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Chapter 10 : Conidial Germination in Aspergillus fumigatus
Category: Clinical Microbiology; Fungi and Fungal Pathogenesis
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This chapter discusses the physiological and biochemical aspects of conidial germination in Aspergillus fumigatus and the regulatory pathways used to activate the process. In a study on genetic analysis of conidial germination in Neurospora crassa and A. nidulans, three of five genes encoded proteins involved in translational initiation (sgdA) and elongation (sgdB and -C), and a fourth (sgdE) encoded a protein involved in protein stabilization and folding. Thus, out of a bewildering array of early biochemical events, an unbiased genetic approach was able to specifically highlight a central role for translation in conidial germination. The results of this genetic screen suggest that conidial germination may be controlled by signaling pathways that directly activate protein translation. The chapter analyses the findings that link cyclic AMP (cAMP) signaling to conidial germination in Aspergillus species. In both A. nidulans and A. fumigatus, overexpression of a dominant negative form of rasA causes a delay in germination, whereas overexpression of dominant active rasA causes initiation of germination, including spore swelling, adhesion, and nuclear decondensation in the absence of a carbon source. The contribution of known and recently reported cell wall-associated proteins (CWPs) to conidial germination is discussed. Recently, several proteins involved in cell wall biosynthesis were implicated in controlling the speed of conidial germination in A. fumigatus. Analysis of the transcriptional and translational control of some of cAMP/protein kinase A (PKA), RAS, and mitogen-activated protein kinase (MAPK) genes may reveal how they are regulated by the above-described pathways, as well as other signal transduction pathways.
Germination of A. fumigatus conidia is inhibited at high conidial concentrations. A. fumigatus conidia at high (1 × 107 conidia/ml) and low (5 × 105 conidia/ml) concentrations were incubated for 8 h at 37°C in YAG rich medium, and the percentage of germinating conidia with emergent hyphal tubes was counted (n = 300).
Germination of A. fumigatus conidia. Cells were fixed on glass coverslips and visualized following calcofluor (cell wall) and 4′,6′-diamidino-2-phenylindole (nuclear) staining. The morphological and biochemical changes that occur during the first 2 h are the subject of this review.
The cetA/calA-K/O1 double mutant has a defective cell wall and autolyses during germination. Dormant wild-type (WT) control and cetA/calA-K/ O1 mutant conidia (0 h) and conidia that were allowed to germinate in liquid minimal medium for 6 h at 37°C (6 h) were fixed and analyzed by scanning electron microscopy.
Disruption of AfuEcm33 results in rapid conidial germination. AF293 wild-type and AfuEcm33-disrupted cells were fixed on glass coverslips and visualized following calcofluor (cell wall) staining. Note the early germination and cell-cell clumping in the mutant strain (lower panel) relative to the control AF293 wild-type strain (upper panel).
Tentative model of conidial germination in A. fumigatus. A carbon, phosphate, and nitrate source activates the cAMP/PKA, RAS, and MAPK pathways. The main bottleneck controlling conidial germination is the initiation of protein synthesis. Rapid assembly of polysomes onto prestored mRNA is followed by translation of key enzymes and proteins necessary for tighter adhesion, metabolic activation, conidial uncoating, nuclear decondensation, and isotrophic growth.
A. fumigatus conidial germination in different media