Microbial competitive outcomes depend on interactions between insect vectors and the abiotic environment

Background/Question/Methods

Insect vectors directly change biological diversity when they transmit microbial species among habitats; vectors may also indirectly change microbial diversity when they physically alter habitat patches. We tested competitive outcomes among fruit-inhabiting yeasts on substrates that mimicked insect-altered habitats. We focused on the yeast Saccharomyces paradoxus, a wild sister species to the human-associated S. cerevisiae. Saccharomyces yeasts consume sugars through respiration (oxygen-dependent) and fermentation (not dependent on oxygen). We hypothesized that spatial structure imposed by insect vectors alters oxygen flux in a habitat; we expected changes in oxygen flux to then determine competitive outcomes among S. paradoxusand its fungal competitors.

We grew S. paradoxus in the presence of two competitors (Hansennispora uvarum and Metschnikowia sp.) on apple based substrates. We compared competitive outcomes across several treatments that included liquid and solid cultures. Different treatments mimicked vector behavior: vectors may deposit microbes on a substrate, inject microbes inside a substrate, or deposit microbes on holes in a substrate.

Results/Conclusions

Identities of competitors and substrate spatial structure altered competitive outcomes. Saccharomyces paradoxus was a poor competitor in our oxygenated liquid cultures. Presence of either competing yeast depressed S. paradoxus growth in liquid cultures, but presence of S. paradoxus did not depress competitor growth. In contrast, when we introduced spatial structure and oxygen limitation, S. paradoxus growth was a function of oxygen limitation and competitor identity. In general, S. paradoxus performed better in treatments where oxygen was limited. Competitive outcomes also depended on the identities of competing organism: S. paradoxus depressed Metschnikowia compared to controls, and H. uvarum depressed S. paradoxus compared to controls. Continuing research is investigating whether differences in competitive outcomes are due only to oxygen limitation, or to oxygen limitation plus spatial structure. Insect vectors and abiotic conditions interact and influence microbial competition in nature. Insect vectors facilitate Saccharomyces yeasts’ access to the oxygen-limited habitats, including human commensal habitats, in which they can dominate.