Talking to themselves: autoregulation and quorum sensing in fungi

Extracellular autoinducing compounds in the supernatants of microbial cultures were first recognized for their roles in the induction of genetic competence in gram-positive bacteria (17, 94) and in the regulation of light production in marine vibrios (60). In 1994, this form of population-level regulation in microbes was dubbed 'quorum sensing' since it enabled bacterial cells to chemically measure the density of the surrounding population (18). Subsequently, many examples of cell density-dependent regulation by extracellular factors have been found in diverse microorganisms. The widespread incidence of diverse quorum-sensing systems strongly suggests that regulation in accordance with cell density is important for the success of microbes in many environments (see references 25 and 95 for reviews).

Cell density-dependent regulatory networks in microorganisms generally control processes that involve cell-cell interactions, such as group motility (10, 37) and the formation of multicellular structures (67, 93, 95). In a wide array of environmental and medically relevant bacteria, the development, maintenance, and dispersion of multicellular, surface-associated biofilms are in part controlled by quorum-sensing regulatory pathways (for reviews, see references 67 and 93). The uptake of extracellular DNA is often regulated in accordance with cell density presumably to enhance the chances of taking up DNA from closely related strains. For some bacteria, a link between the competence and biofilm formation has been established (69, 93). Both pathogens and symbionts that live in association with plant or animal hosts often use quorum sensing to regulate factors involved in microbe-host interactions (20, 86, 96). Quorum-sensing regulation may allow host-associated microbes to delay detection until an effective population has formed in the appropriate niche within the host.

Recently, it has become apparent that fungi, like bacteria, also use quorum regulation to affect population-level behaviors such as biofilm formation and pathogenesis. Considering the extent to which quorum-sensing regulation controls important processes in many distantly related bacterial genera, it is not surprising that cell density-dependent regulation also appears to be prevalent in diverse fungal species. Because fungal quorum sensing has been most extensively studied in Candida albicans, a dimorphic opportunistic pathogen, the majority of this review focuses on the details of quorum-sensing regulation in this fungus. A discussion of other fungi that appear to use quorum-sensing regulation and the identities of other known fungal signaling molecules are presented, and potential connections between mating in fungi and cell density-dependent regulation are explored. The review will conclude with points relating to the ecology of quorum sensing and a discussion of quorum-sensing networks as potential targets for antifungal therapies.

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