60 DECEMBER • WORLD AQUACULTURE • WWW.WAS.ORG The intensification of aquaculture has escalated the incidence of bacterial epizootics, driving the extensive use of antibiotics as chemotherapeutic agents for prophylaxis, metaphylaxis, and growth promotion. Although effective, indiscriminate application has accelerated the evolution of antimicrobial resistance (AMR), compromised food safety through drug residues, and contributed to ecological imbalances in aquatic environments. This article delineates the principal antibiotic classes, their pharmacological modes of action and therapeutic applications, while critically evaluating the ecological and public health risks associated with misuse. Furthermore, it underscores emerging non-antibiotic interventions that offer sustainable and ecologically sound strategies for mitigating bacterial pathogenesis in aquaculture. Aquaculture now supplies nearly half of the world’s fish for human consumption (FAO, 2018). However, the rapid shift from traditional to intensive farming systems, where large numbers of fish are raised in limited water areas, has heightened concerns about sustainability and the increased risk of infectious disease outbreaks. Disease remains a major challenge to the industry, as fish reared under crowded and stressful conditions become highly prone to bacterial infections, leading to considerable production losses (Santos and Ramos, 2018). In aquaculture, antibiotics are widely used both preventively and therapeutically to control bacterial infections and reduce disease risks (Cabello et al., 2013). Traditionally, antibiotics were described as biologically derived compounds, mainly produced by microorganisms, that could either inhibit the growth of, or destroy, other microbes, even at very low concentrations (Denyer et al., 2004). Based on their action, antibiotics are classified as bactericidal, which eliminate bacteria, or bacteriostatic, which slow down their growth (Walsh, 2003). While the term ‘antibiotic’ is often used for antibacterial agents, it more broadly includes compounds classified as antibacterials, antifungals, or antivirals, depending on the microorganisms being targeted (Hugo et al., 2004). Antibiotics are broadly classified based on their chemical structure, mechanism of action, and spectrum of activity. The major classes include Beta-lactams, Macrolides, Tetracyclines, Quinolones, Aminoglycosides, Sulphonamides, Glycopeptides, and Oxazolidinones. Major Antibiotic Mechanisms Antibiotics exert their antimicrobial effects by targeting distinctive structural or functional components of bacteria. As shown in Figure 1, the primary modes of action include disrupting cell wall formation, damaging cell membrane integrity, interfering with nucleic acid synthesis and function, inhibiting protein production, and blocking essential metabolic pathways (Wright, 2010). Inhibition of Cell Wall Synthesis Most bacteria are protected by a rigid peptidoglycan layer that maintains cell integrity under osmotic stress. Enzymes such as transglycosylases and transpeptidases (PBPs) extend and crosslink this structure. β-lactam antibiotics (penicillins, cephalosporins, carbapenems) block PBP activity, preventing cross-linking. Glycopeptides like vancomycin bind directly to peptidoglycan precursors, inhibiting both transglycosylation and transpeptidation, leading to cell lysis. Disruption of Cell Membrane Function Some antibiotics act by damaging bacterial membranes, which differ in lipid composition across species. Daptomycin, for instance, depolarizes membranes in a calcium-dependent manner, halting macromolecule synthesis. Polymyxins bind to lipopolysaccharides in Gram-negative bacteria, disrupting membrane integrity. Inhibition of Nucleic Acid Synthesis DNA replication and transcription are vital for bacterial survival. Quinolones interfere with enzymes such as helicase, topoisomerase II, and IV, thereby blocking DNA unwinding, replication, and repair. This action also disrupts RNA polymerase activity, preventing RNA synthesis and selectively targeting bacterial cells without affecting mammalian polymerases. Inhibition of Protein Synthesis Bacterial ribosomes (70S, composed of 30S and 50S subunits) differ from eukaryotic ribosomes, allowing selective targeting. Antibiotics act by blocking translation steps: • 50S inhibitors (erythromycin, clindamycin, linezolid, chloramphenicol) prevent initiation or elongation of peptide chains. Antibiotics in Aquaculture: Cure or Curse? Banothu Divya, Banoth Raveendar, Shiga Nagaraj, Konduri Arun and M. Shyam Prasadki Aquaculture now supplies nearly half of the world’s fish for human consumption. However, the rapid shift from traditional to intensive farming systems, where large numbers of fish are raised in limited water areas, has heightened concerns about sustainability and the increased risk of infectious disease outbreaks. Disease remains a major challenge to the industry, as fish reared under crowded and stressful conditions become highly prone to bacterial infections, leading to considerable production losses.
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