Budding yeast star in their own biofilm

L aboratory strains of Saccharomyces cerevisiae enjoy a much more comfortable life than their undomesti-cated relatives. Váchová et al. detail the complex colonies that wild strains of budding yeast form in order to survive the hostile conditions of their natural environment (1). Most microorganisms can form highly organized multicellular structures known as biofilms. These aggregates attach to surfaces and help protect the population from external threats such as chemicals or a host's immune system (2). Although domesticated strains of S. cerevisiae primarily form the simple, fl at colonies familiar to scientists around the world, budding yeast can form much more complex, biofi lm-like colonies in the wild (3). Yet little is known about the development and function of these structures, in part because of the diffi culties associated with looking inside the colonies as they form. Zdena Palková, from Charles University in Prague, the Czech Republic, and her colleagues decided to overcome these problems using two-photon microscopy (4). " We wanted to know what the internal structure of these colonies looks like and to investigate the mechanisms involved in organization and protection, " Palková explains. Váchová et al. followed the development of a wild S. cerevisiae strain from a single cell plated on agar into a complex biofi lm-like colony (1). Over several days, the yeast formed an intricate, three-dimensional structure with an internal cavity. Elongated cells at the base of the colony formed extended fi laments that the researchers termed pseudohyphae, which projected into the agar to keep the colony fi rmly anchored to its substrate. Meanwhile, oval cells on the upper, air-exposed surface of the colony formed multiple ridges, lending the colony a wrinkled appearance. Individual cells within the colonies were connected to each other by long fi bers, which were absent from yeast colonies lacking the cell wall adhesion protein Flo11p. These fi brous connections may be required for the 3D organization of yeast colonies, because Flo11p-defi cient strains fail to form ordered biofi lm-like structures (5). Váchová et al. then found that cells within the colonies are functionally, as well as structurally , diverse. " There are specifi c cells with specifi c functions in specifi c places, " says Palková, comparing the yeast biofi lm to differentiated , multicellular organisms. For example , while cells in the colony interior continue to divide, cells near the upper surface exit the cell cycle after a few days …