How antibiotics work and why resistance spreads

Penicillin worked because Alexander Fleming's mould was producing a compound that broke something bacteria need and human cells do not have at all. That principle — selective toxicity — is the whole basis of antibiotic therapy, and also the reason the drug cabinet is smaller than we would like. There are only so many things a bacterium does differently from us.

Selective toxicity: attacking what we do not have

Bacteria are prokaryotes. They lack a nucleus, their ribosomes are built differently from ours, and most are wrapped in a rigid cell wall made of peptidoglycan, a molecule with no counterpart in human cells. Every useful antibiotic targets one of these differences. The bigger the difference, the safer the drug — which is why cell-wall antibiotics are generally well tolerated, and why drugs that act on targets closer to our own biology carry more toxicity.

This also explains a fact that costs lives every winter: antibiotics do nothing against viruses. A virus has no cell wall, no ribosomes of its own and no bacterial metabolism to interfere with. Taking an antibiotic for a cold treats nothing and selects for resistance in the bacteria a person happens to be carrying.

The four main mechanisms

Cell wall synthesis inhibitors. The beta-lactams — penicillins, cephalosporins, carbapenems — block the enzymes that cross-link peptidoglycan strands. The wall cannot be completed, and internal osmotic pressure bursts the cell. Vancomycin achieves the same end by binding the wall building blocks directly. These drugs kill only growing bacteria, since a cell that is not building wall has nothing to disrupt.

Protein synthesis inhibitors. Bacterial ribosomes are 70S, made of 30S and 50S subunits; human ribosomes are 80S. Drugs exploit the difference: tetracyclines and aminoglycosides bind the 30S subunit, while macrolides such as azithromycin, and clindamycin and chloramphenicol, bind the 50S. Aminoglycosides also cause misreading of the genetic code, producing nonsense proteins that damage the membrane.

Nucleic acid inhibitors. Fluoroquinolones such as ciprofloxacin block DNA gyrase and topoisomerase IV, enzymes bacteria use to unwind and re-coil their chromosome during replication. Rifampicin blocks bacterial RNA polymerase, and is a mainstay of tuberculosis treatment.

Metabolic pathway inhibitors. Bacteria must synthesise their own folate; we get ours from food. Sulfonamides and trimethoprim block two consecutive steps in that synthesis, which is why they are given together — blocking a pathway twice is far harder to escape than blocking it once.

Drugs are also described as bactericidal (they kill) or bacteriostatic (they halt growth and leave the immune system to finish the job). The distinction matters most in patients whose immune defence is weak, or in infections such as endocarditis where the drug must do the killing itself.

A course of antibiotics is not a war against one bacterium. It is a selection pressure applied to every bacterium in the body, including the harmless ones that will pass on what they learn.

Where resistance comes from

Resistance is not something bacteria decide to develop. It is ordinary evolution running at extraordinary speed, because a bacterial generation can be twenty minutes long and a single infection can contain billions of cells.

Bacteria defend themselves in a handful of ways:

• Enzymatic destruction — beta-lactamases cut the beta-lactam ring open before it reaches its target. Extended-spectrum beta-lactamases and carbapenemases such as NDM-1 are the reason once-reliable drugs now fail.
• Target modification — a small change to the binding site leaves the enzyme working but the drug unable to grip. MRSA does exactly this, producing an altered penicillin-binding protein.
• Efflux pumps — membrane proteins that push the drug back out faster than it accumulates. Many pump several drug classes, giving resistance to antibiotics the organism has never met.
• Reduced permeability — losing or narrowing the porin channels through which the drug enters.

What makes resistance spread rather than merely appear is horizontal gene transfer. Bacteria exchange DNA directly: by conjugation, passing plasmids that often carry several resistance genes at once; by transformation, taking up free DNA from the environment; and by transduction, via bacteriophages. A resistance gene that evolves in a harmless gut organism can therefore end up in a pathogen — and it can cross between species, which ordinary inheritance never allows.

Every antibiotic exposure selects for whichever cells survive it. Incomplete courses, unnecessary prescriptions for viral illness, over-the-counter sales without prescription, and the routine use of antibiotics as growth promoters in livestock all apply that pressure at scale. India carries a particularly heavy burden here, and the rise of carbapenem-resistant organisms in hospitals is the visible result.

Antibiotic stewardship is the response: culture and sensitivity testing before prescribing where possible, narrow-spectrum agents in preference to broad, correct dose and duration, and infection control to stop resistant strains moving between patients. The pipeline of genuinely new antibiotic classes has been thin for decades, so preserving the drugs that still work is not a supplement to developing new ones — for now it is the main strategy.

Why it matters for students and researchers

Antimicrobial resistance is on every serious list of long-term threats to global health, and it sits at the intersection of pharmacology, microbiology, genetics, clinical medicine and public policy. For pharmacy and life-science students the subject is unusually practical: it links molecular mechanism directly to prescribing decisions, and it explains why a dosing schedule is a clinical intervention rather than an administrative detail. Research is active on new drug classes and targets, bacteriophage therapy, antimicrobial peptides, resistance-breaker combinations, and rapid diagnostics that could replace empirical prescribing with informed prescribing. Following the peer-reviewed literature is how students and practitioners track resistance patterns and treatment guidelines that change faster than any textbook edition.

Frequently asked questions

How do antibiotics kill bacteria?

They attack structures or processes that bacteria have and human cells do not — the peptidoglycan cell wall, the 70S ribosome, bacterial DNA gyrase, or folate synthesis. Disrupting any of these either kills the cell outright or stops it multiplying so the immune system can clear the infection.

Why do antibiotics not work on viral infections?

Viruses have no cell wall, no ribosomes of their own and no independent metabolism, so there is nothing for an antibiotic to act on. Colds, flu and most sore throats are viral, and treating them with antibiotics gives no benefit while still selecting for resistance.

What causes antibiotic resistance?

Resistance arises from random mutation and is then spread by horizontal gene transfer between bacteria, including across species. Antibiotic use selects for the cells that already carry it — so unnecessary prescriptions, incomplete courses, unregulated over-the-counter sales and agricultural use all accelerate the process.

Why must a full course of antibiotics be completed as prescribed?

Stopping early can leave behind the least susceptible organisms — precisely the ones most likely to carry partial resistance — allowing them to regrow and be transmitted. Course length should follow current clinical guidance for the specific infection, which is why the instruction is to complete the course as prescribed rather than to stop when symptoms improve.