Mesophilic Lactic Acid Bacteria (LAB) refer to a group of Gram-positive, acid-tolerant bacteria that grow optimally at moderate temperatures (20-45°C) and are widely used in food fermentation processes such as cheese, yogurt, and sauerkraut production. Common genera include Lactococcus, Lactobacillus, and Leuconostoc. These bacteria ferment lactose and other sugars into lactic acid, lowering pH and contributing to flavor, texture, and preservation. They are not a single chemical compound but living microorganisms with complex cellular structures including peptidoglycan cell walls, proteins, and nucleic acids. No boiling or melting points apply as they are biological entities. They play essential roles in dairy industry for mesophilic starter cultures.