Aureobasidium pullulans is a polymorphic microfungus that produces extracellular pullulan in culture media with a life-cycle involving blastospores, hyphae, chlamydospores and intermediate forms. The form(s) responsible for polysaccharide synthesis is still uncertain. In order to clarify the relation between pullulan synthesis and the different morphological forms, we carried out studies under different culture conditions, varying nutrients, incubation time, aeration and agitation. The extracellular polysaccharides were analysed by enzymic and acidic hydrolysis and thin-layer chromatography. The number of residues in repeating units was established. The results are reported in relation to the celt density and the percentage of the different morphological types. The best conditions for producing polysaccharides, particularly pullulan, were determined. The results suggest that swollen cells and chlamydospores are the forms that produce extracellular polysaccharides. When significant amounts of chlamydospores are present, authentic pullulan is always observed whatever the culture conditions. The swollen cells are responsible for the synthesis of another polysaccharide capable of being transformed into pullulan; the hyphae do not play a role in extracellular polysaccharide biosynthesis.
