Study of the inability to use maltose and the antimicrobial properties of Streptomyces scabiei

Abstract

So far, most of the research on the phytopathogen S. scabiei 87-22 focused on its pathogenic lifestyle, both on the composition of its virulome and the environmental signals that trigger its expression enabling host colonization. In this work, our aim is to provide a detailed insight on the catabolic processes on the manner in which S. scabiei feeds on its main host (Solanum tuberosum) and on the way in which it establishes itself in its ecological niche thanks to its antimicrobial properties. The first purpose of this master thesis continues a previous study carried out in which an unexpected inability of S. scabiei to use maltose was observed. Our aim is to focus on the occurrence of spontaneous mutants with improved maltose utilization and, more specifically, to the mutation phenomenon developed by S. scabiei to consume maltose. The second purpose of this master thesis is to assess the antimicrobial capacity of S. scabiei against several fungi and 1 oomycete by identifying/validating the activity of volatile or agar- dilusible compounds. Main results: For the first objective, our results revealed an enormous mutation rate (1/2500 colonies) when S. scabiei is grown on minimal medium with maltose as the sole carbon source. Whole genome analysis highlighted 3 mutation hotspots, including one located directly within the malR gene, which possesses 2 functions: repressor of maltose utilization and activator of the amylolytic system. Our results confirm that this mutation is indeed responsible for the emergence of these spontaneous mutants. Nonetheless, the mutation mechanism still needs to be investigated. Prior work has shown that siderophores from Streptomyces scabiei were involved in the growth inhibition of Penicillium restrictum. In agreement with this, we demonstrated for the second purpose that these siderophores were also involved in the growth inhibition of other fungi and one oomycete by targeting iron homeostasis. Analysis of the S. scabiei volatilome also allowed us to identify volatile compounds (known and unknown) with antimicrobial activity. Our experiments confirmed the toxic elect of these volatile compounds by using the compounds directly on the strains tested.