Date Awarded


Document Type


Degree Name

Master of Science (M.Sc.)




Helen Murphy

Committee Member

John Swaddle

Committee Member

Kurt Williamson


Microbial communities are ubiquitous in every ecosystem on earth, but the social interactions within these communities have only recently become a topic of investigation. Biofilms, the most common growth pattern found in nature, offer an exciting opportunity to study these interactions in a complex, spatially structured environment. This series of investigations explored the relationships between chemical warfare, a common competitive strategy, the three-dimensional spatial structure found in biofilms, and the phenotypic variation common in natural communities using Saccharomyces cerevisiae as a model organism. In the first set of experiments, the competitive effects of toxin production and biofilm formation were measured in well-mixed liquid environments, simple agar colonies, and complex biofilm colonies. Spatial structure strongly influenced the final community composition, with significant differences found between the liquid and agar substrates in the majority of cases. Toxin proved to be most effective against competing strains in spatially structured environments, regardless of biofilm formation. In the second set of experiments, the effect of adding a third, toxin-resistant strain was assayed in the same sets of growth conditions. It was expected that in spatially structured conditions, the resistant strain would passively form a physical barrier between the sensitive and toxin-producing strains, thus increasing the fitness of the sensitive strain. The results from these trials indicated that the relationships within these three-strain communities were much more complex than in two-strain communities. The resistant strain acted primarily as a competitor in all cases, and variation in the competitive outcomes of the trials was high. These results suggested when studying communities with multiple lineages, a combination of cell-level fitness metrics and whole-colony spatial analysis is a more appropriate analysis framework. These investigations contributed to an area of study with high ecological and evolutionary relevance but little existing knowledge, and opened new avenues for future research concerning microbial communities.



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