BIODETERIORATION OF DIFFERENT TYPES OF WOOD

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ABSTRACT

 

A total of Five (5) different types of wood were used for this research. The used and unused wood samples were gotten from buildings that had been used after standard microbial screening. A total of six (6) bacterial and four (4) fungal species were isolated from the used and unused wood screened for microbes involved in deteriorating used wood samplesThe total heterotrophic plate count ranged from 4.1 x 10cfu/g to 7.6 x105cfu/g, the total coliform count ranged from 3.9 x105 cfu/g to 6.7 x105 cfu/g while the total fungal count ranged from 1.7 x10cfu/g to 4.7 x10cfu/g. The mean microbial counts from the unused wood samples. The total heterotrophic plate count ranged from 0.9 x 10cfu/g to 6.1 x10cfu/g, the total coliform count ranged from 2.9 x105 cfu/g to 7.9 x105 cfu/g while the total fungal count ranged from 0.7 x10cfu/g to 2.1 x10cfu/g. Esch1erichia coli had the highest percentage occurrence at (20%) while the least occurring bacterial isolates are Staphylococcus aureus (15%) each. The percentage of occurrence of bacterial isolates for unused wood samples revealed that Citrobacter sp. (36%) were more predominant while Bacillus (18%) had the least occurence. The percentage of occurrence of fungal isolates from used wood samples showed that Aspergillus niger, Rhodotorula sp.  and Mucor sp. had the highest percentage occurrence  at (33, while the least occurring fungal isolates were Aspergillus fumigatus at (5). On the other hand, the percentage of occurrence of fungi isolates from used wood samples revealed Aspergillus niger (67%) being predominant while Aspergillus fumigatus (33%) was least predominant.







TABLE OF CONTENTS

Title Page                                                                                                                                i

Certification                                                                                                                           ii

Dedication                                                                                                                              iii

Acknowledgements                                                                                                                iv

Table of Contents                                                                                                                   vi

List of Tables                                                                                                                          ix

Abstract                                                                                                                                  x

1.0                   CHAPTER ONE                                                                                           1

1.1                   Introduction                                                                                                    1

1.1.1                Wood                                                                                                              1

1.2                   Types of Wood                                                                                               3

1.2.1                Beech Wood                                                                                                   4

1.2.2                Ash Wood                                                                                                       4

1.2.3                Fibreboard                                                                                                      4

1.2.4                Plywood                                                                                                          4

1.2.5                Veneer                                                                                                            5

1.3                   Classification of Wood                                                                                   5

1.3.1                Hardwoods                                                                                                      6

1.3.1.1             Maple                                                                                                              6

1.3.1.2             Mahogany                                                                                                       6

1.3.1.3             Cherry                                                                                                             7

1.3.1.4             Walnut                                                                                                            7

1.3.2                Soft Woods                                                                                                     7

1.3.2.1             Pine                                                                                                                 7

1.3.2.2             Ash                                                                                                                  7

1.3.2.3             Birch                                                                                                               8

1.3.2.3             Cedar                                                                                                              8

1.3.2.4             Redwood                                                                                                         8

1.3.2.4             Hemlock                                                                                                         9

1.4                   Factors That Promote Growth of Microorganisms in Wood

1.4.1                Moisture                                                                                                          9

1.4.2                Temperature                                                                                                   9

1.5                   Wood and Its Component                                                                               10

1.5.1                Cellulose                                                                                                         11

1.5.2                Hemicellulose                                                                                                 11

1.5.3                Lignin                                                                                                             11

1.6                   Aim and Objectives                                                                                        13

2.0                   CHAPTER TWO                                                                                          14

2.1                   Literature Review                                                                                           14

2.1.1                Microbial Colonization of Wood                                                                   14

2.2                   Degradation of Wood by Fungi                                                                      14

2.2.1                White-rot Fungi                                                                                              15

2.2.2                Brown-Rot Fungi                                                                                            16

2.3                   Diversity of Bacterial Communities in Wood                                                17

2.3.1                Edaphic and Atmospheric Sources of Bacteria                                              18

2.3.2                Bacterial Endophytes                                                                                      19

2.4                   Bacterial Wood Decomposition                                                                     19

2.5                   Bacterial Nitrogen Fixation in Wood                                                             20

2.6                   Isolation and Characterization of Microorganisms Involved

in Degradation of Wood                                                                                 22

3.0                   CHAPTER THREE                                                                                      24

3.0                   Materials and Method                                                                                     24

