Blastocystis: Harmless or Harmful? The Short Answer is…It Depends!

Here’s a topic I haven’t written about much: single-cell eukaryotes. A eukaryote is a living organism whose cells contain a nucleus (in which genetic material is contained) within a membrane.  Thus, in the world of living things, only bacteria and archaea (which is sort of a kind of bacteria) are not eukaryotes.  Protozoa, which I wrote about once before, are eukaryotes and as I pointed out, there are many single-celled organisms in the human biome that are not yet well studied.

One that is pretty well-known though – which many of you may have heard of – is Blastocystis, which up until now, was generally thought to be benign.  Bearing that in mind, I thought it interesting that in my work as a nutritionist, I have come across many people who were thought to be experiencing GI issues from the organism.  I have often wondered why it’s generally harmless – and yet, at times seems pathogenic.  I took a wild guess that perhaps it had to do with the state of the biome, in general:  perhaps, in people with inflammatory issues and dysbiosis, the organism caused a different reaction???… kind of like H.pylori which at times, is benign, at times, most certainly not.

As it turns out, the answer may be simpler. A group of researchers in Singapore have discovered a subtype of Blastocystis, ST7, that destroys probiotic bacteria in the gut.[i]  Thus far, it’s only been found in Singapore, Japan and in one Danish study.  But that’s simply because no one has been looking for it.

Blastocystis is, according to this paper, incredibly common, with at estimates claiming it is present in at least 1 billion people worldwide.

This strain of Blastocystis apparently induces oxidative stress in the gut.  The molecules released (reactive oxygen species) kill Bifidobacterium.  The Blastocystis also reduces Lactobacillus, although the mechanism of action for this is not yet known.  Worse still, this type of Blastocystis also works with E.coli to kill even more probiotic bacteria.  Remember a few weeks ago when I described gut bacteria carrying their own viral killers?  It turns out that both E.coli and Bifidobacterium help Blastocystis to grow better.

“The results of this study indicated that Blastocystis can disrupt gut microbiota populations particularly decreasing the content of Bifidobacteria and Lactobacillus but increasing Escherichia coli. Possible explanations of these occurrences point to oxidative stress caused by Blastocystis as well as host factors induced by the parasite. Our data indicates that while Blastocystis spp. may be a member of healthy gut microbiota, specific isolates or rare ST may disrupt homeostasis leading to pathological states in the host.”

Blastocystis ST7 also injures the gut lining by triggering an inflammatory response, “…causing lesions (ulcers) and a disordered structure of the intestinal lining…Add to this the loss of the protective good bacteria, an infection with Blastocystis ST7 could be a recipe for long-term damage to the gut lining, possibly contributing to inflammatory bowel disease, irritable bowel syndrome, as well as gastrointestinal and colon cancers.”[ii]

Worst of all, it is resistant to metronidazole (Flagyl), which is the medicine most often used to treat Blastocystis.  By the way, in case you are wondering, the organism is transmitted via food that has been contaminated with feces from infected animals (especially birds).  Nice thought.  (not)

Other subtypes of Blastocystis are strongly associated with a healthy gut.  Several studies support this idea, one of which did look at subtypes of the organism and found ST1 and ST3 which “…are associated with asymptomatic infections.”  Thus, the authors point out, the other studies – upon which the notion is based that Blastocystis is commensal – could have used subjects “…colonized with subtypes associated with lower pathogenic potential and may be associated with healthy gut microbiota.”   One other epidemiological study though did find that the subtype ST4 (which has “moderate pathogenic potential”) was associated with irritable bowel syndrome and dysbiosis (and the populations infected had significantly lower levels of Bifidobacterium and Lactobacillus).  Overall then, these scientists conclude that these findings, “…corroborate with the results of this study, both suggesting that virulent subtypes of Blastocystis are more likely to be associated with dysbiosis, and its pathological outcomes, including IBD and IBS.”

This could I imagine be a huge finding for those suffering from these illnesses.  The question that is not addressed in the paper though is: as it’s resistant to the currently used medication, what do you do about it?!


[i] ohn Anthony Yason, Yi Ran Liang, Chin Wen Png, Yongliang Zhang, Kevin Shyong Wei Tan. Interactions between a pathogenic Blastocystis subtype and gut microbiota: in vitro and in vivo studies. Microbiome, 2019; 7 (1) doi: 10.1186/s40168-019-0644-3


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