Mechanisms
Five mechanisms recur across the experimental literature. None of them proves that every Blastocystis carrier is ill, but together they describe a credible route from colonisation to symptoms in susceptible people carrying particular strains.
The best-characterised virulence factors are proteases. Blastocystis releases cysteine proteases and a surface legumain that activates cathepsin B. These enzymes degrade host proteins, increase paracellular permeability in human intestinal cell monolayers and can be blocked in vitro by protease inhibitors such as E-64. Protease activity is not uniform: it varies both between subtypes and between isolates of the same subtype.
Secretory IgA is the antibody that keeps microbes at a distance from the mucosa. Blastocystis lysate and its conditioned medium cleave human secretory IgA through protease activity, which would allow the organism to persist in the gut and may also loosen control over other resident microbes.
In cell and animal models Blastocystis induces apoptosis of intestinal epithelial cells through caspase 3 and 9, rearranges F-actin, lowers transepithelial electrical resistance and disorganises the tight-junction proteins ZO-1 and occludin. Some of this happens without direct contact, pointing to secreted factors. Signalling through Rho kinase is implicated, and simvastatin prevented the effect in one study.
Subtype 7 is the most consistently damaging in laboratory work: strongly adherent ST7 strains bind epithelial junctions, break down ZO-1 and occludin and raise permeability, while subtype 4 did not in the same assays. Isolates from symptomatic patients show higher caspase-like activity than isolates from asymptomatic carriers. This is the strongest explanation for why identical stool findings can mean very different things clinically.
ST7 infection in people with diarrhoea was associated with lower bacterial diversity and enrichment of Enterobacteriaceae and Escherichia–Shigella. In IBS cohorts, carriage has been linked to fewer protective bacteria such as Bifidobacterium, although at least one study found the IBS-associated shift regardless of Blastocystis status. Inflammatory readouts — IL-17 in Hashimoto's patients, TNF-α and the gut-permeability marker IFABP2 in rheumatoid arthritis — rose with carriage in the studies below.
Puthia MK, Vaithilingam A, Lu J, Tan KSW · Parasitology Research
Blastocystis lysate and conditioned medium cleave secretory IgA through cysteine and aspartic protease activity — a plausible mechanism of mucosal immune evasion.
Open in PubMed · PMID 16151742Mirza H, Tan KSW · Parasitology Research
Cysteine protease activity differs strongly both between subtypes and between isolates of the same subtype, which helps explain why some carriers are symptomatic and others are not.
Open in PubMed · PMID 18846388Wu B, Yin J, Texier C, Roussel M, Tan KSW · Journal of Biological Chemistry
A surface-localised legumain protease is required for parasite survival, identifying a virulence-related enzyme and a possible drug target.
DOI 10.1074/jbc.M109.049064Nourrisson C, Wawrzyniak I, Cian A, et al. · Parasitology
A secreted, legumain-activated cathepsin B increases permeability of human intestinal cell monolayers; the effect is blocked by the protease inhibitor E-64.
DOI 10.1017/S0031182016001396Puthia MK, Sio SWS, Lu J, Tan KSW · Infection and Immunity
Even without direct contact, Blastocystis triggers apoptosis, actin rearrangement, lower transepithelial resistance and higher permeability in intestinal epithelial cells.
Open in PubMed · PMID 16790785Wu Z, Mirza H, Teo JDW, Tan KSW · BioMed Research International
Subtype 7 — but not subtype 4 — significantly increases apoptosis in human gut cells via caspase 3/9 and disorganises the tight-junction protein ZO-1.
DOI 10.1155/2014/209163Balakrishnan DD, Kumar SG · Parasites & Vectors
Isolates taken from symptomatic patients showed higher caspase-like activity and apoptosis than isolates from asymptomatic carriers.
DOI 10.1186/1756-3305-7-219Mirza H, Wu Z, Kidwai F, Tan KSW · Cellular Microbiology
Blastocystis cysteine proteases compromise the epithelial barrier through Rho-kinase signalling (falling resistance, ZO-1 and actin disruption); simvastatin prevents it in vitro.
DOI 10.1111/j.1462-5822.2012.01814.xWu Z, Mirza H, Tan KSW, et al. · PLoS Neglected Tropical Diseases
Strongly adherent ST7 strains bind intestinal cell junctions, disrupt ZO-1 and occludin and raise permeability; adhesion strength tracks with virulence.
DOI 10.1371/journal.pntd.0002885Deng L, Lee JWJ, Tan KSW · Parasites & Vectors
In patients with diarrhoea, ST7 carriage was linked to lower bacterial diversity and enrichment of Enterobacteriaceae and Escherichia–Shigella.
DOI 10.1186/s13071-022-05435-zNagel R, Traub RJ, Allcock RJN, et al. · Microbiome
IBS patients showed more Firmicutes and fewer Bacteroidetes regardless of Blastocystis status — carriage alone did not explain the phylum-level shift.
DOI 10.1186/s40168-016-0191-0Nourrisson C, Scanzi J, Pereira B, et al. · PLoS ONE
Blastocystis carriage was associated with reduced protective bacteria (including Bifidobacterium) in the stool of IBS patients compared with controls.
DOI 10.1371/journal.pone.0111868Piperni E, Nguyen LH, Manghi P, et al. · Cell
In a very large multi-country metagenomic study, carrying Blastocystis was associated with plant-rich diets, lower BMI and better cardiometabolic markers.
Open in PubMed · PMID 38981480Beghini F, Pasolli E, Truong TD, Putignani L, Cacciò SM, Segata N · ISME Journal
Across 1,689 subjects there was no positive association with disease; Blastocystis was less common in colorectal cancer and Crohn's-associated dysbiosis.
DOI 10.1038/ismej.2017.139Stensvold CR, Sørland BA, Berg RPKD, et al. · Microorganisms
Blastocystis carriers had significantly higher microbial diversity and more eubiosis-associated genera such as Prevotella and Faecalibacterium.
Open in PubMed · PMID 35208781