WWW.WAS.ORG • WORLD AQUACULTURE • DECEMBER 2025 61 • 30S inhibitors (tetracyclines, streptomycin, spectinomycin) block aminoacyl-tRNA binding. Aminoglycosides are bactericidal, while most others are bacteriostatic, though some (e.g., chloramphenicol) can be bactericidal in specific species. Inhibition of Key Metabolic Pathways Some antibiotics act as metabolic analogs. Sulphonamides and trimethoprim mimic substrates in folic acid synthesis, a pathway essential for nucleic acid and amino acid production. By outcompeting the natural substrate, they block folate metabolism, leading to impaired DNA and protein synthesis. Beta-lactams (penicillins, cephalosporins, carbapenems, monobactams) inhibit cell wall synthesis and remain widely used, though resistance is increasing. Macrolides (erythromycin, azithromycin) and tetracyclines (doxycycline, minocycline) target protein synthesis, with macrolides often prescribed for penicillin-allergic patients and tetracyclines restricted in children due to teeth discoloration. Quinolones (ciprofloxacin, levofloxacin) disrupt DNA replication, effective in urinary and respiratory infections but limited by toxicity concerns. Aminoglycosides (streptomycin, gentamicin, tobramycin) are potent against Gram-negative bacteria but can be toxic. Sulphonamides were the first synthetic antibiotics, effective against a wide range of bacteria but associated with adverse effects. Glycopeptides like vancomycin are vital for treating resistant Gram-positive infections, while Oxazolidinones such as linezolid are newer synthetic drugs effective against multidrug-resistant strains, especially in respiratory and skin infections (Etebu and Arikekpar, 2016). Antibiotic Resistance Antibiotic resistance is an escalating global concern, with “superbugs” emerging as bacteria adapt to survive exposure to drugs that once controlled them. These pathogens acquire resistance through genetic mutations or by developing mechanisms such as producing enzymes that inactivate antibiotics, altering the target sites, or accumulating multiple resistance genes. Some strains are now resistant to five or more antibiotics, severely limiting treatment options. A major driver of this crisis is the widespread misuse of antibiotics, such as using them against viral infections or administering them improperly. An even greater contributor is their extensive application in livestock production, where nearly 80% of antibiotics are used to promote growth and prevent disease. Residues excreted by animals enter soil and water systems, maintaining antimicrobial activity and spreading resistance across ecosystems (Fymat, 2017). Bacteria resist antibiotics through multiple strategies that interfere with drug activity, limit drug entry, or alter drug targets. Among them, β-lactam resistance is most widespread, largely due to the production of β-lactamases, enzymes that cleave the characteristic β-lactam ring and neutralize the antibiotic. These enzymes, thought to have originated from penicillin-binding proteins (PBPs), are now common across Gram-positive, Gram-negative, and even mycobacterial species. Depending on their genetic origin, they may be chromosomal or plasmid-borne and are often associated with mobile elements such as integrons and transposons, facilitating rapid dissemination. Other antibiotic groups also face distinct resistance mechanisms. Aminoglycoside resistance is largely enzymatic, mediated by acetyltransferases (AAC), nucleotidyltransferases (ANT), and phosphotransferases (APH), which chemically modify the drug and limit its activity. These enzymes contribute to complex patterns of cross-resistance, for example between kanamycin, neomycin, and streptomycin. For trimethoprim and sulfonamides, resistance develops through mutations in dihydrofolate reductase (DHFR) or dihydropteroate synthase (DHPS), reducing drug affinity, or through plasmid-encoded resistant variants of these enzymes. Integrons frequently co-localize such resistance determinants with genes conferring resistance to unrelated antibiotics, accelerating multidrug resistance. Fluoroquinolone resistance usually arises from chromosomal mutations in DNA gyrase (gyrA, gyrB) or topoisomerase IV (parC, (CONTINUED ON PAGE 62) FIGURE 1. Different target sites of antibiotics. Etebu and Arikekpar 2016. Antibiotic resistance is an escalating global concern, with “superbugs” emerging as bacteria adapt to survive exposure to drugs that once controlled them. These pathogens acquire resistance through genetic mutations or by developing mechanisms such as producing enzymes that inactivate antibiotics, altering the target sites, or accumulating multiple resistance genes. Some strains are now resistant to five or more antibiotics, severely limiting treatment options.
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