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Chapter 64 : Yeasts Are Complex . . .
Microbiologists categorized yeasts as solitary cells simply because this was the form Saccharomyces cerevisiae and its relatives adopted when compelled to proliferate in the artificial environment of the shake flask, fermentor, or chemostat. The prevailing view was that yeasts were "planktonic" creatures, teeming populations of individual cells thriving on the nourishing warmth of laboratory glassware. As microbiology progressed, these were the organisms that spawned numerous advances in biochemistry and other specialties. Especially prized was Schizosaccharomyces pombe, which provided major insights into the operation of the eukaryotic cell cycle. At the most genuinely elementary level, communication between S.cerevisiae cells has been known for some years. There are two haploid mating types, producing corresponding pheromones and having surface receptors for the opposite type. Each pheromone arrests the cell cycle of the opposing type. Going beyond this primitive sexuality, Richard Dickinson highlighted the recognition of quorum sensing in Candida albicans. Dickinson also drew attention to research on the production of bicarbonate by S.cerevisiae during sporulation. This work indicated that the yeast is not a free-living unicellular eukaryote but a social organism, whose diploid cells "tell" others to sporulate. Dickinson made a compelling case for yeasts to be categorized not as free-living unicellular eukaryotes but as "social, colonial organisms with cell-to-cell communication. The "rediscovery" of filamentation in the last decade has been accompanied by recognition that yeasts also thrive as biofilms.