I was recently encouraged by one of my readers to do a blog post on
Blastocystis
and animal experimental models. This is not exactly my core competence, which probably boils down to the fact that animal models have only been scarcely used in
Blastocystis research for reasons that I will try to account for below.
Animal models (mice, rats, guinea pigs) have often been
used to study interactions between hosts and microbes as well as the effect of
chemotherapeutic interventions. Therefore, one might assume that animal models
are an obvious way of potentially establishing a link between
Blastocystis and pathology. But
currently, the rationale for carrying out some types of
Blastocystis experiments on,
say, mice or rats is limited. Why? Well, first and foremost because of at least three major
issues.
1) Lack of correlation between in vitro and in vivo evidence. Experimental infections of laboratory mice (Elwakil and Hewedi, 2010) resulted in tissue invasion - something never reported in humans. Another study showed increased oxidative stress in
Blastocystis infected rats (Chandramathi et al., 2010), again something not linked to human colonisation. Studies that provided evidence for induction of cytokines, contact mediated apoptosis, and barrier disruption all used axenic
Blastocystis and in vitro mammalian cell cultures with no evidence that these effect occur in vivo.
2) Host specificity. Blastocystis exhibits extreme genetic diversity and
multiple, genetically very different variants (species, subtypes) exist. These subtypes
exhibit moderate host specificity. This means that some subtypes are common in
one type of host, whereas other subtypes are common in other types of hosts. For
instance, ST5 is very common in pigs, but we rarely see it in humans. ST4 is
common in rodents, and in some human populations (mainly Europe it seems), but
otherwise extremely uncommon. And so on. This means that some subtypes may be
difficult to establish in experimental animals. It also means that any pathology
detected in the animal, may not be “reproducible” in another host, - maybe due
to the fact that this host has adapted to this particular subtype or even
strain. Blastocystis is common in a huge variety of animals, and different
animals may have adapted do different subtypes. It is not unlikely that this is
due to co-evolution, and therefore it may not turn out to be a big surprise if
Blastocystis per se is not usually directly associated with disease. It may
still be so, however, that for humans, some subtypes or strains may be
associated with disease, preliminary data point in this direction.
3) Study design. Another issue is the use of appropriate controls –
for example, experimental infection of animals with
Blastocystis from cultures growing with bacteria need to have the
appropriate controls - namely infection with the accompanying bacterial flora alone
– before it can be concluded that
Blastocystis
is responsible for any effects seen. It is extremely difficult to
axenise (i.e. make sterile)
Blastocystis strains, so they will always be
accompanied by some bacterial species. Hence, any effect noticed after
challenge with a
Blastocystis strain will be difficult to interpret, - is it
due to
Blastocystis or to accompanying bacterial strains? (If you want to see what
Blastocystis look like in culture, go to my previous blog post
here.)
So, results from scientific studies using animal experimental
models should be interpreted cautiously. In vitro experimental models using
enterocyte mono-layers for instance may constitute a more attractive
alternative, but the problems of using xenic (i.e. unsterile) strains are
evident also here. A great challenge ahead is the development of a standardised
method for axenising (sterilising) strains… so far, such a method does not exist.
Our French colleagues recently published the genome of
Blastocystis sp. ST7. Functional genomic analysis is key to understanding the
extent to which Blastocystis is capable of exerting any direct pathological
effect, and will assist us in studying the potential pathogenicity of
Blastocystis in the absence of a suitable animal model. Indirect pathological
effects may be more difficult to identify and probably require studies of the
interaction between the host, the parasite and the rest of the gut microbiota
(bacteria). Given our recent technological advances, I believe that a pathway
to knowledge lies in the study of Blastocystis in an ecological context. I
think that we should get an understanding of: 1) Who are colonized with
Blastocystis, 2) From where do we get it, 3) For how long do we have the
parasite, and do we establish symptoms in the very beginning, only to adapt to
the presence of the parasite later on, 4) does Blastocystis require a
particular flora to establish (and are there differences between subtypes (in humans and animals)), 5) could Blastocystis be seen as a proxy for a
given gut microbiota (biomarker), and/or does Blastocystis select for a given
microbiota phenotype (metatranscriptomic analysis of the intestinal flora accompanying
Blastocystis might be useful to study how the bacteria “behave” (i.e. gene
expression) in the presence/absence of Blastocystis),
6) are any Blastocystis-induced
symptoms related to parasite abundance, etc.; this can be explored in rough
detail by using real-time PCR, of which two have been published.
So, while animal models may not be immediately suitable
in our quest to study
Blastocystis
pathogenicity, our “omics” methodologies and data analyses may sooner than we
know help us answer many of the questions that we have been pondering for
decades.
Having said that, I think that for instance a pig experimental model might be useful in terms of studying the effect of chemotherapeutic intervention. Obvious studies include those aiming to identify drugs capable of eradicating
Blastocystis, but it could also be interesting to study the structure and function (gene expression profiling) of the accompanying microbiota before and after intervention.
Since pig feed often contains a range of antibiotics, it could be interesting to test whether pigs on diets +/- antibiotics differ in terms of
Blastocystis colonisation... a recent PNAS paper demonstrates a shift in the structure and function of the microbiome in medicated pigs compared to pigs fed a diet void of antibiotics.
Further reading:
Chandramathi
S, Suresh KG, Mahmood AA, & Kuppusamy UR (2010). Urinary
hyaluronidase activity in rats infected with Blastocystis
hominis--evidence for invasion? Parasitology research, 106 (6), 1459-63 PMID: 20358228
Elwakil HS, & Hewedi IH (2010). Pathogenic potential of Blastocystis hominis in laboratory mice. Parasitology research, 107 (3), 685-9 PMID: 20499092
