An analysis of the metabolism and cell wall composition of Candida albicans during germ-tube formation

Abstract

The uptake of nutrients (glucose, glutamine, and N-acetylglucosamine), the intracellular concentrations of metabolites (glucose-6-phosphate, cyclic AMP, amino acids, trehalose, and glycogen) and cell wall composition were studied in Candida albicans. These analyses were carried out with exponential-phase, stationary-phase, and starved yeast cells, and during germ-tube formation. Germ tubes formed during a 3-h incubation of starved yeast cells (0.8 X 10(8) cells/mL) at 37 degrees C during which time the nutrients glucose plus glutamine or N-acetylglucosamine (2.5 mM of each) were completely utilized. Control incubations with these nutrients at 28 degrees C did not form germ tubes. Uptake of N-acetylglucosamine and glutamine was inhibited by cycloheximide which suggests that de novo protein synthesis was required for the induction of these uptake systems. The glucose-6-phosphate content varied from 0.4 nmol/mg dry weight for starved cells to 2-3 nmol/mg dry weight for growing yeast cells and germ tube forming cells. Trehalose content varied from 85 nmol/mg dry weight (growing yeast cells and germ tube forming cells) to 165 nmol/mg weight (stationary-phase cells). The glycogen content decreased during germ-tube formation (from 800 to 600 nmol glucose equivalent/mg dry weight) but increased (to 1000 nmol glucose equivalent/mg dry weight) in the control incubation of yeast cells. Cyclic AMP remained constant throughout germ-tube formation at 4-6 pmol/mg dry weight. The total amino acid pool was similar in exponential, starved, and germ tube forming cells but there were changes in the amounts of individual amino acids. The overall cell wall composition of yeast cells and germ tube forming cells were similar: lipid (2%, w/w); protein (3-6%), and carbohydrate (77-85%). The total carbohydrates were accounted for as the following fractions: alkali-soluble glucan (3-8%), mannan (20-23%), acid-soluble glucan (24-27%), and acid-insoluble glucan (18-26%). The relative amounts of the alkali-soluble and insoluble glucan changed during starvation of yeast cells, reinitiation of yeast-phase growth, and germ-tube formation. Analysis of the insoluble glucan fraction from cells labelled with [14C]glucose during germ-tube formation showed that the chitin content of the cell wall increased from 0.6% to 2.7% (w/w).

A hyphal-specific chitin synthase gene (CHS2) is not essential for growth, dimorphism, or virulence of Candida albicans.

“…This fungus is capable ofdimorphic growth where growth can occur by unicellular budding or filamentous hyphal extension and branch formation (3). The hyphal cell wall has three to five times the chitin content of the yeast cell wall (4,5) and the cells have up to 10 times the in vivo chitin synthase activity (6). Three genes encoding chitin synthases have been cloned and sequenced in C. albicans (7,8,37).…”

Compounds active against cell walls of medically important fungi

“…The cell wall of C. albicans shares many similarities with those of saprophytic yeasts; it has a budding form composed of chitin, beta-glucan, and mannoprotein, with the latter two components constituting approximately 80% of the wall mass (232). This yeast, however, has a slightly higher component of chitin, constituting 0.6 to 1% of the cell wall (50,232).…”

“…This yeast, however, has a slightly higher component of chitin, constituting 0.6 to 1% of the cell wall (50,232). Although 90% of the chitin is restricted to the bud scar site, the remainder can be found distributed throughout the cell wall, contributing to its strength (250).…”

“…Besides the obvious morphologic differences in the two growth forms, there are some important differences in the physical compositions of their cell walls. The mycelial phase is known to contain three to five times as much chitin as the yeast phase (30,50,232), possibly a reflection of the higher levels of chitin synthetase activity during germ tube formation (54). While the overall composition of glucan, mannan, lipid, and protein is similar in both growth phases (232), the proportions of specific glucans are not.…”

“…The mycelial phase is known to contain three to five times as much chitin as the yeast phase (30,50,232), possibly a reflection of the higher levels of chitin synthetase activity during germ tube formation (54). While the overall composition of glucan, mannan, lipid, and protein is similar in both growth phases (232), the proportions of specific glucans are not. The activity of beta-(1,3)-D-glucan synthase is significantly higher in the mycelial form, while exo-(1,3)-beta-D-glucan hydrolase activity is greatly reduced (170).…”