3.1                   Study Area                                                                                                      24

3.2                   Collection of Samples                                                                                    24

3.3                   Sterilization of Materials                                                                                24

3.4                   Preparation of Culture Media                                                                         25

3.5                   Isolation of Microorganisms from Wood                                                       25

3.6                   Identification of the Isolates                                                                           25

3.7                   Gram Staining                                                                                                26

3.8                   Biochemical Test                                                                                            26

3.8.1                Catalase Test                                                                                                   26

3.8.2                Indole Test                                                                                                      26

3.8.3                Citrate Utilization Test                                                                                   27

3.8.4                Hydrogen Sulphide (H2S) Production Test                                                    27

3.8.5                Starch Hydrolysis                                                                                           27

3.8.6                Motility, Indole, Urease (MIU)                                                                      27

3.8.7                Coagulase Test                                                                                               28

3.8.8                Oxidase Test                                                                                                   28

3.9                   Identification of Fungal Isolates                                                                     29

3.9.1                Wet Preparation                                                                                              29

3.9.2                Colonial Morphology                                                                                     29

4.0                   CHAPTER FOUR                                                                                        30

4.0                   Results                                                                                                            30

5.0                   CHAPTER FIVE                                                                                          41

5.0                   Discussion, Conclusion and Recommendation                                             41

5.1                   Discussion                                                                                                       41

5.2                   Conclusion                                                                                                      43

5.3                   Recommendation                                                                                           43

                             References                                                                                                      44

                        Appendix                                                                                                        50

 

 

 

 

 

 

LIST OF TABLES

 

TABLE

TITLE

PAGE NO

1a

Mean Microbial Counts from Deteriorating Used Wood Samples

32

 

1b

Mean Microbial Counts from Unused Wood Samples

33

2

Morphological Identification of Bacterial Isolates from Deteriorating Wood Samples

 

34

3

Cultural Morphology and Microscopic Characteristics Fungal Isolates from Deteriorating Wood Samples

 

35

4

Biochemical Identification, Gram Reaction and Sugar Utilization Profile of Bacterial Isolates

 

36

5a

Percentage of Occurrence of Bacterial Isolates from Deteriorating Used Wood Samples

 

37

5b

Percentage of Occurrence of Bacteria Isolates from Unused Wood Samples

38

6a

Percentage of Occurrence of Fungi Isolates from Used Wood Samples

39

6b

Percentage of Occurrence of Fungi Isolates from Deteriorating Unused Wood Samples

40

 

 

 

 

 

                                             CHAPTER ONE

                                                      INTRODUCTION

1.1.1 Wood

Wood is a porous and fibrous structural tissue found in the stems and roots of trees and other woody plants. It is an organic material  a natural composite of cellulose fibers that are strong in tension and embedded in a matrix of lignin that resists compression. Wood is sometimes defined as only the secondary xylem in the stems of trees, or it is defined more broadly to include the same type of tissue elsewhere such as in the roots of trees or shrubs (Hoppe et al., 2015).  In a living tree it performs a support function, enabling woody plants to grow large or to stand up by themselves. It also conveys water and nutrients between the leaves, other growing tissues, and the roots. Wood may also refer to other plant materials with comparable properties, and to material engineered from wood, or wood chips or fiber.  Wood has been used for thousands of years for fuel, as a construction material, for making  tools  and  weaponsfurniture and paper. More recently it emerged as a feedstock for the production of purified cellulose and its derivatives, such as cellophane and cellulose acetate. As of 2005, the growing stock of forests worldwide was about 434 billion cubic meters, 47% of which was commercial. As an abundant, carbon-neutral renewable resource, woody materials have been of intense interest as a source of renewable energy. In 1991 approximately 3.5 billion cubic meters of wood were harvested. Dominant uses were for furniture and building construction (Pan et al., 2005).

