Biosynthesis of novel exopolymers by Aureobasidium pullulans

Abstract

Aureobasidium pullulans ATCC 42023 was cultured under aerobic conditions with glucose, mannose, and glucose analogs as energy sources. The exopolymer extracts produced under these conditions were composed of glucose and mannose. The molar ratio of glucose to mannose in the exopolymer extract and the molecular weight of the exopolymer varied depending on the energy source and culture time. The glucose content of exopolymer extracts formed with glucose and mannose as the carbon sources was between 91 and 87%. The molecular weight decreased from 3.5 x 10(6) to 2.12 x 10(6) to 0.85 x 10(6) to 0.77 x 10(6) with culture time. As the culture time increased, the glucose content of the exopolymer extract formed with glucosamine decreased from 55 +/- 3 to 29 +/- 2 mol%, and the molecular weight increased from 2.73 x 10(6) to 4.86 x 10(6). There was no evidence that glucosamine was directly incorporated into exopolymers. The molar ratios of glucose to mannose in exopolymer extracts ranged from 87 +/- 3:13 +/- 3 to 28 +/- 2:72 +/- 2 and were affected by the energy source added. On the basis of the results of an enzyme hydrolysis analysis of the exopolymer extracts and the compositional changes observed, mannose (a repeating unit) was substituted for glucose, which gave rise to a new family of exopolymer analogs.

FIG. 1

Production of exopolymers with glucose (●), mannose (■), and glucosamine (⧫) as a function of time of cultivation of A. pullulans ATCC 42023. (a) Cell growth. (b) Exopolymer production. (C) Molar ratio of glucose to mannose in exopolymers.

FIG. 2

GPC chromatograms of exopolymers purified from 5-day cultures grown on glucose (A), mannose (B), and glucosamine (C).

FIG. 3

Molecular weight fractional patterns as a function of culture time. (A) Glucose. (B) Mannose. (C) Glucosamine. Symbols: ●, high-molecular-weight fraction; ■, medium-molecular-weight fraction; ▴, low-molecular-weight fraction.

FIG. 4

Patterns of molecular fraction areas as a function of culture time when glucose (A), mannose (B), and glucosamine (C) were the carbon sources. Black bars, high-molecular-weight fraction; gray bars, medium-molecular-weight fraction; white bars, low-molecular-weight fraction. MW, molecular weight.

FIG. 5

Treatment of pullulan (○), exopolymers synthesized with glucose (molar ratio of glucose to mannose, 89:11) (●), exopolymers synthesized with glucose and glucosamine (molar ratio of glucose to mannose, 79:21) (■), and exopolymers synthesized with glucosamine (molar ratio of glucose to mannose, 55:45) (▴) with pullulanase.