1
A: Lysozyme in saliva prevents microbial infections = TRUE ✓
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R: Lysozyme degrades polysaccharide (peptidoglycan) of bacterial cell walls = TRUE ✓
R explains A: by degrading cell wall → bacteria lyse → infection prevented
Answer: Both A and R correct, R explains A
Lysozyme: cleaves NAM-NAG bonds in bacterial peptidoglycan → cell wall degradation → bacterial lysis
Found in: saliva, tears, nasal mucus, breast milk, egg white, leukocytes
Theory: Human Physiology / Immunology
1. Innate Immunity — First Line Defences
First line: physical and chemical barriers. Skin: tough keratinised barrier. Mucous membranes: trap and remove pathogens. Cilia: sweep trapped particles. Antimicrobial secretions: lysozyme, IgA, lactoferrin, defensins in saliva, tears, nasal mucus, breast milk. Stomach acid: pH 1-3 kills most ingested pathogens. Normal microbiota: compete with pathogens.
2. Lysozyme in Different Body Secretions
Tears: protect eyes (crying can reduce eye infections). Saliva: oral defence. Nasal mucus: respiratory tract defence. Breast milk/colostrum: protect neonate gut. Egg white (albumin-rich): protects egg from infection (natural antimicrobial environment). Neutrophil granules: kill phagocytosed bacteria intracellularly. Macrophage lysosomes: contain lysozyme to degrade bacteria after phagocytosis.
3. Other Antimicrobial Enzymes and Peptides
Besides lysozyme: Phospholipase A2: degrades bacterial phospholipid membranes. Defensins: cationic antimicrobial peptides; disrupt bacterial/fungal membranes; found in neutrophils, epithelial cells. Lactoferrin: iron-chelating protein; deprives bacteria of iron; also directly antimicrobial. Cathelicidins (LL-37): antimicrobial peptide; disrupts bacterial membranes. Complement system: cascade of proteins that lyse bacteria, opsonise for phagocytosis.
4. Clinical Applications of Lysozyme Knowledge
Lysozyme-enhanced products: lysozyme added to some eye drops, wound care products, food preservatives. Laboratory use: lysozyme used to lyse bacterial cells for DNA/protein extraction (add lysozyme → bacterial cell wall degradation → cell contents released). Phage therapy: bacteriophages encode endolysins (phage-encoded cell wall lytic enzymes similar to lysozyme) that lyse bacteria — being explored as alternative to antibiotics. Lysozyme deficiency: rare, associated with increased susceptibility to infections.
Frequently Asked Questions
1. What is lysozyme? ⌄
Lysozyme (muramidase) is an antimicrobial enzyme present in many body secretions: saliva, tears, nasal mucus, breast milk, sweat, egg white, and certain leukocytes (neutrophils, macrophages — important for killing bacteria after phagocytosis). It was discovered by Alexander Fleming in 1922 (6 years before penicillin). It was the first antibacterial enzyme discovered.
2. How does lysozyme work? ⌄
Lysozyme cleaves the beta-1,4 glycosidic bond between N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG) in the peptidoglycan (murein) layer of bacterial cell walls. This hydrolysis weakens and degrades the cell wall. Without an intact cell wall, bacteria in isotonic or hypotonic environments undergo osmotic lysis (water rushes in → bacteria burst). Gram-positive bacteria (thick peptidoglycan) are generally more susceptible than Gram-negative bacteria (thinner peptidoglycan shielded by outer membrane).
3. What is peptidoglycan? ⌄
Peptidoglycan (murein) is the structural polymer forming the bacterial cell wall: alternating N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG) residues linked by beta-1,4 glycosidic bonds (cross-linked by peptide bridges). Functions: maintains bacterial shape, withstands osmotic pressure, provides rigidity. Gram-positive: thick peptidoglycan (20-80 nm). Gram-negative: thin peptidoglycan (2-7 nm) + outer membrane. Target of: lysozyme (breaks NAM-NAG bonds), penicillin (inhibits cross-linking), vancomycin (blocks peptide cross-link formation).
4. What other innate immune components are in saliva? ⌄
Saliva contains multiple antimicrobial components: Lysozyme (bacterial cell wall degradation), IgA (mucosal antibody, prevents pathogen binding), Lactoferrin (chelates iron, limiting bacterial growth), Amylase (digests starch; also minor antimicrobial activity), Defensins (antimicrobial peptides that disrupt bacterial membranes), Mucins (glycoproteins that trap microorganisms), Peroxidase system (oxidative antimicrobial activity). Together these form the first line of defence in the oral cavity.
5. Who discovered lysozyme? ⌄
Lysozyme was discovered by Alexander Fleming in 1922. He noticed that a drop of his nasal mucus dissolved bacterial colonies on a Petri dish. He identified the responsible enzyme as lysozyme. This discovery preceded his more famous discovery of penicillin (1928) and demonstrated that the body has its own antimicrobial substances — an important concept in immunology.