Germ tube growth of Candida albicans.

The clinical pathogen Candida albicans is a budding yeast that is capable of forming a range of polarized and expanded cell shapes from pseudohyphae to true nonconstricted hyphae. Filamentous forms consist of contiguous uninucleated compartments that are partitioned by septa. It has long been held that the so-called "dimorphic transition" from a budding to a filamentous form may aid the fungus to penetrate epithelia and may therefore be a virulence factor. This review summarized new information regarding the physiology and ecology of hyphal growth in C. albicans. New evidence has demonstrated that hyphae of C. albicans have a sense of touch so that they grow along grooves and through pores (thigmotropism). This may aid infiltration of epithelial surfaces during tissue invasion. Hyphae are also aerotropic and can form helices when contacting solid surfaces. Growing evidence supports the view that hyphal growth is a response to nutrient deprivation, especially low nitrogen and that filamentous growth enables the fungus to forage for nutrients more effectively. Further insights into the growth of C. albicans have come from the analysis of genes and mutations of Saccharomyces which have begun to reveal the molecular mechanisms underlying the mechanisms of bud site selection, cell polarity and signal transduction pathways that lead to pseudohyphal development in this and other organisms. For example, it is now clear that a MAP-kinase cascade, homologous to the mating pathway in Saccharomyces, regulates filamentous growth in both fungi. However, this must be only one of several overlapping or separate signal transduction pathways for hyphal development because filamentous growth still occurs in mutants of Candida and Saccharomyces which are blocked in this pathway. Cell cycle analyses have shown that hyphal phase cell cycle of Candida is distinct from that in budding and pseudohyphal formation and so pseudohyphal growth of Saccharomyces is not a true model of germ tube growth in Candida. Pseudohyphal growth in both Candida and Saccharomyces involves synchronous division of mother cells and their daughters. In contrast, during germ tube growth of Candida, cytoplasm is unequally partitioned at cytokinesis so that apical cells inherit more cytoplasm and sub-apical cells have a single nucleus but are extensively vacuolated. As a result, apical cells grow and divide while sub-apical cells are apparently arrested in the cell cycle until they can regenerate sufficient cytoplasm to re-enter the cell cycle. Although current studies still fall short of verifying the status of yeast-hypha dimorphism as a virulence factor, they suggest that the cell biology of germ tube growth of C. albicans is well suited for the invasive growth of the fungus in vivo.