الفهرس 
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Abstract
The history of chitosans dates back to the 19th century when rouget discussed the deacetylated form of chitosan in 1859.
Chitosan is a copolymer consisting of β-(1→4)-2-acetamido-D-glucose and β-(1→4)-2-amino-D-glucose units, with the latter usually
exceeding 80%. However, chitosan has an apparent pKa of 6.0 and is only soluble in
acidic solutions with pH values lower than 6.0. This interferes with its biomedical
applications of this polymer, especially at the physiological pH value (7.4) where
chitosan is insoluble and consequently less effective. However, its poor solubility in
aqueous water at pH above 6.5 and most of common-used organic solvents limits its
utilizations. As an efficient solution to improve its solubility and antibacterial activity in
aqueous water, numerous modifications of chitosan have been reported such as N-carboxymethylation, quaternization, sugar modification, and alkylation. Typically,
quaternization is a promising kind of modification due to its multifunction along with
favorable solubility at all pH range. Quaternised chitosan derivatives have shown higher
activity against bacteria.
Recently, studies have attracted interest for converting chitosan to their oligomers.
chitosan oligosaccharides have low viscosity and low molecular weight and are soluble in
neutral aqueous solutions. They exhibit various interesting biological activities. Chain
length and degree of deacetylation are considered the most important factors influencing
the biological activities of COS. Chitosan could be depolymerized by physical, chemical or
enzymatic methods. The chemical hydrolysis can be performed using HCl, H
2SO4
, H2O2
,
HNO3
. There are many problems existing in the chemical processes, such as the production
of a large amount of short-chain oligosaccharides including a monomeric unit which
requires an additional process for the fractionation of longer oligosaccharides, low yields
of oligosaccharides, high cost in separation, and also environmental pollution. Enzymatic
depolymerization based either on specific or non-specific enzymes finds advantages over
other methods. Non-specific enzymes such as chitinase, lysozyme, cellulase, lipase,
papain, pepsin, pectinase, and pronase are inexpensive and commercially available, result
mainly in the formation of COS.
Generally, chitosanases have been recognized as enzymes that attack chitosan but not
chitin. In 2004, the Enzyme Commission amended the definition of chitosanase, and it is
now defined as the enzyme performing endohydrolysis of β-1,4-linkages between D-glucosamine residues in a partly acetylated chitosan. Chitosanases are able to hydrolyze all
kinds of linkages in chitosan except the GlcNAc-GlcNAc bond. chitosanase enzyme
isolated from Streptomyces sp. cleaves GlcN-GlcN and GlcNAc-GlcN linkages. On the
other hand, chitosanase enzyme isolated from some Bacillus sp. cleaves only GlcN-GlcN
linkages;
Chitosan and its derivatives display antibiotic activity against microorganism, both
bacteria and fungi. The antibacterial activity of Trimethyl chitosan is now attracting great
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interests. With positive charged N-atoms, the antibacterial activity of Trimethyl chitosan is superior to chitosan due to permanent quaternary moieties and enhanced solubility.
It is well known that the molecular weight of chitosan is one of the most important
factors affecting on the bactericidal activity. Chitosan was more effective than chi tosan
oligosaccharides in inhibiting growth of bacteria.
Variations in chitosan’s bactericidal efficacy arise from various factors. According to
roles playing, these factors can be classified into four categories as follow: (1) microbial
factors, related to microorganism species and cell age; (2) intrinsic factors of chitosan,
including positive charge density, Molecular weight, concentration,
hydrophilic/hydrophobic characteristic and chelating capacity; (3) physical state, namely
water-soluble and solid state of chitosan; (4) environmental factors, involving ionic
strength and pH.
In the present study, we isolate chitosanase producing bacteria and use the chitosanase
enzyme in depolymerization of chitosan to produce COS. Also, we modified the chitosan
chemically using dimethyl sulfate to increase the permanent positive charge of chitosan
and solubility at all pH range through the formation of trimethyl chitosan. Finally, we are
tested the antibacterial activity of chitosan and its derivatives (trimethyl chito san, chitosan
oligosaccharides, and trimethyl chitosan oligosaccharides) against the Gram-positive and
Gram-negative representative bacteria, Enterococcus fecalies and E. coli, respectively.
The interaction between chitosan and microbial cells could be on the cell surface,
which leads to increased permeability of cell wall and leakage of intracellular components,
or inside the cell, which inhibits DNA and RNA synthesis and directs cells into death.
Several proposed mechanisms all involve some kind of damage or interaction with the cell
membrane. Chitosan also act as chelating agents that selectively can bind certain trace
metals
and thus inhibit microbial growth. Chitosan can activate several defense processes in the
host tissue, act as a water binding agent and inhibit various enzymes. Even binding of
chitosan with DNA and inhibition of mRNA synthesis has been shown to occur through
chitosan penetration toward the nuclei of the microorganisms and interfering with the
synthesis of mRNA.