Exploring Blastocystis: hidden complexity in a common intestinal microbe

Blastocystis is an anaerobic stramenopile that colonizes the large intestine of humans and many animals worldwide. Small-subunit rRNA-based typing reveals deep genetic diversity: at least 44 genetic variants, namely, subtypes (STs), are recognized, with ST1-ST4 accounting for most human carriage. Its medical relevance remains contested. Blastocystis carriage is frequently asymptomatic, and while associations with non-specific gastrointestinal symptoms (e.g. diarrhoea, abdominal pain and bloating), irritable bowel syndrome and extraintestinal manifestations such as urticaria have been proposed. In parallel, multiple microbiome studies, including a recent population-scale analysis, link Blastocystis carriage to higher bacterial diversity, healthier diets and favourable cardiometabolic profiles, suggesting that in many settings, it may be a marker of a resilient gut ecosystem rather than a primary pathogen. Transmission is primarily faecal-oral, most likely via environmentally resilient cysts, with potential for zoonotic and waterborne spread. Diagnosis using quantitative PCR is the most sensitive method; microscopy has limited sensitivity and can be confounded by morphological plasticity, though its diagnostic accuracy varies considerably according to the experience of the microscopist. Treatment is typically based on excluding alternative causes rather than confirmed causality; while metronidazole is commonly used, clinical outcomes are highly variable due to potential co-infections, inconsistent links between parasite eradication and symptom relief and reports of reduced drug susceptibility.