Evolution at its fastest
Bacteria reproduce in minutes. A single E. coli cell can divide every 20 minutes, producing a billion descendants in under 10 hours — each one a potential evolutionary experiment. Mutations arise at a rate of roughly one per billion base pairs per replication, and with billions of cells dividing constantly, every possible single-point mutation arises somewhere in the population every few days.
When an antibiotic is introduced, it acts as a powerful selective pressure: bacteria without resistance die; any that happen to carry a resistance mutation survive and reproduce. This is natural selection in real time — identical in mechanism to how finch beaks changed in the Galápagos, but operating on a timescale of days rather than millennia.
Bacteria also transfer resistance genes horizontally — between cells that are not parent and offspring — via plasmids, bacteriophages and direct cell-to-cell contact. This allows a resistance trait that evolved in one species to spread rapidly across an entire microbial community.
How resistance works biochemically
Resistance is not one mechanism but many:
- Drug inactivation — bacteria produce enzymes that destroy the antibiotic. Beta-lactamases break the beta-lactam ring of penicillins and cephalosporins, rendering them harmless.
- Target modification — bacteria mutate the protein the antibiotic binds to. MRSA produces an altered penicillin-binding protein (PBP2a) that builds the cell wall normally but has low affinity for beta-lactam drugs.
- Efflux pumps — membrane proteins that actively expel antibiotics before they reach their target. Efflux pumps can confer low-level resistance to multiple drug classes simultaneously.
- Reduced permeability — bacteria reduce the number or size of membrane channels (porins) through which the antibiotic must enter.
For background on how enzymes work as catalysts and how their inhibition is clinically significant, see our enzymes guide.
The pipeline problem
Since the 1980s, no truly new class of antibiotic has reached widespread clinical use. Drug development is expensive, resistant strains emerge before developers can recoup costs, and regulatory pathways have been slow to adapt. The WHO classifies 13 pathogen families as priority threats requiring urgent new treatments.
Several approaches are under active development:
- Phage therapy — bacteriophages (viruses that infect only bacteria) can be engineered to target specific strains. Clinical trials are running in Georgia (the country), Belgium and the US.
- CRISPR antimicrobials — CRISPR-Cas systems are being engineered to cut bacterial chromosomes selectively. See our CRISPR gene editing article for how the technology works.
- Deep-sea natural products — extreme environments are a rich source of novel compounds. The antibiotic teixobactin was discovered in soil bacteria that could not previously be cultured. See our deep-sea discovery article on analogous organisms.
- Antivirulence drugs — rather than killing bacteria, these block the toxins or attachment proteins they use to cause disease, applying lower selective pressure.
What individuals and institutions can do
The WHO's World Antimicrobial Awareness Week (WAAW) runs every November and focuses on reducing unnecessary antibiotic use. Key actions include:
- Complete every prescribed antibiotic course — stopping early leaves the most resistant survivors alive.
- Never demand antibiotics for viral infections (colds, flu, most sore throats).
- Support campaigns to reduce agricultural antibiotic use.
- Promote vaccination, which reduces demand for antibiotics by preventing bacterial complications of viral disease.
The evolutionary principles behind resistance are identical to those driving all of life's diversity. Our evolution guide covers the mechanisms of natural selection and genetic drift that underlie this public health crisis.