Wood, in the strict sense, is yielded by trees, which increase in diameter by the formation, between the existing wood and the inner bark, of new woody layers which envelop the entire stem, living branches, and roots. This process is known as secondary growth; it is the result of cell division in the vascular cambium, a lateral meristem, and subsequent expansion of the new cells. These cells then go on to form thickened secondary cell walls, composed mainly of cellulosehemicellulose and lignin. Where the differences between the four seasons are distinct, e.g. New Zealand, growth can occur in a discrete annual or seasonal pattern, leading to growth rings; these can usually be most clearly seen on the end of a log, but are also visible on the other surfaces. If the distinctiveness between seasons is annual (as is the case in equatorial regions, e.g. Singapore), these growth rings are referred to as annual rings. Where there is little seasonal difference growth rings are likely to be indistinct or absent. If the bark of the tree has been removed in a particular area, the rings will likely be deformed as the plant overgrows the scar. If there are differences within a growth ring, then the part of a growth ring nearest the center of the tree, and formed early in the growing season when growth is rapid, is usually composed of wider elements. It is usually lighter in color than that near the outer portion of the ring, and is known as early wood or springwood. The outer portion formed later in the season is then known as the latewood or summerwood. However, there are major differences, depending on the kind of wood.   As a tree grows, lower branches often die, and their bases may become overgrown and enclosed by subsequent layers of trunk wood, forming a type of imperfection known as a knot (Stokland et al., 2012).

The dead branch may not be attached to the trunk wood except at its base, and can drop out after the tree has been sawn into boards. Knots affect the technical properties of the wood, usually reducing the local strength and increasing the tendency for splitting along the wood grain,   but may be exploited for visual effect. In a longitudinally sawn plank, a knot will appear as a roughly circular "solid" (usually darker) piece of wood around which the grain of the rest of the wood "flows" (parts and rejoins). Within a knot, the direction of the wood (grain direction) is up to 90 degrees different from the grain direction of the regular wood.  In the tree a knot is either the base of a side branch or a dormant bud. A knot (when the base of a side branch) is conical in shape (hence the roughly circular cross-section) with the inner tip at the point in stem diameter at which the plant's vascular cambium was located when the branch formed as a bud (Frey-Klett et al., 2005).

In grading lumber and structural timber, knots are classified according to their form, size, soundness, and the firmness with which they are held in place. This firmness is affected by, among other factors, the length of time for which the branch was dead while the attaching stem continued to grow. Knots materially affect cracking and warping, ease in working, and cleavability of timber. They are defects which weaken timber and lower its value for structural purposes where strength is an important consideration. The weakening effect is much more serious when timber is subjected to forces perpendicular to the grain and/or tension than when under load along the grain and/or compression. The extent to which knots affect the strength of a beam depends upon their position, size, number, and condition. A knot on the upper side is compressed, while one on the lower side is subjected to tension. If there is a season check in the knot, as is often the case, it will offer little resistance to this tensile stress. Small knots, however, may be located along the neutral plane of a beam and increase the strength by preventing longitudinal shearing. Knots in a board or plank are least injurious when they extend through it at right angles to its broadest surface. Knots which occur near the ends of a beam do not weaken it. Sound knots which occur in the central portion one-fourth the height of the beam from either edge are not serious defects (de Boer et al., 2005).


1.2       TYPES OF WOOD

Wood is one of the most commonly used materials in the world, and almost any type of wood can be used to build furniture. Each type of wood has its own unique characteristics, which in turn can add different degrees of warmth, emphasis and beauty to its surrounding decor (James et al., 2012). The types of wood are as follows;

 

1.2.1    Beech Wood

Beech is a hard, strong and heavy wood. It has a fine, tight grain and even texture. Beech wood is very light in colour and has a high shock resistance. It is a popular wood for furniture and will give your room a warm feeling. With its smooth finish it is a great wood to polish. Wood produced by Gmelina arborea is an example of beech wood that can be found in Nigeria (de Boer et al., 2005).


1.2.2    Ash Wood

Ash is a tough hardwood which is known for its excellent bending abilities. It is primarily used for bent pieces of furniture such as a chair with curved backrests. Ash is light brown in colour with a straight grain. The Oil Bean tree produces such type of wood and it can be found in abundance in Nigeria (de Boer et al., 2005).


1.2.3    Fibreboard

Fibre board is an inexpensive manufactured wood made from the breaking down of hard or soft woods into fibres which are then bonded together with wax, resin and heat to create a dense piece of wood. One of the most popular fibreboards is constructed of medium density fibres that are known for their strength and durability and lend themselves ideally to furniture products. Medium Density Fibreboard is very strong and is considerably more popular than people think. In-fact many will be surprised as to how much medium density fibres furniture is around them (de Boer et al., 2005).


1.2.4    Plywood

Plywood is a very strong manufactured wood as it is build-up of layers of wood veneers which are bonded together to create a flat smooth sheet of wood. It is popular in the furniture and flooring industries due to its inherent strength and resistance to warping due to the bonded cross-ply construction (de Boer et al., 2005).

