Natural Antimicrobial Systems in Milk & Inherent Defense Factors

Assertion (A) is false because the bovine lactoperoxidase system consists of three components: lactoperoxidase enzyme, thiocyanate ion ($SCN^-$), and hydrogen peroxide ($H_2O_2$)—not water and thiamin. Reason (R) is true because the resulting oxidation products (primarily hypothiocyanite, $OSCN^-$, and hypothiocyanous acid, $HOSCN$) oxidize bacterial sulfhydryl ($-SH$) groups in vital metabolic enzymes and damage cell membrane integrity.

Analysis of lysozyme distribution in milk and antibacterial susceptibility:
Statement (I) is incorrect: In bovine (cow) milk, lysozyme is present only in minuscule trace amounts (approximately $0.013\text{ to }0.07\text{ mg/100 ml}$), whereas human breast milk contains extraordinarily high concentrations of lysozyme (approximately $30\text{ to }40\text{ mg/100 ml}$, roughly 3,000 times higher than cow milk), conferring critical mucosal immunity to breastfed infants.
Statement (II) is correct: Lysozyme (muramidase) enzymatically hydrolyzes the $\beta(1\to 4)$ glycosidic bonds between N-acetylmuramic acid and N-acetylglucosamine in bacterial peptidoglycan. In Gram-positive bacteria, the thick peptidoglycan cell wall is directly exposed. In Gram-negative bacteria, the complex outer membrane rich in lipopolysaccharides (LPS) acts as a physical barrier preventing lysozyme from reaching the periplasmic peptidoglycan layer, rendering Gram-negative bacteria inherently far more resistant.

Antimicrobial defense factors present in milk:
Natural antimicrobial systems in milk are broadly categorized into non-specific and specific protective systems:

• Non-specific factors include lactoferrin, lactoperoxidase (lactenins L1, L2, L3), and lysozyme, which act indiscriminately against a wide range of bacteria.
• In traditional dairy immunology classifications and the official examination key, the designated specific factors are identified as Bifidus factor and complement (option 4), which function synergistically in targeted host defense and gut microflora conditioning.

Lactenins are natural antibacterial substances originally discovered in fresh raw milk active against Streptococcus pyogenes. They comprise three distinct fractions: Lactenin 1 ($L_1$, predominantly found in colostrum, identified as agglutinins/antibodies), Lactenin 2 ($L_2$, in normal milk, identified as the lactoperoxidase-thiocyanate system), and Lactenin 3 ($L_3$). Because these three fractions differ significantly in molecular identity, thermal stability, specificity, and mechanism of action, stating that they exhibit similar activity is NOT true.

Option analysis:

• True: Lactoperoxidase system requires oxygen/peroxide and shows minimal or no activity under strictly anaerobic conditions.
• True: Reducing agents (such as cysteine, glutathione, or thiols) reduce the hypothiocyanite and inhibit lactenin activity.
• NOT true (Correct answer): The three lactenins differ markedly in heat resistance, chemical identity, and antimicrobial spectrum.
• True: Lactenins were originally characterized by their potent bacteriostatic action against Streptococcus pyogenes.