General Microbiology - Schlegel, H. 1987

Degradation of Natural Substances
Pectins

Acting as intercellular substances, pectins play a vital role in the Tissues of young plants; berries and stone fruits are particularly rich in them. The Significance of pectins is primarily due to their ability to impart necessary mechanical strength to plant tissues. These substances are constituents of the middle lamellae that form between the walls of adjacent plant Cells.

Pectins are polygalacturonides—unbranched chains composed of D-galacturonic acid residues linked by α-1,4-glycosidic bonds. The carboxyl groups of the acids are fully or partially esterified with methanol. In insoluble pectins, the chains are largely cross-linked to form a continuous network. Microorganisms degrade pectins using pectolytic Enzymes, namely esterases and depolymerases. Pectin esterases cleave the ester bonds, releasing methanol and polygalacturonic acids. The latter are subsequently broken down by specific Hydrolases into oligomers and monomers of D-galacturonic acid. Calcium salts of polygalacturonic acids are used in the preparation of fruit jellies.

The ability to degrade pectin is characteristic of many Fungi and Bacteria. The pathogenicity of various plant microorganisms (such as Botrytis cinerea and Fusarium oxysporum f. lycopersici) depends on their secretion of pectin-solubilizing enzymes. Erwinia carotovora causes tissue breakdown in lettuce, carrots, celery, and similar crops. In soil, the population of pectin-degrading microorganisms is extremely high (10⁵ cells per 1 g of soil). The most active among them include the spore-forming bacteria Bacillus macerans and B. polymyxa. Pectins are also degraded by representatives of various fungal groups.

Pectin-degrading organisms play a crucial role in the retting of flax and hemp. The purpose of this process is to separate bundles of Cellulose fibers from the remaining plant tissues. Fungal species are involved in aerobic dew retting, whereas anaerobic Water retting is carried out primarily by bacteria. Among the latter, Clostridium pectinovorum and C. felsineum appear to be of the utmost importance. Pectolytic enzymes used for various industrial Applications (such as the clarification of fruit juices) are predominantly obtained from fungi cultivated on pectin-containing media.