Nisin is a polycyclic antibacterial peptide produced by the bacterium Lactococcus lactis that is used as a food preservative. It has 34 amino acid residues, including the uncommon amino acids lanthionine, methyllanthionine, didehydroalanine, and others. Nisin is a member of a class of molecules known as lantibiotics, and it is related to other lantibiotics such as subtilin and epidermin. Nisin is obtained from the culturing of L. lactis on natural substrates, such as milk or dextrose, and it is not chemically synthesized.

Nisin is a "broad-spectrum" bacteriocin that is effective against many Gram-positive organisms, including lactic acid bacteria, Listeria monocytogenes, Staphylococcus aureus, Bacillus cereus, Clostridium botulinum, and others. It is also particularly effective against spores. Nisin is soluble in water and can be effective at levels nearing the parts-per-billion range. Nisin concentration can be measured using various techniques such as chromatography or by a simple agar diffusion bioassay.

Nisin is used as a natural preservative for many food products, mainly in dairy and meat products. It inhibits pathogenic foodborne bacteria such as Listeria monocytogenes and many other Gram-positive food spoilage microorganisms. Nisin is commonly used in processed cheese, meats, beverages, etc. during production to extend shelf life by suppressing Gram-positive spoilage and pathogenic bacteria. In foods, it is common to use nisin at levels ranging from ~1-25 ppm, depending on the food type and regulatory approval.

Recent studies have shown that nisin influences the gut microbiota and its functionality by modulating the abundance of specific bacterial groups. Nisin is classified as a Type A lantibiotic and has been shown to have potential biomedical applications.