Microbial Degradation of Cellulose, starch, lipids, legnin,pectin & proteins

CHAPTER: 7

Microbial Degradation

·      Cellulose

·      Starch

·      Lipids

·      Lignin

·      Pectin

·      Proteins

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Cellulose:

• Cellulose is the most abundant organic matter in nature.
• It is a polysaccharide composed of glucose molecules linked together in a linear chain of 1-4-glycosidic linkage.
•  Several microorganisms are capable of degrading Cellulose:

   1. Fungi: Aspergillus, Fusarium, Trichoderma, Curvularia

   2. Bacteria: Bacillus, Achromobacter, Pseudomonas, Vibrio,Cellulomonas, Streptomyces, Nocardia

•   Cellulase splits the long chain cellulose to yield glucose or may split into cellbiose.
• Cellobioseis a two molecules of glucose linked together. Cellobiose in turn can be split by cellobiase to yield two molecules of glucose.
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         Hemicellulose:

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•            Hemicullose are water soluble polysaccharides and consists of hexoses, pentoses and uronic acids.
•        Glucose, galactose, mannose, xylose, arabinose, glucoronic acid, and galactouronic acids are    commonly found in hemicellose plants.
•          The molecules contains aromatic ring as the building blocks.
• Hemicellulase degrade the hemicellulose and release the constituent components

•     Bacillus, Paenibacillus, Streptomyces or the actinobacteria group, while the Gramnegative strains  were assigned to the genera Pseudomonas, Acinetobacter, Ochrobactrum, and to genera belonging to  the family Enterobacteriaceae are generally involved in hemicellolose digestion.

Lignin:

•      Lignin is one of the most resistant organic substances for the microorganisms  to degrade.

•      Many Basidiomycetes have been found to possess special capacity in degradation of lignin

•      Only rarely bacteria have been found to reduce lignin.•      The fungi commonly found are : Fomes, Ganoderma, Agaricus, Armilaria, Polyporous, etc.

•      These fungi utilizes high lignin containing materials.

•      The enzymatic breakdown of lignin may result in simpler aromatic compounds syringaldehyde, vanillin, p-hydroxy-benzaldehyde and ferulic acid.

•      The final cleavage of the aromatic ring may take place with the involvement of other fungi and bacteria to yield low molecular weight organic acids, carbondioxide, methane etc.

•      Fungi degrade lignin by secreting enzymes collectively termed “ligninases”. Ligninases can be classified as either phenol oxidases (laccase) or heme peroxidases [lignin peroxidase (LiP), manganese peroxidase (MnP) and versatile peroxidase (VP)]

Starch:

•      Starch serves as a storage products of plants and is present in several specialized parts such as tubers, bulbs, rhizome, and seed.

•      It contains two components:

                                                1. Amylose

                                                2. Amylopectin

•      In amylose, the glucose molecules are linked together by an α-1,4- glycosidic bondage whereas in amylopectin, besides the same linkage of glucose molecules, side chains are attached through α-1,6- glycosidic bondage.

•      Aerobically, the microbes fully utilize starch to produce carbondioxide and low molecular weight organic acids.

•      Anaerobically, fermentation takes place to yield methane, acetic acids, lactic acids and butyric acids.

•      Bacteria, actinomycetes and fungi have the physiological capacity to utilize starch as a carbon source for growth and multiplication.

            

        Pectin:

•      Pectic substances are polysaccharides found in the constituents of middle lamella and in the primary and secondary cell walls of plants.

•      They are made up of galactourinic acid units bound in a long chain.

•      There are three types of pectic substances, viz. pectin,  protopectin, and pectic acid.

•      These compounds, though, closely related requires a specific enzymes to get hydrolysed.

•      Microorganisms involved in pectin degradation:

                                    Bacillus, Clostridium, Pseudomonas, Erwinia,

                                    Several Fungi

                                    Streptomyces

•      The enzymes involved in pectin digestion are collectively known as pectinases.

       Chitin:

•      Chitin is a polysaccharide whose basic unit is an amino sugar.

•      It is structural components giving mechanical strength to several plants and animals.

•      It is also present in fungal cell wall and insects.

•      This is one of the hardiest organic molecules for microbial action in soil.

•      The organisms involved are:

                                    Streptomyces, Nocardia, Micromonospora, Aspergillus, Penicillium,            Mucor, Trichoderma, etc

•      The breakdown products are: glucosamine, acetic acid, ammonia, carbondioxide,

•      Chitinase initiates the process to reduce chitin to diacetylchitobiase, and chitobiase reduces the latter into acetylglucosamine.

Proteins:

•      The proteins are made up of amino acids.

•      These are the structural components of plants as well as many other living things.

•      The residual proteins that are found in soil are hydrolyzed by several organisms.

•      The microorganisms splits protein into polypeptides and finally into simpler amino acids.

•      These amino acids are either utilized by the microorganisms for body build-up, or acted upon by some bacteria and fungi producing ammonia, carbondioxide, various organic acids, and alcohol.

•      Under anaerobic conditions, various amino acids and mercaptans are formed, producing offensive odour.

•      The organisms mainly involved are:

                                    E. coli, Bacteroides, Propionibacterium, Streptococcus, Clostridium,           Bacillus, Staphylococcus.

Fats & Oils (Lipids):

•      Lipids are hydrolyzed to glycerol and fatty acids                 

•      The organisms involved are: Clostridium. Acinetobacter, Bacillus etc

The enzyme involved in lipid degradation is lipases.