What Are Antibiotics?

Antibiotics are chemical compounds that kill bacteria or inhibit their growth, used clinically to treat bacterial infections and studied across pharmacology, medicinal chemistry, and biomedical engineering. The term covers a broad structural and functional class of molecules, including naturally derived compounds such as penicillin, semi-synthetic derivatives, and fully synthetic agents such as fluoroquinolones. Since Alexander Fleming's 1928 observation of the antibacterial effect of Penicillium mold on Staphylococcus cultures, antibiotics have become one of the most consequential classes of drugs in medicine, enabling routine surgery, cancer chemotherapy, and the management of infectious disease.

Antibiotics are classified by their chemical structure, their target within the bacterial cell, and whether they are bacteriostatic (inhibiting growth) or bactericidal (killing bacteria outright). This classification matters clinically because bactericidal agents are generally preferred for infections in immunocompromised patients or those involving the bloodstream and central nervous system.

Mechanisms of Action

Antibiotic mechanisms fall into several structural target categories. Beta-lactam antibiotics, including penicillins and cephalosporins, bind to and inhibit penicillin-binding proteins that cross-link peptidoglycan chains in the bacterial cell wall, causing lysis under osmotic pressure. Aminoglycosides, including gentamicin and streptomycin, bind to the 30S ribosomal subunit and cause misreading of messenger RNA, producing defective proteins that insert into the membrane. Macrolides such as erythromycin bind the 50S ribosomal subunit and block translocation of the peptide chain. Fluoroquinolones inhibit DNA gyrase and topoisomerase IV, enzymes required to relieve supercoiling during replication. The molecular detail of these and other mechanisms is described in a PMC review of antibiotic action and resistance, which provides a guide organized by target class.

Antibiotic Resistance

Antibiotic resistance arises when bacteria acquire or develop mechanisms that neutralize a drug before it can reach or disable its target. The four primary resistance strategies are enzymatic drug inactivation, target modification, reduced membrane permeability, and active efflux. Beta-lactamase enzymes produced by resistant bacteria cleave the beta-lactam ring before it can bind penicillin-binding proteins; extended-spectrum beta-lactamases and carbapenemases have further extended this resistance to later-generation cephalosporins and carbapenems. Efflux pumps encoded by resistance genes, which can transfer horizontally between species via plasmids, actively expel a wide range of antibiotic classes from the bacterial cell. A molecular review in Nature Reviews Microbiology maps the full range of resistance mechanisms and the structural biology underlying each. The World Health Organization's priority pathogen list identifies organisms for which new antibiotic development is most urgently needed.

Engineering New Antibiotics

The pipeline of novel antibiotic classes approved for clinical use has remained thin since the 1980s, driving research into new discovery and design strategies. Genome-based target identification uses whole-genome sequencing of pathogen populations to find conserved genes essential for viability that are absent in the human genome. Fragment-based and structure-aided drug design computationally screen compound libraries against crystallographically resolved binding sites to propose candidates for synthesis and testing. Phage therapy, the use of bacteriophages to lyse specific pathogens, has re-emerged as a potential adjunct to antibiotic treatment in cases of extreme drug resistance. Research cataloged in Frontiers in Pharmacology on antibiotic action and resistance covers these approaches alongside updated resistance surveillance data.

Applications

Antibiotics have applications in a range of fields, including:

• Clinical infectious disease, for treating bacterial infections from pneumonia to sepsis
• Surgical prophylaxis, for preventing post-operative infections during and after invasive procedures
• Agricultural and veterinary medicine, for managing bacterial disease in livestock and aquaculture
• Drug delivery research, for investigating conjugated antibiotic-nanoparticle systems with improved tissue penetration
• Biosensor development, for detecting antibiotic residues and resistant organisms in food and water supplies