Purpose: Chronic co-infection with Pseudomonas aeruginosa and Aspergillus fumigatus is a common cause of lung function decline and mortality in cystic fibrosis patients. Previous work reported that P. aeruginosa produces dimethyl sulphide (DMS), a volatile sulphur compound (VSC), which promotes A. fumigatus growth in vitro. The aim of this study was to investigate the role of VSCs for A. fumigatus growth and the relevance of P. aeruginosa derived VSCs in the co-infection process.
Methods: Gas chromatography mass spectrometry was used to identify the major VSCs derived from methionine catabolism that facilitate A. fumigatus growth. The production and assimilation of these VSCs by A. fumigatus was then characterised using an in vitro volatile setup. A Galleria mellonella model allowed an analysis of the influence of P. aeruginosa volatiles on A. fumigatus during co-infection in vivo. Finally, a leukopenic murine model was used to investigate the sulphur sources utilised by A. fumigatus during infection of the mammalian lung.
Results: Here we show that A. fumigatus degrades methionine using aminotransferases, producing a number of VSCs which can be exploited as sole sulphur source. These VSCs need to be further catabolised to H2S for assimilation, as proved by the inability of a cysBΔcysDΔ strain to utilise them. One of the major VSCs released is DMS, which was previously reported to be produced by P. aeruginosa and to promote A. fumigatus growth in vitro. We demonstrate that pure and Pseudomonas derived DMS are exploited as a S-source by A. fumigatus.
The cysBΔcysDΔ strain cannot utilise any inorganic S-source, including DMS, but is completely virulent in both a leukopenic model of pulmonary aspergillosis and the alternative mini-host model of Galleria mellonella. This strongly suggests that A. fumigatus exploits organic S-sources during in vivo growth. In the Galleria model, co-infection with P. aeruginosa and A. fumigatus results in increased mortality. Interestingly, co-infection with wild-type A. fumigatus enhances mortality to a much higher degree than co-infection with the cysBΔcysDΔ strain. This suggests that P. aeruginosa is producing DMS in vivo and this can be exploited as an additional sulphur source by A. fumigatus wild-type, but not the mutant strain, promoting its growth and in turn increasing mortality. Further investigations are ongoing to investigate this hypothesis.
Conclusion: A. fumigatus exploits organic sulphur sources during in vivo growth. Increased mortality caused by A. fumigatus – P. aeruginosa co-infection can partially be attributed to the positive effect of P. aeruginosa derived VSCs on A. fumigatus growth due to its utilization as an additional S-source.
Full conference title:
- AAA 8th (2018)