Dear Group, especially Judy and Michael,
As one of the authors, I thought I should give you the citation of the
Nature paper to stop Michael from having to wade through his piles of
Silberman, J.D., Sogin, M.L., Leipe, D.D., Clark, C.G. 1996 Human
parasite finds taxonomic home. Nature 380: 398
Blasto's closest relative is actually Proteromonas, an intestinal
flagellate of reptiles and amphibia (commensal). Presumably Blasto its
has lost its flagella secondarily, leading to the rather boring looking
organism we see today.
I have studied Blasto almost exclusively in culture, so perhaps I am
out of my depth here, but, like Stenzel and Boreham, I suspect that a
lot of the morpgological forms described in the literature are actually
artefacts - cells in the process of degeneration following exposure to
oxygen. Certainly that is the implication of a short paper by Vdovenko
(Parasitol Res 2000 Jan;86(1):8-10 Blastocystis hominis: origin and
significance of vacuolar and granular forms).
As far as its role in disease is concerned, I believe we will never
find out until people start taking into account the extraordinary
genetic diversity of the organism. Looking at ribosomal RNA gene
sequences, we found that isolates from humans can differ by up to 7%
which is a huge amount for this gene (Clark, C.G. 1997 Extensive genetic
diversity in Blastocystis hominis. Mol. Biochem. Parasitol. 87: 79-83).
I described 7 genetic types in that paper and I and others have
subsequently found a number of additional types. Blastocystis hominis is
clearly a species complex. But given that almost all Blasto
identification is based on microscopy alone, if only one or a few of the
genetic types are capable of causing disease it is not surprising that
population surveys find no link between Blasto and symptoms. If anyone
has any ideas on how to persuade funders that this is a worthwhile
project I'd love to hear from you!
Glad to know someone else out there finds this organism to be
C. Graham Clark, Ph.D.
Infectious and Tropical Diseases,
London School of Hygiene & Tropical Medicine, Keppel Street, London
WC1E 7HT, England, G.B.
Entamoeba Homepage- www.lshtm.ac.uk/itd/units/pmbbu/enta/