Eukaryotes learn how to count: quorum sensing by yeast: Figure 1.

Signaling mechanisms that govern physiological and morphological responses to changes in cell density are common in bacteria. Quorum sensing, as such signal transduction processes are called, involves the production of, release of, and response to hormone-like molecules (autoinducers) that accumulate in the external environment as the cell population grows. Quorum sensing is found in a wide variety of bacteria, both Gram positive and Gram negative, and the spectrum of physiological functions that can be regulated is impressive indeed. Variation in the nature of the extracellular signal, in the signal detection machinery, and in the mechanisms of signal transmission demonstrates the evolutionary adaptability of quorum sensing systems for multiple uses. One testament to the range of these uses and to the general importance of quorum sensing in bacterial physiology comes from a PubMed search using “quorum” as the query: Well over 1000 articles have been published in the last 10 years. Here, we review the universal strategies that characterize bacterial quorum sensing and provide a glimpse of the diverse physiological settings in which it operates. We then discuss recent evidence for quorum sensing in different fungal species and conclude with an especially compelling case in the budding yeast Saccharomyces cerevisiae, described by Chen and Fink (2006) in this issue of Genes & Development.