Background: Blastocystis is a typical anaerobic colon protist in humans with controversial pathogenicity and has relation with alterations in the intestinal microbiota composition (dysbiosis), whose eventual indicator is the Firmicutes/Bacteroidetes ratio (F/B ratio); this indicator is also linked to complications such as diabetes, obesity, or inflammatory bowel disease. The present study investigated the prevalence of Blastocystis and its association with Firmicutes/Bacteroidetes ratio in healthy and metabolic diseased subjects. Methods: Fecal and blood samples were collected consecutively from 200 healthy subjects and 84 subjects with metabolic disease; Blastocystis and its most frequent subtypes were identified by end-point PCR and the two most representative phyla of the intestinal microbiota Firmicutes and Bacteroidetes by real-time PCR. Results: The prevalence of Blastocystis in healthy subjects was 47.0, and 65.48% in subjects with metabolic disease; the most prevalent subtype in the total population was ST3 (28.38%), followed by ST1 (14.86%), ST4, ST5, and ST7 (each one of them with 14.19% respectively), and finally ST2 (8.78%). The low F/B ratio was associated with the prevalence of Blastocystis in the two cohorts FACSA (OR = 3.78 p < 0.05) and UNEME (OR = 4.29 p < 0.05). Regarding the subtype level, an association between the FACSA cohort ST1 and ST7 with low Firmicutes/Bacteroidetes ratio was found (OR = 3.99 and 5.44 p < 0.05, respectively). Conclusions: The evident predatory role of Blastocystis over Firmicutes phylum was observed in both cohorts since the abundance of bacterial group’s Bacteroidetes increases in the groups colonized by this eukaryote and, therefore, may have a beneficial effect.

The role played by Blastocystis in humans has been a subject of discussion due to its intestinal effects and modifications in the intestinal microbiota. We aimed to analyze the relationship between Blastocystis subtypes ST1-4 and 7, the Firmicutes to Bacteroidetes ratio (F/B ratio) of fecal microbiota, and chronic stress in university students. This study had a cross-sectional design with a sample of 202 students. We analyzed fecal and hair samples, and stress inventories were applied to the students. The results showed a frequency of Blastocystis-colonized students of 52.97%. Regarding fecal microbiota, a median RAU of 0.801 for Firmicutes and 0.82 of Bacteroidetes were obtained, with an F/B ratio of 0.83. A low F/B ratio (66.04%) was more frequent in Blastocystis-colonized students, whereas a high F/B ratio (68.09%) (p = < 0.0001) was found in the Blastocystis-non-colonized. Only Blastocystis ST3 did not significantly correlate with a low F/B ratio (p = 0.290). The ST4 was associated with lower values of cortisol (p = 0.030), psychological stress (p = 0.040), and lower frequency of constipation (p = 0.010). Only two students with the ST1 had abdominal pain (p = 0.007). Our results suggest that colonization by Blastocystis subtypes can modify the intestinal microbiota due to a decreased ratio between the two most representative phyla (F/B). Also, the results of this study show that ST4 colonization is related to a lower level of chronic stress. Graphical abstract

Blastocystis is a parasite commonly found in the gut of humans and animals; there are 22 known subtypes (STs). STs 1-9 and 12 have been found in humans. This parasite has a faecal-oral route of transmission; its high infection prevalence in developing countries is due to poor hygiene practices, exposure to infected animals, and intake of contaminated water or food. Its pathogenicity has not been established, because it has been found in symptomatic and asymptomatic patients. The goal of this study was to analyze the frequency of Blastocystis and its subtypes (1, 2, 3, 4, 5, and 7), and assess the relationship between these subtypes and abdominal pain and distension. 202 university students participated in this study. A questionnaire was applied to assess the gastrointestinal symptoms, and subsequently the students were asked to provide faecal samples. The presence of parasites was determined by optical microscopy. Blastocystis-positive samples had their DNA extracted and end-point PCR was performed to corroborate the presence of Blastocystis and determine its subtypes. Among the samples, 47.03% were positive according to PCR analysis. The most prevalent subtypes were ST3 (29.79%), ST4 (16.84%), and ST1 (14.89%). We found a relationship between ST1 and abdominal pain (OR = 0.196; CI = 0.0533–0.7318; p = 0.015), and between ST4 and abdominal distension (OR = 0.2928; CI = 0.1017–0.8429; p = 0.023). However, the presence of this parasite and the probable relationship with gastrointestinal symptoms suggest the need to determine its role within intestinal microbiota in order to confirm whether its eradication is really necessary or not.

