Bacteria become resistant when genetic changes or genes acquired from other bacteria give them ways to survive an antibiotic. The drug then selects for those bacteria: susceptible ones are more likely to die, while resistant survivors can multiply. Resistant bacteria and resistance genes can both spread, by different routes.
How antibiotic exposure selects for resistant bacteria
A bacterial population is not always genetically identical. Some bacteria may already carry a resistance trait because of a genetic change or a resistance gene they acquired. When an antibiotic is used, susceptible bacteria may be killed or held back while bacteria with a trait that helps them survive are more likely to remain and reproduce. Their descendants can inherit that trait.
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The antibiotic does not teach an individual bacterium how to resist it or make a deliberate adaptation happen on demand. Rather, exposure changes which bacteria survive and reproduce. Resistance can also arise or enter a population while no antibiotic is being used; antibiotic exposure can then favor bacteria carrying it.
The Centers for Disease Control and Prevention (CDC) defines the broader phenomenon this way: “Antimicrobial resistance happens when germs like bacteria and fungi can defeat the drugs designed to kill them.” This article focuses on bacteria and antibiotics; antimicrobial resistance also includes other organisms and medicines.
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How bacteria resist antibiotics
Resistance may originate in a genetic change or in a resistance gene acquired from another bacterium. The specific defense depends on the bacterium and the antibiotic; a resistant bacterium does not necessarily use every mechanism.
| Resistance strategy | What it does |
|---|---|
| Reduce entry | Limits how much antibiotic gets into the bacterial cell, making it harder for the drug to reach its target. |
| Pump the drug out | Uses efflux pumps to remove antibiotic from the cell before enough can act. |
| Change the target | Alters the part of the bacterium that the antibiotic must bind to, reducing the drug’s ability to act. |
| Inactivate or alter the drug | Produces a substance that destroys or modifies the antibiotic so it no longer works effectively. |
These defenses can prevent a drug from reaching its target, stop it binding effectively, or render it inactive. The resistance strategy involved varies by organism and medicine.
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How resistance genes move between bacteria
A bacterium can pass a resistance trait to its descendants as it reproduces. Resistance genes can also move horizontally between bacteria, including bacteria that have not themselves been exposed to the antibiotic. This means a trait can cross bacterial lineages rather than spreading only through reproduction within one lineage.
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- Conjugation: bacteria transfer DNA, often on plasmids, from one cell to another.
- Transformation: a bacterium takes up DNA from its surroundings.
- Transduction: a bacteriophage—a virus that infects bacteria—carries DNA between bacterial cells.
Gene transfer can occur between strains and, in some cases, between different bacterial species or genera. Whether a particular gene moves, and which bacteria can receive it, depends on the organisms and circumstances.
How resistant bacteria spread
Gene transfer is not the only route. A resistant bacterium can move from one host or setting to another, carrying its resistance traits with it. CDC describes spread through everyday contact and healthcare settings, as well as routes involving animals, food, and the environment. The World Health Organization (WHO) also describes connected pathways among people, animals, food, plants, and environmental settings such as water, soil, and air. This is why resistance is not solely a hospital problem.
In healthcare settings, hands or contaminated surfaces can contribute to transmission; procedures and medical devices can also be routes. Patients carrying resistant bacteria may move between facilities. CDC provides further detail on how antimicrobial resistance spreads in healthcare settings.
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These two kinds of spread can reinforce one another: a resistant bacterium can travel to a new person or setting, while its resistance gene can also move into other bacteria. A newly exposed bacterium does not have to encounter the antibiotic first to acquire a resistance gene.
What the burden estimates say—and what they do not
CDC’s January 31, 2025, “About Antimicrobial Resistance” page cites global estimates for 2019 of at least 1.27 million deaths caused by bacterial antimicrobial resistance and nearly 5 million deaths associated with it. “Caused by” and “associated with” describe different estimates; the larger figure should not be read as deaths caused solely by resistance.
The same CDC page reports more than 2.8 million antimicrobial-resistant infections and more than 35,000 resulting deaths annually in the United States, attributing those figures to CDC’s 2019 Antibiotic Resistance Threats Report. They are not estimates for 2025 or 2026.
What helps slow resistance and its spread
No single action can guarantee resistance will not emerge or spread. CDC and WHO describe a combined approach: reduce infections, use antibiotics appropriately, and prevent resistant bacteria from passing between people and across connected settings.
Quick Recap
- Prevent infections: hand hygiene, routine vaccination, safer food preparation, and safe sex practices can reduce opportunities for infections—and therefore the need for antibiotics.
- Use antibiotics appropriately: prescribing and use should follow qualified clinical judgment and current local guidance. For an individual illness or prescription, ask a qualified healthcare professional; do not start, stop, share, or save antibiotics based on a general explainer.
- Strengthen infection prevention and control: healthcare facilities and communities can reduce transmission through hygiene and other appropriate infection-control measures.
- Address conditions beyond individual care: WHO identifies inappropriate antimicrobial use, inadequate water, sanitation and hygiene, and poor infection prevention and control as drivers of emergence and spread. Prevention therefore also involves clinical practice, agriculture, public health, and environmental conditions.
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