How Antibiotics Work: Mechanisms That Kill Bacteria

The Principle of Selective Toxicity

Antibiotics exploit fundamental differences between bacterial cells and human cells. Bacteria are prokaryotes with cell walls, 70S ribosomes, and circular DNA. Human cells are eukaryotes with no cell walls, 80S ribosomes, and linear DNA packaged in a nucleus. These differences provide the targets that antibiotics can hit without damaging the host. The more different a bacterial target is from anything in human cells, the safer the antibiotic tends to be. Cell wall synthesis inhibitors are among the safest antibiotics precisely because human cells have no cell wall at all.

Antibiotics are classified as either bactericidal (killing bacteria directly) or bacteriostatic (preventing bacterial growth and letting the immune system finish the job). Beta-lactams, fluoroquinolones, and aminoglycosides are generally bactericidal. Macrolides, tetracyclines, and sulfonamides are generally bacteriostatic. However, this classification is concentration-dependent and organism-dependent: a drug that is bacteriostatic at lower concentrations may become bactericidal at higher concentrations, and the same drug may be bactericidal against one species and bacteriostatic against another.

Cell Wall Synthesis Inhibitors

The bacterial cell wall is a rigid mesh of peptidoglycan (also called murein) that surrounds the cell membrane and prevents the cell from bursting due to osmotic pressure. No human cell has a peptidoglycan wall, making this target ideal for antibiotic exploitation.

Beta-lactam antibiotics are the most widely prescribed antibiotic class worldwide. They include penicillins (amoxicillin, ampicillin), cephalosporins (cephalexin, ceftriaxone, cefepime), carbapenems (meropenem, imipenem), and monobactams (aztreonam). All share a four-membered beta-lactam ring that is essential for their mechanism of action. Beta-lactams work by binding to and inhibiting transpeptidases, also called penicillin-binding proteins (PBPs), the enzymes that catalyze the cross-linking step in peptidoglycan synthesis. Without cross-links, the cell wall weakens. As the bacterium grows and divides, the weakened wall cannot withstand osmotic pressure, and the cell lyses (bursts). Beta-lactams are bactericidal.

Glycopeptides like vancomycin use a different mechanism to inhibit cell wall synthesis. Instead of binding to the transpeptidase enzyme, vancomycin binds directly to the D-Ala-D-Ala dipeptide terminus of the peptidoglycan precursor, physically blocking the transpeptidase from accessing its substrate. Because vancomycin binds the substrate rather than the enzyme, it is effective against bacteria that have altered their PBPs to resist beta-lactams, making it critically important for treating methicillin-resistant Staphylococcus aureus (MRSA).

Bacitracin inhibits an earlier step in cell wall synthesis by blocking the recycling of the lipid carrier (bactoprenol) that transports peptidoglycan precursors across the cell membrane. It is too toxic for systemic use but is widely used as a topical antibiotic in wound care.

Protein Synthesis Inhibitors

Bacterial ribosomes (70S, composed of 30S and 50S subunits) differ structurally from human cytoplasmic ribosomes (80S, composed of 40S and 60S subunits). These differences allow antibiotics to bind bacterial ribosomes selectively and block protein synthesis without affecting human ribosomes. However, human mitochondrial ribosomes are 70S-like (reflecting their bacterial evolutionary origin), which explains some of the side effects of ribosome-targeting antibiotics, particularly with prolonged use.

Aminoglycosides (gentamicin, tobramycin, amikacin, streptomycin) bind to the 30S ribosomal subunit and cause misreading of the mRNA code. The resulting abnormal proteins insert into the cell membrane, disrupting its integrity and causing cell death. Aminoglycosides are bactericidal and are particularly effective against aerobic Gram-negative bacteria. Their major toxicities, ototoxicity (hearing damage) and nephrotoxicity (kidney damage), may relate in part to accumulation in the inner ear and kidney tubular cells, where they can affect mitochondrial ribosomes.

Tetracyclines (doxycycline, minocycline) also bind the 30S subunit but work by blocking the attachment of aminoacyl-tRNA to the ribosome's A site, preventing the addition of new amino acids to the growing polypeptide chain. They are bacteriostatic and have a broad spectrum of activity covering Gram-positive and Gram-negative bacteria, as well as atypical organisms like Chlamydia, Mycoplasma, and Rickettsia.

Macrolides (azithromycin, erythromycin, clarithromycin) bind to the 50S ribosomal subunit and block translocation, the step in which the ribosome moves along the mRNA after each amino acid is added. This halts protein chain elongation. Macrolides are generally bacteriostatic and are commonly used for respiratory tract infections, skin infections, and as alternatives for penicillin-allergic patients.

Chloramphenicol binds the 50S subunit and inhibits the peptidyl transferase reaction, the actual formation of the peptide bond between amino acids. It has excellent tissue penetration, including crossing the blood-brain barrier, making it valuable for bacterial meningitis in some settings. However, its use is limited by the risk of aplastic anemia (a rare but potentially fatal bone marrow failure), likely related to effects on mitochondrial protein synthesis in human bone marrow cells.

Linezolid (an oxazolidinone) binds the 50S subunit at a unique site and prevents the formation of the 70S initiation complex, blocking the very start of translation. It is active against many resistant Gram-positive organisms, including MRSA and vancomycin-resistant enterococci (VRE).

