The mode of action of microbes varies depending on the class of microbes and the nature of the fiber. Natural fibers are more susceptible to microbial attack than synthetic fibers for obvious evolutionary reasons. The major natural plant fibers used in textiles are cotton, flax (linen), jute and hemp, with cellulose being the main polymeric constituent of all of these fibers. The protein fibers used in textiles include wool and silk, and a variety of other animal fibers.
8.2.1 Cellulosic fibers
Cellulose is a polysaccharide consisting of linear chains of several thousand (1- 4)-β-D-glucose units (see Fig. 8.1). Biodeterioration of cellulose occurs through the action of hydrolytic, oxidative or phosphorolytic enzymes (Eriksson et al., 1990). The extracellular enzymes produced by many fungi and bacteria break down the (1-4)-β-D-glucan or glycosidic bonds, causing chain scission and subsequent depolymerization of cellulose to simple sugar molecules (Mitchell and McNamara, 2010, Mussatto et al., 2008). The most important hydrolytic enzymes utilized by microbes are endo-1,4-β-glucanases, exo-1,4-β-glucanase and 1,4-β-glucosidase. Endoglucanases randomly break cellulose chains by at- tacking the β (1-4) linkages. Exoglucanases attack the non-reducing end of cellulose molecules to generate glucose or cellobiose, while glucosidases hydrolyze the cellubiose fragments to glucose (Lucas et al., 2001; Buschle- Diller et al., 1999; Cavaco-Paulo, 1998). Some microbes also utilize an oxidative approach along with hydrolytic breakdown. The major oxidative en- zymes include quinone oxidoreductase and cellobiose oxidase. The quinone oxidoreductase oxidizes cellobiose to cellobiono-δ-lactone during reduction of
Durable antimicrobial textiles: types, finishes and applications 147
8.1 Structure of (1-4)-β-D-glucose (cellulose).
8.2 Typical structure of lignin from hardwood.
quinone and phenoxy radicals. The oxidative action of cellobiose oxidase enzymes on cellobiose produces cellodextrins and their respective acidic deriva- tives. It has been reported that some aerobic and anaerobic bacteria can generate phosphorylases that can also depolymerize cellulose.
Differences in the physical and morphological structures of cellulose fibers alter their susceptibility to microbial attack. The higher the degree of poly- merization, fiber crystallinity or degree of orientation, the less will be the rate of biodeterioration (Mansfield et al., 1999). The presence of branched polysacchar- ides such as hemicellulose and pectins in fibers generally decreases the crystallinity index and opens up the physical structure. This increases the ability of microbes to invade the structure of the fiber (Szostak-Kotowa, 2004). Fibers with higher lignin content are more resistant to microbes (see Fig. 8.2) (Mohan et al., 2006). Among other effects, it is believed that lignin constrains swelling, leading to lower accessibility to the inner macro structure of the fiber (Mitchell and McNamara, 2010).
OH O
HO OH
O O
HO
O
OH n
OH
148 Understanding and improving the durability of textiles
8.3 Structure of α-amino acid, based on the substituent (R). Close to 20 naturally occurring α-amino acids are known.
8.2.2 Protein fibers
The basic unit of protein fibers is the α-amino acid, as seen in Fig. 8.3. There are 19–20 different α-amino acids in wool. The other animal hair fibers in significant use include cashmere, camel, mohair, alpaca, llama and vicuna. The animal hair fibers differ only in their cystine and cysteic acid content. Jones et al. (1998) observed that the amino acid composition of these fibers cannot be used success- fully to distinguish one from the other. The only secreted protein fiber of commercial importance is silk, whose structure is dominated by glycine–alanine–serine sequences in its beta sheet structure.
Wool
Wool is a natural protein fiber, with keratin protein being the major building block.
Keratin protein contains cystine (disulphide linkages) (see Fig. 8.4) and salt bridges between molecular chains which provide stability and strength to the wool fiber. On the molecular level, biodeterioration of wool begins with scission of the disulphide linkages by a reaction called sulfitolysis (Blyskal, 2009). The reaction yields smaller proteins which can be broken down further with extracellular hydrolytic enzymes, such as proteases, to continue the keratinolysis process. This biochemical process leads to morphological changes, fiber fracturing, pitting and eventual disintegration of fiber structure as more and more cortical cells of the fiber are exposed to attack (Mitchell and McNamara, 2010). Fungal attack on wool usually causes more damage than bacterial deterioration, and detection of degrada- tion is difficult to assess until severe degradation has occurred (Gochel et al., 1992).
8.4 Disulfide linkages between polymer chains.
CH C OH
H2N R
O R = -H Glycine R = -CH3 Alanine R = -CH(CH3)2 Valine R = -CH2CH(CH3)2 Leucine R = -CH2SH Cysteine R = -CH2OH Serine
S S S S S S
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Silk
Silk is a natural protein fiber produced by silkworms. Silk is one of the most resistant natural fibers to microbial attack. The exuded natural fiber contains the fibroin protein, which is covered with a protective layer of another protein called sericin (Kadolph and Langford, 2002). The high orientation and crystal- linity of fibroin protein in silk gives strength and chemical resistance to the fiber. However, sericin is an amorphous, polar and water-soluble protein, and is more susceptible to microbial attack. Studies on silk bio-deterioration report that degummed silk (silk without sericin) showed less bacterial growth compared with native silk, indicating that bacteria can break down sericin and use it as a carbon source (Szostak-Kotowa, 2004). Unlike wool, it is reported that silk fiber is more susceptible to biodeterioration by bacteria than by fungi (Seves et al., 1998).