Lysozyme is an enzyme that is often isolated from hen egg, as well as other natural sources, such as certain vegetables, insects, plants, fungi, human colostrum, and milks (Larson, 2005). The antimicrobial activity of lysozyme relies on the ability of hydrolyzing the (3-1,4 linkage between C1 of N-acetylmuramic acid and C4 of N-acetyl glucosamine present in peptido- glycan. Gram-positive bacteria are very susceptible to lysozyme, because their cell wall is made up of 90% of peptidoglycan; while in Gram-negative bacteria, peptidoglycan counts only for 5—10% of the cell wall and lies beneath the outer membrane of the cell envelope, making them not susceptible to lysozyme (Barbiroli et al., 2012). However, the sensitivity of lysozyme to gram-negative bacteria can be increased by adding chelators, such as ethy- lenediaminetetraacetic acid to bind with Ca21 and Mg21 to destroy the integrity of lipopolysaccharide (LPS) layer (Davidson and Zivanovic, 2003). The activity of lysozyme toward Gram-negative bacteria can also be improved by lacroferrin (Barbiroli et al., 2012), which is introduced below.

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