Nutrient diffusion governs S. cerevisiae colony dynamics: Monitoring by optical coherence tomography

Microorganisms are able to form structured aggregates, such as colonies, when they grow on a solid support. In contrast to suspended cultures, growth in these conditions varies in space and time. In some species including Saccharomyces cerevisiae, colonies reach dimensions that have been challenging to experimenters as they pose practical monitoring issues. In this study, optical coherence tomography was employed to assess the growth of Saccharomyces cerevisiae colonies nondestructively for 2 weeks. The evolution of colony diameter, height and volume was monitored for different combinations of nutrient concentrations and thicknesses of the solid agar gel. Our results indicate that the diffusion of nutrients, both in the solid support and in the colony, is key to understanding the dynamics of colony expansion. Radial growth was linear at first sight, but a more careful assessment revealed that it actually decreases as time progresses. Vertical expansion seemed to be asymptotic, and the constant height reached was impacted by the thickness of agar gel. Volume expansion was biphasic, with a phase of nearly exponential increase followed by a phase of slower growth. Finally, an empirical law was proposed for the increase of volume. Taken together, these elements indicate diffusion constraints, gradually altering the colony's development, and therefore its shape, as nutrients become depleted. This study also showed that optical coherence tomography provides relevant quantitative information that complements well the qualitative biological information obtained with other techniques.