Date Thesis Awarded


Access Type

Honors Thesis -- Access Restricted On-Campus Only

Degree Name

Bachelors of Science (BS)




Helen Murphy

Committee Members

Patty Zwollo

Saskia Mordijck


Infectious diseases are often characterized as specialist pathogens evolved to infect a singular host species; however, many virulent microorganisms are actually environmental microbes. If these microbes evolved for survival in the natural setting, then what evolutionary process enabled them to develop pathogenic traits? The “accidental virulence” hypothesis proposes that microorganisms evolve traits for survival in the natural environment, but these traits inadvertently make the microbe more virulent when presented to a host, unrelated to the initial purpose of the trait. For example, the concept of multicellularity has a variety of non-virulent purposes, such as nutrient foraging and protection from environmental insults, but can incidentally make microbes more virulent in hosts. Multicellular phenotypes may not only serve as support for the accidental virulence hypothesis but is of interest to microbial ecology.

Are there particular multicellular traits in a panel of environmental microbes that are associated with increased virulence? Saccharomyces cerevisiae, or baker’s yeast, has served as a model organism for myriad studies within biology over the past century; however, until recently many have been limited to laboratory evolved strains. As an opportunistic pathogen and microorganism of interest to biologists, baker’s yeast poses as a good model to examine both multicellularity and virulence.

In Chapter 1 of this thesis, the prevalence of four multicellular traits in a global panel of 976 isolates of S. cerevisiae was examined. The results show that while “hyper-multicellular” strains that strongly express multiple phenotypes have been seen in the laboratory setting, natural yeast seem to specialize in one or a few multicellular traits. Additionally, multicellular characteristics were observed in isolates from many environments and global locations, with a select few having an association with a particular environment. Thus, multicellularity appeared to be a common and global phenomenon in S. cerevisiae and was not associated with any particular ecological niche.

In Chapter 2, 19 strains from Chapter 1 were used to test if specific multicellular phenotypes were associated with increased virulence. Isolates representative of hyper- multicellularity, no multicellularity, or those expressing only individual phenotypes were injected into Galleria mellonella larvae, a model host for virulence assays, to determine an association with virulence. Consistent with previous findings, high multicellularity is seen to cause increased virulence.

Our results provide greater context for the natural prevalence of multicellular traits in S. cerevisiae globally and gives support for the accidental virulence hypothesis. Understanding the origins of pathogens and their relation to altered environmental selective pressures can be valuable in the context of climate change.

On-Campus Access Only