ABSTRACT
The emergence of resistance to traditional antibiotics has led to an increased interest in alternatives such as antimicrobial peptides and synthetic antimicrobial polymers that can be used for therapy and disinfection. However, issues such as toxicity and cost of production still remain a signicant challenge. Biopolymers and
25.1 Introduction ................................................................................................................................................................... 345 25.2 Antimicrobial properties of chitosan ......................................................................................................................... 346
25.2.1 Molecular weight of chitosan .......................................................................................................................... 346 25.2.2 pH of medium ................................................................................................................................................... 348 25.2.3 Degree of acetylation of chitosan ................................................................................................................... 348
25.3 Antimicrobial properties of chitosan derivatives .................................................................................................... 348 25.3.1 Alkyl chitosan derivatives ............................................................................................................................... 348 25.3.2 Quaternary chitosan derivatives .................................................................................................................... 349 25.3.3 Acyl chitosan derivatives ................................................................................................................................. 353 25.3.4 Hydroxyalkyl chitosan derivatives ................................................................................................................ 353 25.3.5 Carboxyalkyl chitosan derivatives ................................................................................................................. 353
25.4 Antimicrobial assays .................................................................................................................................................... 354 25.4.1 Antibacterial assays .......................................................................................................................................... 354
25.4.1.1 Broth microdilution method ............................................................................................................ 354 25.4.1.2 Time kill assay .................................................................................................................................... 355 25.4.1.3 Optical/turbidimetric method ......................................................................................................... 355 25.4.1.4 Agar dilution method ........................................................................................................................ 356 25.4.1.5 Agar diffusion method ...................................................................................................................... 356
25.4.2 Antifungal assays ............................................................................................................................................. 357 25.4.2.1 Radial growth technique .................................................................................................................. 357
25.4.3 Other methods and general comments ......................................................................................................... 357 25.5 Mechanism of antimicrobial action ........................................................................................................................... 357
25.5.1 Bacterial species and cell wall composition .................................................................................................. 357 25.5.2 Models of antimicrobial action ....................................................................................................................... 358
25.6 Structure-activity relationship ................................................................................................................................... 359 25.7 Applications ................................................................................................................................................................... 360
25.7.1 In wound healing .............................................................................................................................................. 360 25.7.2 In food packaging ............................................................................................................................................. 360 25.7.3 Against plant pathogens .................................................................................................................................. 360 25.7.4 In dentistry ........................................................................................................................................................ 361
25.8 Conclusion ..................................................................................................................................................................... 361 Acknowledgment ................................................................................................................................................................... 361 References ................................................................................................................................................................................ 361
biopolymer derivatives can be used as effective antimicrobial agents and have therefore also been considered alternatives to traditional antibiotics and disinfectants. Amongst the available biopolymers, chitosan is recognized as having signicant antimicrobial properties in addition to other desirable properties such as nontoxicity, biodegradability, and biocompatibility. Chitosan is derived from chitin, which is the most abundant natural polysaccharide of animal origin.1 It is the basic constituent of the exoskeleton in insects and crustaceans like shrimps, crabs, and lobsters. Chitin differs from cellulose, which is the most abundant polysaccharide derived from plants, in that it contains an acetamido group (−NHCOCH3) at the C-2 position instead of a hydroxyl group. Chemical deacetylation of chitin gives rise to its deacetylated form called chitosan, which occurs naturally only in certain fungi (Mucoraceae).2 Chitosan is therefore a linear polymer composed of repeating units of glucosamine and N-acetyl glucosamine units that are linked through β-(1-4) glycosidic bonds. The chemical structure of chitosan contains three types of nucleophilic functional groups: a C-2 NH2 group, a C-3 secondary OH group, and a C-6 primary OH group as shown in Figure 25.1. Chitosan can be polycationic, and the density of cationic charge on the polymer will depend on the pH and the number of free amino groups present. Highly deacetylated chitosan is usually insoluble in aqueous medium, but the increased density of such cationic charges at pH below 6.5 improves its water solubility and also contributes to biological properties such as antimicrobial activity. This polymer can be chemically modied to extend the range of conditions where the polymer is soluble and exhibits antimicrobial activity.
