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Chapter 16 : Aspergillus fumigatus: Survival and Death under Stress
Category: Clinical Microbiology; Fungi and Fungal Pathogenesis
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This chapter summarizes some of the remarkable attributes of the fungus Aspergillus fumigatus that are important for survival in the environment and highlights their potential for overlapping functions in the host. As a thermotolerant fungus, A. fumigatus is well-equipped for growth on diverse substrates throughout the compost cycle, including the thermophilic phase. In A. fumigatus, two of the signaling pathways that are involved in nitrogen and carbon sensing are the Rheb/TOR and protein kinase A (PKA) pathways. The ability of A. fumigatus to thrive on decaying plant and animal debris requires an enzymatic machinery that can efficiently degrade this material into component molecules. This must be accomplished in the presence of numerous other microorganisms that are also competing for access to the same substrates, making nutrient limitation an important stress encountered by A. fumigatus in nature. Transcription factors such as Skn7 and AfYap1 are downstream effectors of oxidative stress response pathways, resulting in the enhanced expression of antioxidant defenses. Programmed cell death (PCD) is a mechanism of cell removal that eliminates excess or damaged cells and is a universally conserved stress response among metazoans. Organisms with increasing complexity have multiple caspase members, two of which have been identified in the A. fumigatus genome, casA and casB. These metacaspases are required for the externalization of phosphatidylserine that occurs during stationary phase stress in A. fumigatus, suggesting that this well-known marker of apoptotic death is metacaspase dependent in A. fumigatus.
The PCD machinery in A. fumigatus. In higher eukaryotes, two types of PCD have been described: type I (caspase dependent) and type II (autophagy dependent). Current evidence suggests that the A. fumigatus metacaspases have a prosurvival function that is active under conditions of ER stress. Autophagy also provides a survival advantage when A. fumigatus is under stress, allowing for conidiophore development when external nitrogen sources are limiting, as well as providing a mechanism to fuel the foraging of hyphae into unexplored substrate. Each of these functions is likely to play an important role in the survival of the fungus in the competitive environment of compost. However, it remains to be determined whether these functions also have death-promoting activities under certain other conditions.