Aspergillosis in the Fruit Fly Drosophila melanogaster as a Model System

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 diseases. 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.