Biological studies into Scedosporium aurantiacum, an opportunistic pathogen colonising human lungs

Scedosporium aurantiacum is a recently identified highly virulent opportunistic pathogen, which is capable of causing a range of infections in immunocompromised people. S. aurantiacum is also known to colonise the respiratory tracts of cystic fibrosis (CF) patients, with a prevalence ranging from 10-17.4% in Australian CF patients. In view of the polymicrobial nature of CF, S. aurantiacum may be frequently encountered with the prokaryotic lung inhabitants such as Pseudomonas aeruginosa. These mixed bacterialfungal interactions or interactions between the fungus and the host lungs may impact the outcome of CF. In this study, a high throughput microtitre plate-based nutrient utilisation assay involving 94 unique substrates was used to characterise four S. aurantiacum strains displaying different virulence levels as determined by the Galleria mellonella larvae model. This approach unveiled five carbohydrate metabolism pathways and a difference in the sucrose and turanose metabolism between high and low virulence S. aurantiacum strains. One high and one low virulence strain were further co-cultured with clinical CF isolates of P. aeruginosa in a medium mimicking human CF sputum to explore the mechanisms of interactions between P. aeruginosa and S. aurantiacum using plate tests and confocal fluorescence microscopy. A genetic transformation system was also developed for S. aurantiacum to facilitate detailed observation of the bacterial-fungal interactions. In liquid cultures, the biofilm forming strains of P. aeruginosa exhibited a higher inhibitory effect against growth of S. aurantiacum compared to the non-biofilm forming strains. These results indicated that the ability to form biofilms might be useful for P. aeruginosa to cause inhibition of S. aurantiacum growth when in direct contact with the fungus. However, these biofilm forming P. aeruginosa strains could inhibit the growth of S. aurantiacum strains even in the absence of a direct physical contact i. e. in co-cultures involving separating membranes. These results suggest that in addition to biofilm formation, P. aeruginosa could also utilise diffusible extracellular metabolites for inhibiting the fungal growth. Further on, respiratory epithelial cells derived from a lung carcinoma were infected with a high virulence S. aurantiacum strain and the response of cells was determined using electron microscopy and transcriptomics-based approaches. S. aurantiacum invaded the respiratory epithelial cells by an initial adherence to the cell surface followed by penetration into the cell using conidial germ tubes. The respiratory cells exhibited a protective response against the fungal infection by up-regulating the inflammatory pathways leading to the release of cytokines. n conclusion, this work expands the existing knowledge of the emerging fungal pathogen S. aurantiacum and thus provides valuable insights into its yet unanswered role in CF, ultimately leading to a better understanding of pathophysiology of the disease.