Interspecies between Bacillus subtilis and Pseudomonas

Mark Lyng*

*Corresponding author for this work

Research output: Book/ReportPh.D. thesis

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Most of the biomass on Earth is comprised of microorganisms which shape ecology as we know it. Bacteria literally form mountains and determine the fates of living beings. But they also lend themselves to biotechnological exploitation. Particularly plant-growth promoting rhizobacteria carry the potential to alleviate hunger and environmental pollution from synthetic fertilizer and pesticides. Strains from the Bacillus and Pseudomonas genera have shown great promise in this field, and there is an increased interest in developing mixed-species consortia to attain synergistic biostimulation. However, the information on intermicrobial interactions between Bacillus spp. and Pseudomonas spp. is currently sporadic and unstructured, and despite both genera being environmentally ubiquitous and often co-isolated, there is no clear consensus on their compatibility. The purpose of this PhD has been to bridge this gap. We wanted to better understand what happens when members of these two genera come into contact and whether their interactions can be predicted from their taxonomy.

To structure the current information on Bacillus and Pseudomonas ecology, we conducted a review of the available literature on pairwise interactions. Most often interactions were found reported as negative, mediated by the production of bioactive secondary metabolites. We also concluded that the available information is too sporadic to infer interactions patterns from. Therefore, we screened a collection of 720 Pseudomonas soil isolates for their impact on Bacillus subtilis pellicle formation. With this study, we found that interactions between B. subtilis and fluorescent pseudomonads followed similar ecological rules as for other organisms. Interactions became negative as medium richness was reduced. However, we also demonstrated that we could predict the fate of B. subtilis in coculture from Pseudomonas taxonomy. Pseudomonas protegens proved a strong inhibitor of B. subtilis due to the production of the antimicrobial 2,4-diacetylphloroglucinol, while Pseudomonas lini could increase pellicle production in dilute medium.

We found a conserved negative interaction mediated by the presence of the B. subtilis siderophore bacillibactin, and thus investigated the molecular mechanism behind this interaction. We found that B. subtilis could restrict the growth of phytopathogenic Pseudomonas marginalis and cause a down regulation of secondary metabolism but only when producing its primary siderophore. The loss of bacillibactin production conferred an advantage to P. marginalis and reversed the interaction. We hypothesized this interaction to be an iron tug-of-war between bacillibactin and the Pseudomonas siderophore pyoverdine.

This thesis provides a structured collection of interactions between bacilli and pseudomonads and contributes novel insight to how B. subtilis interacts with plant-associated pseudomonads. I present several interactions that are conserved intraspecifically, but also interspecifically across Pseudomonas species. Future work could build further on these findings and determine the degree of conservation and their implications for applied agricultural consortia.
Original languageEnglish
Place of PublicationKgs. Lyngby, Denmark
PublisherDTU Bioengineering
Number of pages151
Publication statusPublished - 2023


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