For a long time, finding parasites in a stool sample was almost automatically interpreted as a sign of infection or disease. However, advances in molecular diagnostics and sequencing have shown that some unicellular eukaryotes are more common, diverse and ecologically complex in the human gut than previously thought (Stensvold, 2025). Among them, Blastocystis has stood out for its potential role in modulating the gut microbiota, a topic that gained attention at the 2026 Gut Microbiota for Health World  Summit.

 

Blastocystis, gut microbiota and a shift in perspective

For more than a century, Blastocystis was viewed mainly through the lens of parasitology. First described in 1911 as Blastocystis enterocola and, one year later, named Blastocystis hominis after being isolated from human feces, this intestinal protist became associated with nonspecific gastrointestinal symptoms such as diarrhea, constipation, abdominal pain, flatulence and bloating. However, recent research is challenging the interpretation of Blastocystis as a simple intestinal pathogen, suggesting that its role in the human gut ecosystem is more complex and context-dependent than previously thought.

The human gut microbiota varies widely between individuals and is shaped by diet, geography, lifestyle, antibiotic use, disease, age and, to a lesser extent, genetics. Within this ecosystem, Blastocystis is one of the unicellular eukaryotes most frequently detected in human stool samples.

A pioneering study found that people colonized by Blastocystis had a more diverse gut microbiota, suggesting that its presence is not always linked to dysbiosis or disease. Later studies supported this idea: in individuals without symptoms, Blastocystis was associated with a distinct microbiota profile, changes in the production of short-chain fatty acids and lower fecal levels of markers related to immunity and inflammation, such as calprotectin and IgA. Rather than being merely a “passenger” in the gut, Blastocystis may therefore act as a possible modulator of the microbial community.

From a functional perspective, Blastocystis may interact with the gut ecosystem in several ways: by competing for nutrients, interacting with commensal bacteria, modulating mucosal immune responses and influencing important metabolites, such as short-chain fatty acids and tryptophan-derived compounds. Recent reviews emphasize that its effects probably vary according to subtype, colonization load, the surrounding microbiota, diet, geography and host immune status. This variability helps explain why Blastocystis may be linked to greater microbial diversity in some contexts and to changes in intestinal barrier function or immune responses in others, as discussed below.

 

Not all Blastocystis are the same

Part of the complexity surrounding Blastocystis comes from the fact that it is not a single, homogeneous organism. Epidemiological studies have identified several subtypes in humans, especially ST1 to ST4, which account for most detected cases. In addition, the same person can carry more than one subtype, making it difficult to link each variant to specific clinical effects.

This diversity helps explain why studies on Blastocystis do not always point in the same direction. In some contexts, its presence has been associated with conditions such as irritable bowel syndrome, inflammatory bowel disease and urticaria. In others, especially in asymptomatic individuals, Blastocystis has been linked to greater bacterial diversity and to microbiota profiles that do not resemble the classical dysbiosis observed in metabolic, inflammatory or infectious gastrointestinal diseases.

Large-scale metagenomic analyses have further expanded this discussion. In a study of 56,989 human microbiome samples from 32 countries, Blastocystis was detected worldwide in the gut of healthy adults. One notable finding was the geographic distribution of ST4, which was prevalent in Europe and North America, rare in Asia and not detected in South America or Africa in that analysis, suggesting a possible relationship with regional patterns of lifestyle, environmental exposure or diet.

 

Diet, geography and metabolic health: what do large studies show?

The presence of Blastocystis varies widely across populations. In general, it is less common in industrialized countries and more frequent in developing countries or rural populations, suggesting that factors such as diet, lifestyle, environment and microbial exposure may play a role. Among these factors, diet appears to be particularly important.

In the study by Piperni and colleagues, which analyzed data from thousands of participants across different countries, the presence of Blastocystis was associated with dietary patterns considered healthier, including higher consumption of plant-based and minimally processed foods. In addition, in a group followed before and after a personalized dietary intervention, improvements in diet quality were accompanied by increases in the prevalence and abundance of Blastocystis. Blastocystis-positive individuals also showed more favorable glycemic and lipid profiles and lower body mass index across different cohorts. Although these findings do not prove that Blastocystis causes better metabolic health, they suggest that its presence may reflect, accompany or possibly contribute to a gut ecosystem associated with better diet quality and cardiometabolic health.

Data from non-westernized populations support this interpretation. In a rural community in Colombia, Blastocystis-positive individuals showed differences in gut microbiota composition and a higher presence of some bacterial groups often associated with fiber fermentation, immune modulation and intestinal homeostasis. The study also suggested that colonization intensity matters: not only the presence, but also the abundance of Blastocystis may influence the ecological organization of the gut microbiota.

 

Take-home messages

The emerging view is not that Blastocystis is universally beneficial or harmful, but that its clinical and ecological meaning depends on context. Its detection in stool should not be automatically interpreted as evidence of disease, dysbiosis, or a need for eradication, especially in asymptomatic individuals. At the same time, its presence should not be overlooked, as it may carry relevant information about diet, lifestyle, microbial ecology and host physiology.

In other words, Blastocystis shows that the gut microbiota cannot be understood simply by dividing microorganisms into “good” and “bad”. Its role depends on the ecological relationships it establishes within the gut.

 

References:

  1. Audebert C, Even G, Cian A; Blastocystis Investigation Group; Loywick A, Merlin S, Viscogliosi E, Chabé M. Colonization with the enteric protozoa Blastocystis is associated with increased diversity of human gut bacterial microbiota. Scientific Reports. 2016;6:25255. doi:10.1038/srep25255.
  2. Castañeda S, Tomiak J, Andersen LO, Acosta CP, Vasquez-A LR, Stensvold CR, Ramírez JD. Impact of Blastocystis carriage and colonization intensity on gut microbiota composition in a non-westernized rural population from Colombia. PLoS Neglected Tropical Diseases. 2025;19(5):e0013111. doi:10.1371/journal.pntd.0013111.
  3. Deng L, Tan KSW. From parasite to partner: unravelling the multifaceted role of Blastocystis in human health and disease. The Lancet Microbe. 2025;6(9):101155. doi:10.1016/j.lanmic.2025.101155.
  4. Nieves-Ramírez ME, Partida-Rodríguez O, Laforest-Lapointe I, Reynolds LA, Brown EM, Valdez-Salazar A, Morán-Silva P, Rojas-Velázquez L, Morien E, Parfrey LW, Jin M, Walter J, Torres J, Arrieta MC, Ximénez-García C, Finlay BB. Asymptomatic intestinal colonization with protist Blastocystis is strongly associated with distinct microbiome ecological patterns. mSystems. 2018;3(3):e00007-18. doi:10.1128/mSystems.00007-18.
  5. Piperni E, Nguyen LH, Manghi P, et al. Intestinal Blastocystis is linked to healthier diets and more favorable cardiometabolic outcomes in 56,989 individuals from 32 countries. Cell. 2024;187(17):4554-4570.e18. doi:10.1016/j.cell.2024.06.018.
  6. Popruk S, Adao DEV, Rivera WL. Epidemiology and subtype distribution of Blastocystis in humans: A review. Infection, Genetics and Evolution. 2021;95:105085. doi:10.1016/j.meegid.2021.105085.
  7. Stensvold CR. Aspects of Genetic Diversity, Host Specificity and Public Health Significance of Single-Celled Intestinal Parasites Commonly Observed in Humans and Mostly Referred to as ‘Non-Pathogenic’. APMIS. 2025 Sep;133(9):e70036. doi: 10.1111/apm.70036.