What are Cytotoxin A, Cytotoxin B, and Glutamate Dehydrogenase (GDH) in Clostridium difficile associated colitis?

Understanding Cytotoxin A, Cytotoxin B, and GDH in Clostridioides difficile Infection

Primary Virulence Factors: Toxins A and B

Toxins A and B are the primary virulence determinants of C. difficile that cause disease through direct cellular damage, with toxin B now recognized as essential for virulence. 1

Mechanism of Action

• Both toxins A and B function as glucosyltransferases that catalyze the transfer of glucose onto Rho family GTPases inside host cells 1, 2
• This glucosylation promotes activation of Rho GTPases, leading to disorganization of the cytoskeleton of colonocytes and eventual cell death 1, 3
• The toxins are pro-inflammatory, cytotoxic, and enterotoxic in the human colon 2

Clinical Significance

• Only toxigenic strains cause disease—non-toxigenic C. difficile strains are non-pathogenic since CDI is a toxin-mediated infection 1
• Toxin B is essential for virulence, contrary to earlier beliefs that toxin A was the major virulence factor 2
• Both toxins have 63% amino acid sequence similarity and work together to induce colonocyte death and colitis 1, 2
• Epidemic strains (NAP1/027) produce 16-23 times higher concentrations of toxins A and B compared to non-epidemic strains, explaining increased disease severity 4

Glutamate Dehydrogenase (GDH): A Screening Marker

GDH is a metabolic enzyme produced by all C. difficile strains—both toxigenic and non-toxigenic—making it useful as a highly sensitive screening test but not diagnostic of active infection. 1

Diagnostic Role

• GDH detection serves as a first-step screening test with high sensitivity (90.8%) but cannot distinguish between toxigenic and non-toxigenic strains 1, 5
• A negative GDH test reliably excludes CDI with a negative predictive value of 99.6% 5
• GDH testing is rapid (20 minutes) and easy to perform, making it practical for initial screening 5

Diagnostic Algorithm: Two-Step Approach

Current guidelines recommend a two-step testing algorithm to overcome the limitations of individual tests, starting with high-sensitivity screening followed by confirmatory toxin detection. 1, 6

Recommended Testing Strategy

• Step 1: Screen with GDH or nucleic acid amplification test (NAAT/PCR) for high sensitivity (91%) 6
• Step 2: Confirm positive screens with toxin A/B detection for higher specificity (98%) 6
• This approach addresses the low positive predictive value problem when CDI prevalence is low (5%), where PPV ranges only 0.28-0.77 for single tests 1

Critical Pitfalls to Avoid

• Never test asymptomatic patients or those with formed stools—this detects colonization rather than infection 6
• Do not rely solely on toxin EIA testing, which has suboptimal sensitivity (70-80%) 6
• Testing should only be performed on patients with clinically significant diarrhea (≥3 unformed stools in 24 hours) 6
• A "test of cure" is not recommended as patients may shed spores for up to 6 weeks after successful treatment 6

Reference Standard Methods

• Cell culture cytotoxicity assay (CCA) detects toxin B by observing cytopathic effects (cell rounding) on cultured cells after 24-48 hours 1
• Toxigenic culture involves culturing C. difficile on selective media followed by in vitro toxin detection, providing higher sensitivity than CCA 1
• Both reference methods are time-consuming and require specialized facilities, explaining why rapid EIAs and molecular tests have replaced them in clinical practice 1