER (CLOstridium difficile Vaccine Efficacy tRial) Study Earlier preclinical work using recombinant fragments of TcdB combined with full-length detoxified TcdA had demonstrated protection in hamster challenge models equivalent to traditional toxoid vaccines.21PubMed. Development of a recombinant toxin fragment vaccine for Clostridium difficile infection A phase 2 dose-optimization trial found that the high-dose formulation with adjuvant achieved seroconversion rates above 90% for both toxins.22PubMed. Defining the optimal formulation and schedule of a candidate toxoid vaccine against Clostridium difficile infection
Whether protective antibodies against TcdA add meaningful clinical benefit on top of anti-TcdB immunity remains an open question, but the conservative approach of targeting both has prevailed in vaccine design.
Bile Acids Can Dial Toxin Production Up or Down
An underappreciated wrinkle in C. diff pathogenesis is that the environment in the gut, particularly the mix of bile acids, can dramatically influence how much toxin a given strain produces. Primary bile acids like cholic acid (CA) and taurocholic acid (TCA) can boost toxin levels by as much as 28-fold in certain strains, while the secondary bile acid chenodeoxycholic acid (CDCA) can suppress toxin output, in some strains reducing it by more than 30-fold.23Journal of Microbiological Methods. Different bile acids have versatile effects on sporulation, toxin levels and biofilm formation of different Clostridioides difficile strains
This matters because the bile acid profile in a person’s gut is heavily influenced by their resident microbiome. Antibiotic treatment, the very thing that predisposes people to C. diff infection, also depletes the bacteria that convert primary bile acids into secondary ones. The result is a gut awash in primary bile acids that promote both C. diff spore germination and toxin production. This connection helps explain why microbiome-disrupting antibiotics set the stage for severe disease and why fecal microbiota transplantation, which restores a healthy bile acid profile, is so effective at preventing recurrence. Both TcdA and TcdB levels are affected by bile acids, though the magnitude of the response varies between strains and toxin types.
Beyond the Gut
C. diff has traditionally been viewed as a purely intestinal pathogen, but emerging evidence suggests its toxins can cause damage beyond the colon. In animal models, researchers have documented a clear link between detectable toxin in the bloodstream (toxemia) and systemic organ damage, including cardiac, kidney, liver, and neurological effects. TcdB appears to be the more important player in these extra-intestinal effects, consistent with its broader receptor range and greater cell-type versatility, although TcdA has also shown organ-specific toxicity in experimental settings.24Hygiene and Environmental Health Advances. Expansion of a versatile pathogen: Clostridioides difficile
Animals without detectable circulating toxin did not develop systemic disease, suggesting that the toxins themselves, rather than a generalized immune response to the infection, are driving the organ damage. This is a relatively new area of investigation, and whether systemic toxin effects contribute meaningfully to outcomes in human patients, particularly the multiorgan failure seen in fulminant C. diff colitis, is not yet settled. But it adds another dimension to understanding why TcdB, with its wider receptor-binding capabilities and greater subtype diversity, may be the more dangerous molecule overall.
Why TcdB Has So Many Faces
The genomic region encoding both toxins, called the pathogenicity locus (PaLoc), is one of the most dynamic stretches of the C. diff genome. Evidence points to it being acquired through large-scale recombination events after the major lineages of C. diff had already diverged, rather than being a stable ancestral feature. The gene for TcdB has been shaped by extensive intragenic recombination, producing mosaic sequences where different segments of the gene can trace their ancestry to different source lineages.9Cell Reports. From colonization to infection: Genomic evolution of Clostridioides difficile pathogenesis
One hypothesis suggests the PaLoc may have originated from an ancient phage integration event, supported by the presence of a phage-like gene (tcdE) sitting between the toxin genes. Under this model, there may have originally been separate single-toxin pathogenicity islands that eventually merged into the two-toxin locus we see today. Whatever the exact history, the practical result is that TcdB varies far more between strains than TcdA does, and that variability has real clinical significance. Different TcdB subtypes bind different receptors, modify different host proteins with different efficiencies, and may respond differently to therapeutic antibodies. This diversity is one reason why developing broadly protective treatments and vaccines against C. diff remains so challenging.