An antimicrobial peptide made by bacteria to kill competing bacteria—nisin is the most commercially important example.Bacteriocins are ribosomally synthesized antimicrobial peptides produced by bacteria that inhibit closely related species. Unlike antibiotics (secondary metabolites), bacteriocins are gene-encoded and target specific bacterial populations. Nisin, produced by Lactococcus lactis, is the most studied bacteriocin—a 34-amino acid lantibiotic that binds lipid II (peptidoglycan precursor) and forms membrane pores. Bacteriocins are classified into classes based on structure and modification: Class I (lantibiotics like nisin), Class II (unmodified), Class III (large, heat-labile), and Class IV (complex).