|dc.description.abstract||Aspergilloses are diseases caused by fungi of the genus Aspergillus, which affect
humans and a broad spectrum of other animals. Aspergillus flavus is one of the most
important pathogenic fungi, responsible for about 30% of all aspergillosis cases in
humans. Aspergillosis is often fatal. Currently mammalian models are the standard for
aspergillosis studies. However, because their use is expensive and laborious, recently the
fruit fly Drosophila melanogaster has emerged as a powerful alternative.
Even though aspergillosis is well-characterized, issues regarding the virulence of
A. flavus, defense gene in flies expressed against A. flavus, and the diversity of
microorganisms in wild Drosophila with potential use as probiotics against aspergillosis
are not well understood. Focusing on these issues will increase our understanding about
the pathogenicity of opportunistic fungal pathogens of clinical relevance; reveal new
defensive genes; and help find novel alternative to combat aspergillosis and other
The main objective of this first chapter is to provide a brief background to put the
following chapters in context. The second chapter is to study induced aspergillosis in
flies to determine the virulence of A. flavus strains. Virulence of A. flavus (as measured
by mortality of Drosophila) was very variable, ranging from 15% to >90%. Clinical
strains were significantly less virulent than environmental strains, probably because of
differences in original isolation date. Mean virulence did not differ between MAT1–1 and
MAT1–2 mating types and the phylogeny of A. flavus isolates did not predict virulence.
In the third chapter, I identify genes related to immune defense in flies that may
help to provide insight into factors that lead to human aspergillosis. 1,081 of the 14,554
gene regions detected in flies not infected and infected with A. flavus strains of different virulence levels were significantly and differentially expressed. Some of these up/downregulated
genes were previously shown to be involved in defense responses against
different pathogens. Some are known to be involved in vetilline membrane formation in
flies. Six unknown expressed genes were also reported in previous bacterial and fungal
infections; they are promising candidates for future experiments on pathogenicity.
In the fourth chapter I isolate microorganisms (including some isolated from wild
Drosophila) to elucidate diversity and richness as well as potential humans pathogens. 9
morphospecies of fungi and 12 morphospecies of bacteria were isolated from wild flies.
The most abundant were the yeast Candida inconspicua and the bacterium Klebsiella sp.
species richness was higher in fungi but diversity was lower than in bacteria. We
identified bacterial and fungal human pathogens in flies.
The fifth chapter uses microbes isolated from wild Drosophila and others from
American Type Culture Collection (ATCC) as potential probiotics that help protect hosts
against aspergillosis disease. Bacillus cereus (ATCC 13061), Issatchenkia hanoiensis and
Candida inconspicua decreased mortality of flies infected with A. flavus. Heat-killed
microorganisms did not protect flies, suggesting that the probiotic effect observed was
not caused by improved nutrition.
The sixth chapter is dedicated to general conclusions of all the work described
here. Drosophila melanogaster is an attractive model to test fungal pathogenicity and
could be useful for identifying genes involved in virulence. Gene expressed during the
progress of the infection in flies may be related to those expressed in human aspergillosis,
with potential to improve our knowledge of human innate immunity. D. melanogaster is a also a good model organism to study microbial diversity, microbe-host interactions and
effects of probiotics against systemic pathogens.||