ISOLATION AND CHARACTERISATION OF GEOTRICHUM CANDIDUM (OR EXTRACELLULAR POLYGALACTURONASE PRODUCTION.

ZINEB GUESSOUS  M. OUHSSINE  A. MOKHTARI  M. FAID et M. ELYACHIOUI

Abstract

Fifty microorganisms including seven fungi and 43 bacteria were isolated from citrus fruit rot. The iodine test and enzyme activity assay showed pectinolytic activity in 38 of the selected micro-organisms, this activity being higher in yeasts. The two species that present the highest activity were identified as Candida gualermonde and Geotrichum candidum. Geotrichum candidum produces an extracellular polygalacturonase that is active on pectin and on poly-galacturonic acid. The enzyme is strongly induced by these two substrates, and this induction is glucose-repressible. No activity is detectable during culture with simple oses as the sole source of carbon. The biosynthesis of the enzyme is strongly inhibited by the presence of glucose in culture medium. Maximum polygalacturonase activity (reducing-group-releasing activity) is as high as 2,150 pmole.^L-1. min^-1 and is reached affer 48 h of growth in a fermentor  at 30°C and pH 5, in the presence of polygalacturonic acid 7.5 g/L and an appropriate nitrogen source.