Variation in pH gradients and FLO11 expression in mat biofilms from
environmental isolates of the yeast Saccharomyces cerevisiae
Abstract
Saccharomyces cerevisiae produces a multicellular phenotype, known as a
mat, on a semi-solid medium. This biofilm phenotype was first described
in the lab strain 1278b and has been analyzed mostly in this same
background. Yeast cells form a mat by spreading across the medium and
adhering to each other and the surface, in part through the variegated
expression of the cell adhesin, FLO11. This process creates a
characteristic floral pattern and generates pH and glucose gradients
outward from the center of the mat. Mats are encapsulated in a liquid
which may aid in surface spreading and diffusion. Here, we examine
thirteen environmental isolates that vary visually in the phenotype. We
predicted that mat properties were universal and increased morphological
complexity would be associated with more extreme trait values. Our
results showed that pH varied significantly among strains, but was not
correlated to mat complexity. Only two isolates generated significant
liquid boundaries and neither produced visually complex mats. In five
isolates, we tracked the initiation of FLO11 using GFP under the control
of the endogenous promoter. Strains varied in when and how much GFP was
detected, with increased signal associated with increased morphological
complexity. Generally, the signal was strongest in the center of the mat
and absent at the expanding edge. Our results show that traits
discovered in one background vary and exist independently of mat
complexity in natural isolates. The environment may favor different sets
of traits, which could have implications for how this yeast adapts to
its many ecological niches.