Organization and cell-cell interaction in starved Saccharomyces cerevisiae colonies

Abstract

Cell growth in yeast colonies is a complex process, the control of which is largely unknown. Here we present scanning electron micrographs of Saccharomyces cerevisiae colonies, showing changes in the pattern of cell organization and cell-cell interactions during colony development. In young colonies (</=36 h), cell density is relatively low, and the cells seem to divide in a random orientation. However, as the colonies age, cell density increases and the cells seem to be oriented in a more orderly fashion. Unexpectedly, cells in starved colonies form connecting fibrils. A single connecting fibril 180 +/- 50 nm wide is observed between any two neighboring cells, and the fibrils appear to form a global network. The results suggest a novel type of communication between cells within a colony that may contribute to the ability of the community to cope with starvation.

FIG. 1

SEM micrographs of colonies at various growth stages reveal an age-related increase in cell organization. (A) Colonies of strain SUB62 (3, 6, 7, 9, 13), grown on YPD, were fixed at the indicated time points postplating and visualized by SEM. Original magnification, ×5,000. The bar at 8 h represents 10 μm; at other time points, it represents 5 μm. (B) Top-view SEM micrographs of colonies at the indicated time points were enlarged, and the cells were counted. A total of 200 to 400 cells were counted per time point.

FIG. 2

Cell organization depends on colony age and on the location within the colony. (a) (A to F). Top-view micrographs were taken from the colony periphery (left panels) or from the colony center (right panels). Bars represent 5 μm. (G to and H). Cells that had been grown logarithmically in liquid culture were collected, and the pellet was taken by a toothpick and placed on agar to form a colony-like structure. The artificial “colony” was immediately fixed and further processed according to the same protocol used in panels A to F. Cont., control. Micrographs of the periphery (G) and center (H) are shown. Bars represent 10 μm. (b) Top-view SEM micrographs of various locations (200 μm apart) across the radius were taken, and cell densities were determined as described for Fig. 1B and plotted as a function of the distance from the edge. In addition, cells were classified as unbudded or budded and the percent budding was similarly plotted.

FIG. 3

Intercellular connecting fibrils are found in starved colonies. Colonies were grown on plates containing synthetic medium (SD) supplemented with the standard amounts of various amino acids (14), except that the leucine concentration was limiting (1/10 that of the standard). After 3 weeks, the colonies were fixed and visualized by SEM. (A) Top view. (C) Higher magnification (originally ×10,000). Bars in panels A and B represent 10 μm, and the bar in panel C represents 5 μm.