How Do Antibiotics Work?

Antibiotics are specialized medicines designed to combat infections caused by bacteria. The term “antibiotic” signifies “against life.” Antibiotic medications are commonly used for treating and preventing infections. Two classes of antibiotics are employed to halt bacterial growth: bacteriostatic and bactericidal. The primary distinction lies in the method of action. Bactericidal antibiotics cause direct bacterial death by preventing the production of bacterial cell walls, leading to an irreversible effect. Examples include beta-lactam antibiotics, cephalosporins, and vancomycin. On the other hand, bacteriostatic antibiotics prevent bacterial DNA replication and protein synthesis, with reversible effects. Examples include tetracyclines, spectinomycin, chloramphenicol, sulphonamides, trimethoprim, lacosamides, and macrolides. At high concentrations, bacteriostatic antibiotics may also exhibit bactericidal effects.

These drugs are available in various forms, such as tablets, capsules, liquids, creams, and ointments. While many antibiotics require a prescription, certain creams and ointments are available over the counter. Their primary function is to eliminate or hinder the growth of bacteria in the body. Antibiotics combat bacterial infections by attacking the bacterial wall, interfering with reproduction, or blocking protein production.

Bacterial cells typically consist of a cell wall, cell membrane, and nucleus. The cell wall, an outer layer made up of peptidoglycan with cross-linked polymers, is crucial for resistance mechanisms and virulence factors, shaping the bacteria. Multiple layers of peptidoglycan, composed of glycans and peptide chains, form the bacterial cell wall. N-acetyl glucosamine and n-acetyl muramic acid combine to form the cell wall glycans, facilitated by transglycosidases. In the presence of penicillin-binding proteins (PBPs), glycine residues cross-link the d-alanyl-d-alanine section of the peptide chain. The bacterial cell wall can be likened to a hard outer layer composed of linked protein and sugar blocks, essential for the bacteria’s shape, strength, and resistance to hazardous substances.

β-lactam antibiotics, including penicillins and cephalosporins, commonly function by preventing the formation of bacterial cell walls. The main focus of β-lactam agents is on penicillin-binding proteins (PBPs). The β-lactam ring in these antibiotics mimics the d-alanyl d-alanine segment, a typical binding site for PBPs. Interaction with the β-lactam ring prevents PBPs from participating in new peptidoglycan synthesis, ultimately causing the breakdown of the peptidoglycan layer and bacterial lysis.

Folic acid is essential for DNA and RNA synthesis, as well as for the processes of growth and multiplication. Humans obtain folic acid from the diet, while bacteria need to produce their own. Trimethoprim and sulphonamides inhibit different stages of folic acid production. Sulphonamide binds to the enzyme dihydropteroate synthase, preventing the conversion of para-aminobenzoic acid (PABA) into dihydrofolate (DHF). Trimethoprim inhibits the enzyme dihydrofolate reductase, necessary for producing tetrahydrofolate (THF). Together, trimethoprim and sulphonamide work synergistically to lower the rate of resistance mutation development.

Antibiotics that block protein synthesis typically target bacterial ribosomes, essential for protein synthesis. Bacterial ribosomes have two asymmetrical subunits, 30S and 50S. Aminoglycosides like gentamicin and streptomycin bind to the 30S subunit, preventing the formation of the initiation complex. This binding causes mRNA misreading and the addition of incorrect amino acids to the growing polypeptide chain, leading to bacterial cell death by releasing toxic or nonfunctional proteins. Understanding these mechanisms of action is crucial for selecting the right antibiotics for specific infections and avoiding resistance development.


Name: Vrushali Shantaram Dongare

Class: M. Pharm (Sem- III)

Department: Pharmacology