PhD Defense Heiko Thomas Kiesewalter

Secondary metabolites are compounds which are not directly involved in the growth, development, or reproduction of the producing organisms. It has been shown that they have a great impact on microbial interactions under laboratory conditions. However, their real role in nature still needs to be unravelled. In literature, contradictory opinions exist, some describe them as microbial weapons and others designate them as signalling molecules.

B. subtilis and other Bacillus spp. have excellent biocontrol properties by promoting plant growth and reducing plant diseases caused by both plant pathogenic fungi and bacteria. Furthermore, they produce various secondary metabolites of which the most prominent and bioactive ones are nonribosomal peptides (NRPs).

We demonstrated that anti-fungal properties depend primary on the two NRPs plipastatin and either plipastatin or surfactin, depending on the tested fungus or the tested B. subtilis environmental isolate. Interestingly, the production of the NRPs varied among co-isolated B. subtilis strains due to missing core genes or potentially altered gene regulation highlighting the existing natural diversity of secondary metabolite production in this species.

In addition, we investigated the antifungal spectrum of a recently isolated Bacillus velezensis strain, belonging to the B. subtilis complex, and described its eminent biocontrol properties. It has been shown before that B. subtilis’ plant growth promoting properties are linked to its biofilm formation and root colonization. We could show, that this biofilm development is independent of the NRPs surfactin in the tested strains. Furthermore, we demonstrated that matrix components, important for the establishment of biofilm (aggregation of bacterial cells), are also of high importance for fungal hyphae colonization of both the filamentous black mold fungus, Aspergillus niger and the basidiomycete mushroom, Agaricus bisporus.

To investigate the impact of B. subtilis NRPs on bacterial communities, we established soil-derived semi-synthetic mock communities and supplemented them with an environmental B. subtilis strain and its NRPs mutants. We demonstrated the influence of the wild type strain on community assembly and relative abundance of the genera Lysinibacillus and Viridibacillus, which was reduced in the mutant lacking NRP production.

This study highlights the chemodiversity of recently isolated B. subtilis strains from the environment and the impact of NRPs on the antifungal properties, on biofilm-related properties as well as on bacterial communities.