 

1.2.5    Veneer

A veneer refers to a thin layer of wood which is cut from the circumference of a tree. It is then bonded onto a dense piece of wood, which is typically medium density fibres chipboard or plywood. Veneers are available in many sizes, ranging from 3 to 6mm thick.Many people mistakenly assume that veneered furniture is cheaper than solid wood; however, veneers quite often are used in high end furniture pieces and it can be more costly than solid wood.The way to find out if your piece of furniture is veneered is by looking at the edges, and checking if the grain lines run off the top and over the edges of the wood. As a veneer is real wood, it will accept stains and finishes much like solid wood. Example of such wood in Nigeria is the Iroko wood (de Boer et al., 2005).


1.3       CLASSIFICATION OF WOOD

The more one knows about the unique characteristics of wood and its source, the better one can understand the degree of warmth and beauty that it brings to our everyday décor. Furniture made of wood is one of the few things in the world that all people can own and know that they are the only person in the world who owns that particular grain pattern and its inherent beauty. Each grain pattern is a unique masterpiece of design, texture and splendor. Even what some may view as a defect, like a knot or other natural blemishes, can add more beauty and character to any given piece of furniture (James et al., 2012). The classification of wood has historically always been either hard wood; any leaf bearing tree, and soft wood; any cone bearing tree. These terms can be confusing since some leaf bearing trees can have very soft wood and some coniferous trees can have very hard woods. To make this easier, below you will find a list of different tree types, classification and then individual wood characteristics. There are two basic classification of wood, which are


1.3.1    HARDWOODS

1.3.1.1 Maple

There are 55 species of maple. Only 5 commercially important species grow in Nigeria Two of the five are hard rock maple and sugar maple. Maple is so hard and resistant to shocks that it is often used for bowling alley floors. Its diffuse evenly sized pores give the wood a fine texture and even grain. Maple that has a curly grain is often used for violin backs (the pattern formed is known as fiddle-back). Burls, leaf figure, and birds-eye figures found in maple are used extensively for veneers. The Birds eye figure in maple is said to be the result of stunted growth and is quite rare. Maple is used extensively for American colonial furniture, especially in medium and lower priced categories. It can also be stained to simulate cherry wood, which it resembles.


1.3.1.2 Mahogany

Mahogany, also known as Honduras mahogany is a tropical hardwood indigenous to Africa. There are many different grades and species sold under this name, which vary widely in quality and price. Mahogany which comes from the Caribbean is thought to be the hardest, strongest and best quality. Logs from Nigeria, though highly figured, are of slightly lesser quality. Philippine mahogany has a similar color, but is not really mahogany at all. It is a much less valuable wood, being less strong, not as durable or as beautiful when finished. Mahogany is strong, with a uniform pore structure and poorly defined annual rings. It has a reddish - brown color and may display stripe, ribbon, broken stripe, rope, ripple, mottle, fiddleback or blister figures. Crotch mahogany figures are widely used and greatly valued. Mahogany is an excellent carving wood and finishes well. Mahogany is used extensively in the crafting and production furniture. Mahogany is also used in making styles on woods.


1.3.1.3 Cherry

Cherry is grown in the Western part of Nigeria, it is sometimes called fruitwood. The term fruitwood is also used to describe a light brown finish on other woods. A moderately hard, strong, closed grain, light to red-brown wood, cherry resists warping and checking. It is easy to carve and polish.


1.3.1.4 Walnut

Walnut is one of the most versatile and popular cabinet making woods. It grows in Europe, Africa, America and Asia. There are many different varieties.  Walnut is strong, hard and durable, without being excessively heavy. It has excellent woodworking qualities, and takes finishes well. The wood is light to dark chocolate brown in color with a straight grain in the trunk. Wavy grain is present toward the roots, and walnut stumps are often dug out and used as a source of highly figured veneer. Large burls are common. Walnut solids and veneers show a wide range of figures, including strips, burls, mottles, crotches, curls and butts. European walnut is lighter in color and slightly finer in texture than American black walnut, but otherwise comparable. Walnut is used in all types of fine cabinet work, especially 20th century productions (de Boer et al., 2005).


1.3.2    Soft Woods

1.3.2.1 Pine

Pine is softwood which grows in most areas of the Northern Hemisphere. There are more than 100 species worldwide. Pine is a soft, white or pale yellow wood which is light weight, straight grained and lacks figure. It resists shrinking and swelling. Knotty pine is often used for decorative effect. Pine is often used for country or provincial furniture. Pickled, whitened, painted and oil finishes are often used on this wood.


