The regulation of polarised growth in the pathogenic form of Candida albicans
The dimorphic yeast Candida albicans is capable of growth in a budding yeast form and in a mycelial form. Growing in the yeast form in the digestive and vaginal tracts it normally presents no problems to the host; however, when the dimorphic transition takes place and mycelial growth is initiated, symptoms of candidiasis manifest in the host tissues. This thesis studies the control of the dimorphic response and investigates some of the cellular changes which accompany it. Germ tube formation was induced in a defined amino acids/salts medium. Temperature, pH and culture density were critical factors in stimulating the dimorphic response. Different percentages of term tube formation were recorded for eight different strains of C. albicans. Each strain also acidified the growth medium to a different extent; this was correlated to the extent of filamentation of each strain. Internal pH changes were monitored using the weak and technique and 31P nuclear magnetic resonance spectroscopy during bud and germ tube formation. In mycelial cells there was an initial increase in internal pH from 6.8 to around 8.0, followed by a gradual decrease to neutrality at the actual time of germ tube emergence. Cells in the budding form of growth did not exhibit such a marked increase in cytoplasmic alkalinisation. Mutant strains which were unable to produce germ tubes did not show cytoplasmic alkalinisation under conditions which would normally promote mycelial growth. Activation of the plasmamembrane ATPase may account for this increase in internal pH since diethylstilboestrol inhibited cytoplasmic alkalinisation and germ tube formation without affecting cell viability. Weak bases induced artificially, a rise in internal pH which was accompanied by germ tube formation. Potassium ions were found to enter cells as protons were expelled at the initiation of mycelial growth. Lactic acid prevented any rise in internal pH during germ tube formation. It also collapsed the ΔpH of the cells preventing growth by bud or germ tube formation. This may relate to the observation that endogenous Lactobacilli compete with C. albicans in the vagina and may explain why topically applied live yoghurt cultures soothe vaginal candidiasis. Transport of methionine was found not to be significantly different in cells induced to the budding or mycelial form. Transport or arginine and glutamate was induced by growing cells in the presence of these amino acids. The amino acid pool levels changed during the dimorphic process in C. albicans. A rapid increase in the pool level was shown within the first hour of both bud and hyphal growth followed by a gradual decrease. Little or no evidence was found in support of the hypothesis that the formation of hyphae is a response to nutrient starvation.