Download B. dendrobatidis

Investigation of the life cycle and characterization of phenotypic
pathogenicity of fungal pathogen B. dendrobatidis
Kaitlin McCreery
INTRODUCTION
Batrachochytrium dendrobatidis is an emerging fungal pathogen that has caused
chytridiomycosis in frogs and has been detected in salamander and frog species on every
continent excluding Antarctica, with first population declines occurring in the 1940s and
progressing in devastation over time (Stuart et al. 2004). B. dendrobatidis causes the disease
chytridiomycosis in frogs. Infection by this fungus leads to electrolyte and osmotic imbalance
and ultimately death, depending on the species (Voyles et al. 2010). In adult amphibians, B.
dendrobatidis colonizes epidermal cells on ventral and dorsal sides of host organism. The
sequence of the gene encoding the ribosomal subunit of B. dendrobatidis, including its
occurrence in amphibian hosts, distinguishes that this fungus is unique among other chytrids
(Longcore et al. 1999).
The reproductive cycle in culture includes two distinct stages, one being a motile,
infectious zoospore and the other a fruiting body known as a thallus (Berger et al. 1998). Sexual
reproduction in B. dendrobatidis has not been observed, though genetic variation between strains
has contributed to pathogenicity (Joneson et al. 2011). Asexual reproduction has been observed
in culture and amphibian skin samples. This cycle begins with a single-celled, diploid motile
zoospore. This zoospore possesses a flagellum for motility and can swim up to two centimeters
before encysting to form a sessile germling with rhizoid structures in currently undefined
conditions. Over a period of 48 to 72 hours, the germling enlarges to form a sessile thallus. Once
mature, zoospores discharge from the thallus through an operculum opening (Longcore et al.
1999). Understanding differing stages of B. dendrobatidis is likely to yield important insights. A
more comprehensive measurement of infection rates and direct measurement of the rate of
zoospore release by thalli are essential in understanding further pathogenicity of this fungal
pathogen (Mitchell et al. 2007). Rates of reproduction, encystment, and the production of
zoospores are critical to further the study on pathogenicity (Voyles et al. 2010).
Understanding host-pathogen interactions will provide assistance in developing a cure or
at least lessen the effects for young infections in frog species (Joneson et al. 2011). A culture of
the human embryonic kidney HEK293 cell line has been used to characterize pathogenicity in
several strains of infectious bacteria (Schwandner et al. 1999). HEK293 cells transfect very
easily, making them readily usable. In addition, they have been used in cell biology research for
many years. They do not model normal human cells, cancer cells, or any cell to be used as a
fundamental model of research. They have the purpose to be used in experiments in which the
reaction of the organism is not of interest (Graham and Smiley 1977). To carefully identify
unique aspects of the mode of infection of the unique pathogen B. dendrobatidis, a model system
needs to be developed (Voyles et al. 2011).
Understanding life cycle and mode of infection is the next important aspect in studying B.
dendrobatidis. In this study, a model method was designed to observe phenotypic pathogenicity
of B. dendrobatidis with a focus on maintaining an appropriate budget. A detailed documentation
of the life cycle was also developed in this study. These findings will provide groundwork on
which researchers may build to advance the study on identifying an effective cure of
chytridiomycosis.
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