Nucleic Acid Synthesis Inhibitors

These antibiotics interfere with bacterial DNA replication, transcription (DNA to RNA), or the enzymes that manage DNA topology during these processes.

Fluoroquinolones (ciprofloxacin, levofloxacin, moxifloxacin) inhibit two essential bacterial enzymes: DNA gyrase and topoisomerase IV. DNA gyrase introduces negative supercoils ahead of the replication fork, relieving the torsional strain that would otherwise prevent the DNA strands from unwinding. Topoisomerase IV separates the linked daughter chromosomes after replication. By trapping these enzymes on the DNA in a cleaved complex, fluoroquinolones create double-strand DNA breaks that trigger cell death. They are bactericidal and have broad-spectrum activity. Human topoisomerases are sufficiently different in structure that fluoroquinolones bind them poorly at therapeutic concentrations.

Rifamycins (rifampin, rifabutin) inhibit bacterial DNA-dependent RNA polymerase, the enzyme that transcribes DNA into mRNA. Rifampin binds to the beta subunit of RNA polymerase and physically blocks the elongation of RNA chains beyond 2 to 3 nucleotides. It is bactericidal and is a cornerstone of tuberculosis treatment. Rifampin is also one of the most potent inducers of cytochrome P450 enzymes known, causing clinically significant drug interactions with many other medications.

Metronidazole is activated inside anaerobic bacteria and certain parasites by their unique electron transport proteins (ferredoxins). The activated form damages DNA through the formation of toxic free radicals. It is selectively toxic to anaerobes because aerobic cells lack the low-redox-potential electron carriers needed to activate the drug. Metronidazole is bactericidal and is the treatment of choice for infections caused by anaerobic bacteria and the parasite Giardia.

Folate Pathway Inhibitors

Bacteria must synthesize their own folic acid (vitamin B9) because they cannot absorb it from their environment. Human cells obtain folic acid from the diet and do not have the biosynthetic enzymes that bacteria use. This metabolic difference is exploited by two classes of antibiotics that block sequential steps in the folate synthesis pathway.

Sulfonamides (sulfamethoxazole, sulfadiazine) are structural analogs of para-aminobenzoic acid (PABA), a substrate in the first step of folate synthesis. They competitively inhibit dihydropteroate synthase, the enzyme that incorporates PABA into dihydrofolic acid. Trimethoprim inhibits the next step, blocking dihydrofolate reductase (DHFR), the enzyme that converts dihydrofolic acid to tetrahydrofolic acid. Tetrahydrofolic acid is essential for the synthesis of purines and thymidine, the building blocks of DNA. Without folate, bacteria cannot replicate their DNA and eventually die.

Sulfonamides and trimethoprim are often combined (as co-trimoxazole, or TMP-SMX) because they block sequential steps in the same pathway, producing synergistic bactericidal activity that is more effective than either drug alone. Trimethoprim has some selectivity for the bacterial DHFR over the human version of the enzyme, though at high doses or with prolonged use, folate deficiency can occur in patients (manifesting as megaloblastic anemia), especially if dietary folate intake is low.

Membrane-Disrupting Antibiotics

A small number of antibiotics work by directly disrupting the bacterial cell membrane, causing rapid loss of cellular contents and cell death.

Polymyxins (polymyxin B, colistin) are cationic polypeptides that bind to the lipopolysaccharide (LPS) in the outer membrane of Gram-negative bacteria. They insert into the membrane, disrupting its structure and increasing its permeability, which leads to leakage of intracellular contents and cell death. Polymyxins are bactericidal and are often considered "last resort" antibiotics for multidrug-resistant Gram-negative infections. Their clinical use is limited by nephrotoxicity and neurotoxicity, which result from their general membrane-disruptive properties affecting kidney and nerve cell membranes as well.

Daptomycin is a cyclic lipopeptide that inserts into the Gram-positive bacterial cell membrane in a calcium-dependent manner, forming ion-conducting channels that depolarize the membrane. Loss of membrane potential disrupts multiple essential cellular processes, including ATP synthesis and nutrient transport, leading to rapid cell death without cell lysis. Daptomycin is used for serious Gram-positive infections, including MRSA bacteremia and endocarditis.

Antibiotic Resistance: When Mechanisms Fail

Bacteria evolve resistance to antibiotics through several mechanisms: enzymatic inactivation of the drug (beta-lactamases destroy the beta-lactam ring), alteration of the drug target (modified PBPs in MRSA), reduced drug uptake (porin mutations that prevent drug entry), active efflux (pumps that expel the drug from the cell), and bypass of the inhibited pathway (acquiring a resistant version of the target enzyme). Resistance genes can spread between bacteria through horizontal gene transfer via plasmids, transposons, and bacteriophages, allowing resistance to disseminate rapidly through bacterial populations.

The global rise of antibiotic resistance is driven by selective pressure from antibiotic overuse and misuse. Each time an antibiotic is used, susceptible bacteria are killed while any bacteria carrying resistance genes survive and multiply. The World Health Organization considers antibiotic resistance one of the greatest threats to global health. Strategies to combat it include antibiotic stewardship programs (using antibiotics only when necessary and choosing narrow-spectrum agents when possible), development of new antibiotic classes, and alternative approaches such as bacteriophage therapy and antimicrobial peptides.