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Quorum sensing and the regulation of morphology in the dimorphic fungus Candida albicans

Jacob Michael Hornby, University of Nebraska - Lincoln

Abstract

Quorum sensing in the dimorphic fungus Candida albicans is shown to result from the production of farnesol (C15H 26O, MW = 222.37). The presence of farnesol prevents germ tube formation (GTF) in a cell density-dependent fashion. The inhibition results in actively budding yeasts without influencing cellular growth rates. To begin exploring the mode of action of farnesol, fifty analogs of farnesol were tested for their activity in a differentiation bioassay for GTF. The structural features which were modified include: the chemical nature of the head group, chain length, double bond structure, and substitution of a backbone carbon by S, O, or Se heteroatoms. Of the fifty compounds, 23 showed activity by their ability to reduce GTF by ≥50%. No analog has greater activity than that of farnesol, although several maintain significant activity. The structure/activity relationships were examined in terms of farnesol's mode of action and the likely presence in C. albicans of a farnesol binding receptor protein. Also, a closer examination of commercial samples of farnesol strongly suggests that, of the four possible geometric isomers, only (E, E)-farnesol possesses biological activity. The biosynthetic origin of farnesol has been resolved by treating these cells with zaragozic acid B and fluconazole, potent inhibitors of the sterol biosynthetic pathway. Treatment with zaragozic acid B or fluconazole leads to an 8-fold and 13-fold increase, respectively, in the amount of farnesol produced by C. albicans. Furthermore, C. albicans cell extracts contain enzymatic activity to convert [3H]-farnesyl pyrophosphate to [ 3H]-farnesol. We suggest the fungicidal activity of inhibitors such as zaragozic acid or fluconazole derives in large part from the accumulation of farnesol that accompanies the inhibition of sterol biosynthesis. Additional studies have been conducted to help understand the role farnesol plays in quorum sensing. Examination of the cellular localization of farnesol revealed a nearly equal distribution of intracellular, extracellular, and membrane-associated farnesol. The inoculum size effect in C. albicans also has been shown to depend on media that support growth, presumably due to carbon-flow through the sterol pathway. Lastly, a protocol has been designed to induce pseudomycelia development in C. albicans. Very little is known about pseudomycelia largely due to an inability to produce 100% pseudomycelia. The presence of high levels of phosphate (100–600 mM) induces the formation of nearly 100% pseudomycelia that can be used to study this developmental state. ^

Subject Area

Biology, Microbiology

Recommended Citation

Hornby, Jacob Michael, "Quorum sensing and the regulation of morphology in the dimorphic fungus Candida albicans" (2003). ETD collection for University of Nebraska - Lincoln. AAI3085737.
http://digitalcommons.unl.edu/dissertations/AAI3085737

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