66 JUNE 2023 • WORLD AQUACULTURE • WWW.WAS.ORG hydrophobicity, amphipathic (surfactant) properties and their three-dimensional structures maintained by disulfide bonds. The variations in AMP sequences and the number and nature of cationic residues in their C-terminal domain determine their antimicrobial spectrum. Paradaxin — the first AMP family to be identified in fish (Primor and Tu 1980) — are also called host-defense peptides and offer protection against a wide range of pathogens by disrupting their growth through “lytic” or pore-forming “ionophoric” mechanisms (Smith et al. 2010). Certain AMPs are involved in entrapping bacteria, in addition to their most notable function of being able to form pores and permeabilize membranes in bacteria. Some are able to agglutinate virions into clots, which reduces their pathogenicity. Some fish AMPs are capable of being antioxidant peptides and using reactive oxygen species (ROS) under optimal conditions to enhance their antimicrobial action (Valero et al. 2020). AMPs are secreted in high-risk pathogen targets such as saliva, mucus and circulatory system. AMP Families AMPs are classified into four major families based on their structure; α-helical, β-sheet, linear extended and loop AMPs (Masso-Silva and Diamond 2014, Chaturvedi et al. 2020) α-helical AMPs are the most studied and largest family of AMPs. They consist of approximately 250 peptides that are less than 40 amino acids long and do not contain cysteine residues in their sequence. These peptides have a tertiary structure with a hinge region in the middle. Examples of α-helical AMPs include pleurocidin, pardaxins, buforin, cecropins, melittin and dermaseptin (Tossi et al. 2000). β-sheet AMPs consist of approximately 250 peptides. Their sequences contain cysteine residues that lead to the formation of disulfide bonds. Disulfide bonds stabilize the β-sheet structure, and peptides adopt a nearly cyclic structure due to the presence of these bonds. The cyclic conformation is also crucial for the antimicrobial activity of these peptides. Examples of β-sheet Aquaculture plays a crucial role in providing food security for the growing global population; however, diseases are regarded as a significant threat to the expansion of aquaculture. The use of antibiotics and chemicals in aquaculture has raised concerns among international food buyers. Antibiotics in aquaculture are not as strictly regulated as in other industries and the regulatory framework varies significantly worldwide. Injudicious use of antibiotics in aquaculture has led to the development of antimicrobial-resistant bacteria that can significantly impact human health. Antimicrobial agents can also promote the dissemination of resistant bacteria and genes, which leads to the development of antimicrobial-resistant microorganisms. Multidrug-resistant (MDR) bacteria can develop through several mechanisms, such as reducing intracellular drug concentrations, mutations in the target sites or direct inactivation of antibiotics (Preena et al. 2020). Antibiotic-resistant microorganisms may cause significant changes in microbial communities. The antimicrobial-resistant genes can remain in the environment for years, even after the use of antibiotics had ceased. Prolonged use of antibiotics below the minimum inhibitory concentration can put selective pressure on microbes and lead to horizontal gene transfer among human and fish pathogens (Amarasiri et al. 2020). Antimicrobial peptides (AMPs) are an alternative to antibiotics for controlling pathogenic infections in aquaculture (Bhat et al. 2022a, Bhat et al. 2022b). AMPs offer several advantages over other drug candidates, such as a broad-spectrum activity against bacteria, viruses and fungi, and low potential for developing antimicrobial resistance. Antimicrobial Peptides Antimicrobial peptides play a crucial role in the innate immune system of many organisms in nature. They are small peptides that can protect against a wide range of pathogens. These are the most vital aspects of the body’s innate immune system and perform many biological functions offering immunocompetence and homeostasis to the living organisms (Chaturvedi et al. 2020). AMPs share common chemical features, such as positive charge, Antimicrobial Peptides: A Closer Look at Their Classification and Function Raja Aadil Hussain Bhat, Irfan Ahmad Bhat, Dar Jaffer Yousuf and Mir Ishfaq Nazir Antimicrobial peptides (AMPs) are an alternative to antibiotics for controlling pathogenic infections in aquaculture. AMPs offer several advantages over other drug candidates, such as a broad-spectrum activity against bacteria, viruses and fungi, and low potential for developing antimicrobial resistance. Antimicrobial peptides play a crucial role in the innate immune system of many organisms in nature. They are small peptides that can protect against a wide range of pathogens. Certain AMPs are involved in entrapping bacteria, in addition to their most notable function of being able to form pores and permeabilize membranes in bacteria. Some fish AMPs are capable of being antioxidant peptides.