An opportunistic pathogen with a quiet early life
C. difficile is an opportunistic pathogen. In early infancy, colonization is common: studies have reported C. difficile in up to approximately half of infants, particularly during the first months of life, and most colonized infants do not develop symptomatic disease.
Compared with adults, infants have a developing gut microbiome with a different mix of bacteria, including many Bifidobacterium species. Early colonizing bacteria use oxygen in the intestine, creating an environment where other beneficial bacteria can become established. As a result, C. difficile may be present in infants without causing illness.
In adults, the gut microbiome is more established and diverse. These bacteria help keep C. difficile under control by competing for nutrients, producing substances that limit its growth, and supporting the intestinal lining and immune system. This natural protection is known as colonization resistance. When antibiotics or other factors disrupt the microbiome, C. difficile may multiply and produce toxins that cause disease.
Why older adults carry higher C. difficile infection (CDI) burden
The human microbiome changes throughout life. The direction and magnitude of these changes vary among individuals, but aging may be accompanied by reduced diversity, loss of some beneficial metabolic functions, altered bile-acid metabolism, immune senescence, and greater exposure to healthcare environments and medications. Together, these factors can make the intestinal ecosystem less resilient when it is challenged.
The epidemiology is consistent with this biological model. In CDC surveillance data, the reported 2023 CDI rate was 351.0 cases per 100,000 people aged 65 years or older, compared with 75.5 cases per 100,000 among people younger than 65. Older adults therefore experienced CDI at approximately 4.6 times the rate of younger people. Age itself is not the only explanation. More frequent hospital and long-term-care exposure, multimorbidity, immunologic changes, and repeated antibiotic courses often occur alongside age-related microbiome change.
Antibiotics are the most common trigger of CDI
Antibiotic treatment is one of the clearest triggers for CDI. Broad-spectrum antibiotics can reduce the abundance and function of commensal organisms that normally compete with C. difficile for nutrients and produce metabolites that inhibit the growth of C. difficile. This disruption, often called dysbiosis, can reduce colonization resistance and create an opportunity for C. difficile spores to germinate and for vegetative cells to expand.
C. difficile spores are resistant to environmental stress and are not eliminated by antibiotics that target actively growing bacteria. After antibiotic exposure has weakened the surrounding microbial community, C. difficile spores can germinate in the intestine. Toxigenic strains may then produce toxins, damage the intestinal epithelium and cause diarrhea. In other words, antibiotic exposure removes the resistance barriers that normally keep C. difficile at bay.
Antibiotic treatment can resolve an acute episode of CDI, but some patients remain vulnerable to recurrence because the intestinal microbiome may not fully recover. For patients with recurrent CDI—and, in selected cases, severe or fulminant CDI that does not respond adequately to standard therapy—fecal microbiota transplantation (FMT) or other microbiota-based therapies may be used under specialist guidance. These approaches aim to restore a microbial community that can compete with C. difficile and re-establish functions associated with colonization resistance.
C. difficile test results need to be interpreted within clinical context
The diagnosis of CDI is complicated because C. difficile can be present without causing disease. Both toxigenic and nontoxigenic strains exist. Molecular assays commonly detect genes associated with toxin production, but detection of a toxigenic gene does not prove that toxin is being produced or that C. difficile is the cause of a patient’s diarrhea. A patient with formed stool, no compatible symptoms, or taking laxatives may have a positive nucleic acid amplification test (NAAT) because of colonization rather than infection. Overdiagnosis can lead to unnecessary treatment, additional antibiotic exposure, and unnecessary isolation.
The IDSA/SHEA guideline identifies patients with unexplained, new-onset diarrhea consisting of at least three unformed stools in 24 hours as the preferred population for testing. The guideline also recommends institutional processes to avoid testing patients whose diarrhea may be explained by laxatives and to reject formed stool specimens. CDC guidance similarly recommends stopping laxatives and waiting at least 48 hours before testing if symptoms persist, and it recommends testing only unformed stool.
When specimen submission is restricted to patients with compatible symptoms, a highly sensitive NAAT can be used appropriately. When laboratories receive specimens without reliable clinical screening, a multistep algorithm, such as GDH plus toxin with NAAT arbitration or NAAT plus toxin, can improve the accuracy of diagnosis. The best algorithm depends on local patient-selection practices, test performance, workflow, and institutional policy.
Practical messages for laboratory professionals
- Test the patient, not just the organism. CDI testing should be reserved for patients with unexplained, new-onset diarrhea—typically at least three unformed stools within 24 hours.
- Reject formed stool. Formed stool is generally inappropriate for CDI testing because detection may represent colonization rather than disease.
- Check for laxative use. Laxatives can cause diarrhea and confound interpretation. Discontinue laxatives and wait at least 48 hours before testing if diarrhea persists.
- Do not interpret a positive NAAT as proof of active CDI. NAAT detects DNA from a toxigenic C. difficile strain, but it does not prove that toxin is being produced or that C. difficile is causing the patient’s symptoms.
- Use multistep testing to improve diagnostic confidence. Algorithms that combine GDH and toxin testing, with NAAT used to resolve discordant results, or NAAT plus toxin testing, can improve the positive predictive value of CDI testing by adding evidence of toxin production. This added specificity can be beneficial whether or not the laboratory has strict specimen-submission criteria. The preferred approach should align with local testing practices and current guidelines.
- Avoid testing asymptomatic patients and testing for cure. NAAT can remain positive after clinical resolution, so repeat testing does not reliably indicate treatment success or failure and is not recommended
- Be cautious when testing infants. C. difficile colonization is common during infancy; testing of infants, particularly those younger than 12 months, can produce misleading results.
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References
- Centers for Disease Control and Prevention. C. diff: Facts for Clinicians.
https://www.cdc.gov/c-diff/hcp/clinical-overview/ - Centers for Disease Control and Prevention. Clinical Testing and Diagnosis for C. diff Infection.
https://www.cdc.gov/c-diff/hcp/diagnosis-testing/index.html - McDonald LC, Gerding DN, Johnson S, et al. Clinical Practice Guidelines for Clostridium difficile Infection in Adults and Children: 2017 Update by IDSA and SHEA. Clinical Infectious Diseases. 2018;66(7):e1-e48. https://doi.org/10.1093/cid/cix1085
- Kelly CR, Fischer M, Allegretti JR, et al. ACG Clinical Guidelines: Prevention, Diagnosis, and Treatment of Clostridioides difficile Infections. American Journal of Gastroenterology. 2021;116(6):1124-1147.
https://doi.org/10.14309/ajg.0000000000001278 - Centers for Disease Control and Prevention. Antimicrobial Resistance Threats Report: Clostridioides difficile.
https://arpsp.cdc.gov/artr/cdiff - Van Nood E, Vrieze A, Nieuwdorp M, et al. Duodenal Infusion of Donor Feces for Recurrent Clostridium difficile. New England Journal of Medicine. 2013;368:407-415.
https://doi.org/10.1056/NEJMoa1205037 - Clostridioides difficile Colonization Is Differentially Associated With Gut Microbiome Profiles by Infant Feeding Modality at 3-4 Months of Age. PubMed record.
https://pubmed.ncbi.nlm.nih.gov/31921134/ - The high prevalence of Clostridioides difficile among nursing home elders associates with a dysbiotic microbiome. Gut Microbes. 2021;13(1):1897209.
https://doi.org/10.1080/19490976.2021.1897209
