Antibiotic resistance is the ability of bacteria to withstand antibiotics that would otherwise inhibit their growth or kill them. It is a property of bacteria, not of the person receiving treatment. Resistance can make bacterial infections harder, and sometimes impossible, to treat with available medicines. It is a subset of antimicrobial resistance, a broader term covering resistance in bacteria, viruses, fungi, and parasites to their respective antimicrobial drugs. (cdc.gov)
Biological origins and selection
Resistance is part of bacterial evolution and predates modern antibiotic use. A study published in 2011 identified genes encoding resistance to several antibiotic classes in authenticated DNA from approximately 30,000-year-old permafrost sediments. Functional analysis of an ancient vancomycin-resistance element demonstrated similarity to modern variants. These findings establish that resistance mechanisms existed long before antibiotics became medical treatments. (doi.org)
Bacteria can acquire resistance through mutations or through the acquisition of genetic material from other bacteria. Antibiotic exposure creates conditions for natural selection: susceptible bacteria are inhibited or killed, while bacteria with effective resistance traits can survive and multiply. Antibiotic use therefore influences which bacterial variants become more abundant; it does not imply that bacteria consciously develop defenses. Resistant organisms and their genes can subsequently spread beyond the population originally exposed to the drug. (wwwnc.cdc.gov)
Mechanisms of resistance
Several mechanisms reduce an antibiotic’s ability to reach or affect its bacterial target:
- Drug inactivation: Bacterial enzymes destroy or chemically modify antibiotics. Carbapenemases, for example, break down carbapenems and many other beta-lactam drugs.
- