Nystatin and Gut Bacteria
Nystatin has minimal to no effect on gut bacteria because it is not absorbed from the gastrointestinal tract and acts exclusively against fungi, particularly Candida species, with no appreciable antibacterial activity. 1
Mechanism and Pharmacology
Nystatin is not systemically absorbed when administered orally, with gastrointestinal absorption being insignificant—most orally administered nystatin passes unchanged in the stool. 1
The drug acts specifically on fungal cells by binding to sterols in the cell membrane of susceptible Candida species, causing membrane permeability changes and leakage of intracellular components. 1
Nystatin exhibits no appreciable activity against bacteria, protozoa, or viruses, making it highly selective for fungal organisms without disrupting bacterial flora. 1
Clinical Implications for Gut Microbiome
The lack of systemic absorption and bacterial activity means nystatin functions as a topical antifungal agent within the gut lumen without the collateral damage to commensal bacteria seen with systemic antifungals or antibiotics.
Prophylactic Use in Neonates
Oral nystatin (100,000 units three times daily for 6 weeks) is recommended as an alternative to fluconazole in neonates with birth weights <1500 g when fluconazole availability or resistance is a concern. 2
The mechanism of prophylaxis involves reducing Candida colonization burden in the gut, thereby decreasing the probability of translocation into the bloodstream in vulnerable populations. 2
Studies demonstrate that nystatin reduces fungal colonization and invasive candidiasis rates in premature neonates, though it does not impact overall mortality. 2
Bacterial Flora Preservation
A critical advantage of nystatin over systemic antifungals is its preservation of normal bacterial gut flora:
Unlike broad-spectrum antibiotics that disrupt the gut microbiome and paradoxically increase fungal overgrowth risk, nystatin selectively targets fungi while leaving bacterial populations intact. 1
This selective action makes nystatin particularly valuable in settings where maintaining gut bacterial diversity is important, such as in neonates developing their microbiome or immunocompromised patients already at risk for dysbiosis.
Clinical Effectiveness and Limitations
Efficacy Profile
Nystatin reduces multiple body site colonization and persistent oropharyngeal colonization compared to no prophylaxis, though it may not completely prevent disseminated fungal infections in all high-risk populations. 3
In comparative studies, fluconazole demonstrates superior efficacy to nystatin for preventing Candida infections in neutropenic patients (68% vs 47% success rate), though both are generally well-tolerated. 4
Practical Considerations
A potential concern with oral nystatin in premature infants is inadvertent damage to the fragile gut epithelium, with theoretical risk of necrotizing enterocolitis (NEC), which limits its recommendation strength to B-II grade. 2
Nystatin cannot be administered during ileus, gastrointestinal disease, feeding intolerance, or hemodynamic instability—common situations in low-gestational-age premature infants that limit its broad applicability. 2
Treatment Applications Beyond Prophylaxis
For oropharyngeal candidiasis, nystatin suspension (200,000-400,000 units four times daily) is a first-line topical option that works locally without systemic effects. 2, 5
The suspension should be swished thoroughly in the mouth for at least 2 minutes before swallowing to ensure adequate contact with affected mucosal surfaces. 5
Complete the full 7-14 day course even when symptoms resolve early to prevent recurrence. 5