The genus Pythium comprises
about eighty-five species. Pythium species are common pathogens causing
disease in plants and fishes. The species of this genus are among the
most destructive plant pathogens, inflicting serious economic losses
of crops by destroying seed, storage organs, roots, and other plant
tissues. Pythium insidiosum is the only species reported to
cause infections in mammals. The disease caused by this unique microorganism
has been termed pythiosis (insidiosi) and can cause life threatening
infections in cats, dogs, cattle, equines, captive polar bears, and
humans.
Members of the genus Pythium have been described as "aquatic fungi".
However, they are not true fungi (Kingdom Fungi), they belong to the
Kingdom Stramenopila, Phylum Oomycota, Class Oomycetes, Family Pythiaceae.
In culture, P. insidiosum develops sparsely septate fungal-like
hyphae similar to those produced by the Zygomycetes (true fungi). Like
other Oomycetes, P. insidiosum produces motile zoospores (asexual
stage) when exposed to damp conditions. The zoospores are single cells
with two lateral flagella that swim to find a new plant host where it
completes its lifecycle. Once in contact with the host the zoospores
lose their flagella and encyst. It is believed that zoospores act as
infecting units once in contact with a mammalian host (J. Mycol. Med.
6:151; 1996). Under conditions, still under investigation, P. insidiosum
develops globose oogonia (sexual stage) typical of this species, but
they are rarely observed in most isolates.
Like other oomycetes, P. insidiosum grows relatively well on
a variety of media. On corn meal agar and Sabouraud dextrose, colonies
are colorless to white, submerged with short aerial mycelium and a finely
radiate pattern. The coenocytic hyphae range between 4 and 12 mm in
diameter with 90o perpendicular lateral branches. Septation is only
occasionally observed in early hyphae, but they become abundant in old
viable hyphae. Hyphal swellings, that mimic sporangia measuring 12 to
28 in diameter, are common in laboratory cultures.
Production of zoospores can be induced in water cultures containing
minimal quantities of different ions and grass leaves at 37o C. Zoospore
induction in water without ions is rare. Early sporangia cannot be differentiated
from normal hyphae. These early sporangia, at maturity, flows their
cytoplasm into a discharge tube and form a globose sporangium. The cytoplasmic
content of the sporangium goes through progressive cleavage and biflagellated
zoospores are formed inside a vesicle. The zoospores mechanically break
the sporangium vesicle wall and swim approximately 20-30 minutes and
then encyst. It is believe that P. insidiosum use a plant to
complete its life cycle in nature (lily or grass), but it has not been
confirmed.
6/16/04
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