Diet and gut microbiota are significant determinants of host health, but how dietary quality modulates gut microbiota in Blastocystis-colonised individuals remains underexplored. We studied two contrasting cohorts: university students (FACSA, n = 46) and institutionalised children with their caregivers (PAVILA, n = 37), representing distinct dietary and sociodemographic contexts. Eight participants from each cohort tested positive for Blastocystis; however, two PAVILA samples could not be sequenced, resulting in a final microbiota subcohort of 14 individuals (FACSA n = 8, PAVILA n = 6). Dietary quality was assessed using the Healthy Eating Index-2020 (HEI-2020), and faecal microbiota was characterised through 16S rRNA sequencing. Alpha and beta diversity were analysed, and genus-level transformed data were further evaluated using permutational multivariate analysis of variance (PERMANOVA), principal coordinates analysis (PCoA), and distance-based redundancy analysis (db-RDA). The FACSA cohort exhibited higher microbial richness and diversity (Shannon and Simpson indexes, p < 0.01) compared to PAVILA, with marked differences in microbial composition (PERMANOVA R² = 0.39, p = 0.002). Total diet quality correlated with microbial structure (R² = 0.26, p = 0.016), with protein (R² = 0.23, p = 0.017) and vegetable components (R² = 0.17, p = 0.044) as primary contributors. Multivariate analysis showed that higher protein and vegetable intakes were associated with genera such as Sellimonas, Murimonas, Alistipes, and Desulfovibrio (FACSA group). In contrast, Hydrogenoanaerobacterium, V9D2013_group, and Haemophilus were linked to lower-quality diets (PAVILA group). Our results indicate that diet quality significantly influences gut microbiota composition in individuals colonised by Blastocystis, underscoring its potential as a target for nutritional interventions in vulnerable populations.

Diet and gut microbiota are key determinants of host health, but their interaction in Blastocystis-colonised individuals remains poorly understood. We evaluated the rela-tionship between diet quality, faecal microbial diversity, and Blastocystis colonisation in two cohorts: university students (FACSA) and institutionalised children with their caregivers (PAVILA), all Blastocystis-positive subjects. We assessed diet quality using the Healthy Eating Index-2020 (HEI-2020), and we characterised faecal microbiota by 16S rRNA sequencing. Alpha and beta diversity were analysed, and genus-level trans-formed data were further evaluated using PERMANOVA, PCoA, and db-RDA. The FACSA cohort presented higher microbial richness and diversity (Shannon and Simpson indexes) than PAVILA (p < 0.01), and microbial composition differed signifi-cantly between cohorts (PERMANOVA R² = 0.39, p = 0.002). Total diet quality was as-sociated with microbial structure (R² = 0.26, p = 0.016), with protein (R² = 0.23, p = 0.017) and vegetable (R² = 0.17, p = 0.044) components being the main contributors. Multivariate analysis showed higher protein and vegetable intakes were associated with genera such as Sellimonas, Murimonas, Alistipes and Desulfovibrio (FACSA group). In contrast, Hydrogenoanaerobacterium, V9D2013_group and Haemophilus were related to lower-quality diets (PAVILA group). Despite universal colonisation by Blastocystis, differences in microbial diversity and structure reflect a modulating effect of diet con-ditioned by the sociodemographic environment. These findings suggest the relevance of dietary quality as a modulator of the gut microbiota in populations colonised by Blastocystis and may guide future nutritional interventions in vulnerable groups.

Blastocystis is a very common gastrointestinal protozoan globally distributed; it colonizes humans and non-humans, and in some communities, it reaches prevalences of up to 100%. Blastocystis is transmitted through the fecal-oral route, contaminated food and water, and close contact with animals. There are 34 known subtypes of Blastocystis, and subtypes 1 to 4 (ST1ST4) are the most common in humans. It should be remembered that its pathogenicity is controversial since some studies have shown that Blastocystis is more prevalent in healthy individuals; who have greater diversity and richness of the intestinal microbiota; other studies suggest that Blastocystis infections occur in individuals with intestinal dysbiosis. In America and Africa, a high incidence of ST1 and ST2 is observed in rural areas. Recent data indicate that Blastocystis is linked with specific gut microbiota profiles and health indicators. Convincing information and tools that distinguish asymptomatic colonization from infection in children have yet to be demonstrated. Although this protozoan can cause disease under certain circumstances, but the attention of Blastocystis may change, as the frequency of Blastocystis subtypes in children may vary depending on the geographic area and local health conditions.