Nanna Mee Coops Olsen:
Microorganisms are the most abundant and diverse form of life on Earth and play a vital role in all ecosystems. They have a huge impact on our lives in sickness and health and whether it is to develop strategies to treat microbial infections or to use microbes for biotechnological purposes, it is crucial to understand how they live. Microorganisms most often coexist in spatially structured communities, composed of up to hundreds of different species. It is recognized that the ecology of biofilms is very different from planktonic communities and that both intra- and interspecies interactions are important for shaping microbial communities. However, knowledge is still limited especially when it comes to understanding more complex communities with an increasing number of interacting members. In this thesis, we investigate bacterial interactions on different levels in biofilm model communities and try to elucidate how these interactions affect the individual members and overall features of communities of varying complexity.
In Manuscript I, we addressed the issue of heterogeneity in biofilms and which problems they pose when conducting an analysis. We provide an overview of current approaches, employed to biofilm communities to facilitate the investigation of interactions across heterogeneities and at different scales, focusing on the analysis of synthetic communities. In Manuscript II, we showed that early arrival conferred a growth advantage for good biofilm formers in a four-species synergistic consortium but did not affect the growth of their cohabitants in 24 hour-old biofilms grown in a Drip-flow reactor. Our results indicated that priority effects were not prevailing in our synergistic consortium, under the experimental conditions used. In Manuscript III, we sought to predict the assembly of a community with higher diversity composed of 32 bacterial species. Interestingly, we found that a higher growth rate and growth yield of the individual species in isolation, correlated with higher abundances in the community. Their coexistence was, unexpectedly, not associated with their pairwise interactions. In Manuscript IV, we performed a comparative transcriptional analysis of wild type Xanthomonas retroflexus and an emergent mutant grown in mono- and dual-species biofilms with Paenibacillus amylolyticus, with which Xanthomonas retroflexus interacted mutualistic. We found significant differences in gene expression, including biofilm-related genes, between the ancestor and the wrinkled variant in all conditions. Thus, suggesting that genetic variants in a population respond differently to other species and play an important role in how species can interact.