1.3.2.2 Ash

There are 16 species of ash which grow in the eastern United States. Of these, the white ash is the largest and most commercially important. Ash is a hard, heavy, ring porous hardwood. It has a prominent grain that resembles oak, and a white to light brown color. Ash can be differentiated from hickory (pecan) which it also resembles, by white dots in the darker summerwood which can be seen with the naked eye. Ash burls have a twisted, interwoven figure.  Ash is widely used for structural frames and steam bent furniture pieces. It is often less expensive than comparable hardwoods.


1.3.2.3 Birch

There are many species of birch. The yellow birch is the most commercially important. European birch is fine grained, rare and expensive. Birch is a hard, heavy, close grained hardwood with a light brown or reddish colored heartwood and cream or light sapwood. Birch is often rotary or flat sliced, yielding straight, curly or wavy grain patterns. It can be stained to resemble mahogany or walnut.


1.3.2.3 Cedar

Several species of cedar grow in the southern Nigeria, and Central part of Africa. Cedar is knotty softwood which has a red-brown color with light streaks. Its aromatic and moth repellent qualities have made it a popular wood for lining drawers, chests and boxes. Simple cases and storage closets are also constructed from this light, brittle wood.


1.3.2.4 Redwood

This is indigenous to the Pacific United States, redwood trees grow to more than 300 feet tall and 2,500 years old. The best quality redwood comes from the heartwood which is resistant to deterioration due to sunlight, moisture and insects. It is used to craft outdoor furniture and decorative carvings. Redwood burls have a "cluster of eyes" figure. They are rare and valuable (de Boer et al., 2005).


1.3.2.4 Hemlock

Light in weight, uniformly textured, It machines well and has low resistance to decay and non-resinous. Used for construction lumber, planks, doors, boards, paneling, sub flooring and crates.


1.4 FACTORS THAT PROMOTE GRWOTH OF MICROORGANISMS IN WOOD

1.4.1 Moisture

Moisture and temperature affect the chemical, biological, and mechanical processes of decay. The formation of a moisture layer on the material surface is dependent upon precipitation. It may also be generated as a result of the reaction of adsorbed water with the material surface, deposited particles with the material surface, and deposited particles with reactive gases. On a surface with moisture, particles have a better possibility of adhering and water-soluble gases are more readily captured. Both gas and particle fluxes increase when condensation takes place on the material surface and decrease when evaporation occurs. A moisture layer is a medium for the chemical and photochemical reactions of surface contaminants and is also a conductive path for the electrochemical reactions. Two variables are important from the viewpoint of the damage caused by moisture: dew point and relative humidity of air. Dew point is a characteristic of the water content of the large-scale air mass, whereas relative humidity depends on the local temperature and therefore on the local meteorological parameters. When the temperature of a material is below the ambient dew point, water condenses on the material, a moisture layer (condensation) can form, and the material damage may proceed. In most materials, an increase in relative humidity causes further deterioration due to more prolonged wetness time, higher deposition rates of pollutants and better conditions for biodeterioration.


1.4.2 Temperature

Temperature affects the processes of deterioration of a material gradually and in a variety of ways. Changes in temperature induce a thermal gradient between the surface layer and the inner layer of materials (particularly in materials with lower thermal conductivity), which may result in the degradation of the mechanical properties of the material and can lead to the formation of fine cracks. The formation of cracks is accompanied by a loss of strength and by an increase in material porosity, which may lower the chemical resistance of the material. Temperature fluctuation of materials may influence bulk expansion, such as the expansion of stone grains, the dilatation of different materials in joints, and the expansion of water in material capillaries. Increased ambient air temperature is one of the reasons why the rate of wet deposition is more important for deterioration processes in tropical and subtropical regions than in temperate regions. Higher ambient temperature reduces the effects of freeze–thaw cycles (Kuhad et al., 2007).


1.5       WOOD AND ITS COMPONENT

The major components in wood are cellulose, hemicellulose and lignin. Woody and dead agricultural plant materials constitute more than 60% of the total biomass produced on the earth. The degradation of lignocellulosic materials is considered the most important of biological processes, leading to production of carbon dioxide, water and humic substances in the earth’s carbon cycle (Kuhad et al., 2007). Concern over food and energy sufficiency became apparent for more than 30 years ago, due to the first so-called oil crisis of 1973. Interest in the use of abundant renewable lignocellulose as an energy source has again been kindled worldwide. Lignocellulosic biomasses, such as agricultural and forestry residues, were recognized as a potential source of energy that could replace the future transport of unstable and uncertain petroleum and fossil fuel (Himmel et al., 2002). The most common renewable fuel is ethanol, which could be derived from lignocellulosic material, cellulose and hemicellulose, as an attractive future raw material (Gray et al., 2006). In many countries there are research and development programmes in renewable energy sources; e.g. the European Union has envisions a future in which one fourth of fuel will be derived from biomaterials by 2030 (Mosier et al., 2005) and in Finland there are plans to take lignocelluloses in to use more than ever before.


1.5.1    Cellulose

The main structural component in the cell wall of green plants is cellulose. Cellulose molecules form linear homopolysaccharide chains of β-D-glucopyranose units, which are joined together by β (1–>4)-glycosidic bonds. Cellulose molecules are strongly associated through inter- and intermolecular hydrogen bonds and bundles of cellulose molecules form micro fibrils. They are organized into strong fibrous structures, in which the highly oriented regions are called crystalline cellulose and the less oriented region amorphous cellulose. Most of the plant cell wall cellulose is crystalline. Cellulose micro fibrils are embedded in a lignin-hemicellulose matrix (Daniel, 2003).


1.5.2    Hemicellulose

Hemicelluloses are short chains of frequently branched heteropolysaccharides, containing two or more different sugar monomers, both hexoses and pentose. Hemicelluloses are named after their main sugar constituents, such as galactoglucomannans, which are the principal hemicelluloses in softwoods and which are formed of galactose, glucose and mannose units. Glucuronoxylans predominate in hemicelluloses of hardwood, and are composed of β-D-xylose and 4-O-methyl-α-D-glucopyranosyluronic acid units. Hemicelluloses and lignin give the cell structure rigidity (Kuhad et al., 2007).


1.5.3   Lignin

Lignin is the second most abundant biopolymer next to cellulose in terrestrial vascular plants. Lignin is a highly branched, amorphous and water-insoluble high-molecular weight (MW) natural polymer built of phenyl propane units (Sjostrom, 2003). Lignin is closely associated with cellulose and hemicelluloses in plant cell walls and attaches to polysaccharide polymers in the cell walls both physically and chemically. It binds the fibrous cell walls together and this gives plants mechanical strength. Lignin resists chemical and enzymatic degradation and makes wood resistant against microbial attack. The phenyl propane units of lignin are linked together by different types of bonds (Eriksson et al., 2010). The most common bond is the β-aryl ether (β-O-4) linkage, which may account for 50% and 60% of the inter-monomeric bonds in softwood lignin and hardwood lignin, respectively (Sjostrom, 2003). Lignin is synthesized with the fixation of CO2 in plant cell walls and finally by the polymerization of radicals generated by the one-electron oxidation of lignin precursors (Chiang, 2006). The lignin monomeric precursors, p-hydroxycinnamyl alcohols, are synthesized from L-phenylalanine or L-tyrosine through the shikimic acid pathway. The intermediates are formed by deamination and several successive hydroxylation and methylating with enzyme reactions, leading to p-coumaric, caffeic, ferulic, 5-hydroxyferulic and sinapic acid. Enzyme-mediated reductions create p-hydroxycinnamyl alcohols: coniferyl, sinapyl and coumaryl alcohol, which are the primary precursors and building units of lignin. Of these coniferyl alcohol is the most common precursor (Fengel and Wegener, 2009). Softwood consists mainly of tracheids and ray cells that give strength to wood and provide water transport. Either bordered pits or large simple pits connect the softwood tracheids.Hardwood possesses an array of vessels, fibres and ray parenchyma cells that are connected by bordered or simple pits (Sjostrom 2003). The woody cell wall is composed of secondary and primary walls. The secondary wall has three distinct layers, thin outer (S1) and inner layers (S3) and a thick middle layer (S2) (Sjostrom, 2003). Adjacent cells are separated by the pectin-rich middle lamella. The lignin content is highest in the S2 layer because it is the thickest layer, but the greatest lignin concentration is in the middle lamella between cells (Kuhad et al., 2007). Fungal hyphae penetrate from one cell to another through existing pores and pits in the cell walls (Eriksson et al., 2010).


1.6       AIM AND OBJECTIVES

To characterize and identify microorganisms involved in wood degradation, while the specific objectives are;

       To isolate and identify bacteria present in different wood samples

       To isolate and identify fungi present in different wood samples

 

 

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