Exploring Blastocystis: Hidden Complexity in a Common Intestinal Microbe

Blastocystis is an anaerobic Stramenopile that colonises 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 recognised, 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 (IBS) 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 qPCR is the most reliable and 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 generally reserved for persistent symptoms after alternative causes have been excluded; metronidazole is most commonly used but responses are variable and reduced susceptibility has been reported.