Microsporidia fungus causes chronic diarrhea in travellers, AIDS patients,
and organ transplant recipients
Scanning electron micrograph of a microsporidian spore with an extruded
polar tubule inserted into a eukaryotic cell. The spore injects the
infective sporoplasms through its polar tubule. (Credit: CDC/NCID/DPD
Parasite Image Library)
ScienceDaily (Nov. 3, 2008) — A fungus called microsporidia that causes
chronic diarrhea in AIDS patients, organ transplant recipients and travelers
has been identified as a member of the family of fungi that have been
discovered to reproduce sexually. A team at Duke University Medical Center
has proven that microsporidia are true fungi and that this species most
likely undergoes a form of sexual reproduction during infection of humans
and other host animals.
The findings could help develop effective treatments against these common
global pathogens and may help explain their most virulent attacks.
"Microsporidian
infections are hard to treat because until now we haven't known a lot about
this common pathogen," says Soo Chan Lee, Ph.D., lead author and a
postdoctoral researcher in the Duke Department of Molecular Genetics and
Microbiology. "Up to 50 percent of AIDS patients have microsporidial
infections and develop chronic diarrhea. These infections are also detected
in patients with traveler's diarrhea, and also in children, organ transplant
recipients and the elderly."
Of the 1200 species of microsporidia, more than a dozen infect humans. Their
identity had been obscured because these tiny fungi cannot live outside of
an infected host cell and they have a small number of genes which are
rapidly evolving.
The Duke scientists used two genetic studies to show that microsporidia
apparently evolved from sexual fungi and are closely related to the
zygomycete fungus in particular.
They found that microsporidia share 33 genes out of 2,000 with zygomycetes
which the microsporidia did not share with other fungi. This genomic
signature also shows that microsporidia and zygomycetes likely shared a
common ancestor and are more distantly related to other known fungal
lineages.
In addition, these two types of fungi have the same sex-locus genes – and in
the same order – in their DNA. Other genes involved in sexual reproduction
are also present. The findings suggest that microsporidia may have a
genetically controlled sexual cycle, and may be undergoing sexual
reproduction while they infect the host, Lee said.
Lee said the next step is to explore the sexual reproduction of these
species, which may cause more severe (more virulent) infections because they
use the host's cellular environment and machinery as a safe haven in which
to reproduce.
"These studies resolve the enigma of the evolutionary origins and proper
placement of this highly successful group of pathogens, and provide better
approaches to their experimental study," said senior author Joseph Heitman,
M.D., Ph.D., director of the Center for Microbial Pathogenesis and director
of the Duke University Program in Genetics and Genomics.
The team will pursue further studies with Duke genetic researchers Raphael
Valdivia, Ph.D., and Alejandro Aballay, Ph.D., using cultured cells and C.
elegans, a worm that researchers recently found is a natural host for
microsporidia. "Using this roundworm may prove to be a useful way to study
microsporidia genetics in a living creature," Heitman said.
This work was published online in the Oct. 30 edition of Current Biology,
and was supported by grants from the National Institute of Allergy and
Infectious Diseases of the National Institutes of Health, and by the
Canadian Institutes for Health Research.
Other authors on this study include Nicolas Corradi and Patrick J. Keeling
of the Canadian Institute for Advanced Research, Department of Botany,
University of British Columbia – Vancouver; Edmond J. Byrnes III of the Duke
Department of Molecular Genetics and Microbiology; Santiago Torres-Martinez
of the Departamento de Genetica y Microbiologia, Facultad de Biologia,
Universidad de Murcia, Spain; and Fred S. Dietrich of Duke Molecular
Genetics and Microbiology and the Duke Institute for Genome Sciences and
Policy.
Journal reference:
1.
Lee et al. Microsporidia Evolved from Ancestral Sexual Fungi.
Current Biology, 2008; DOI:
10.1016/j.cub.2008.09.030 |
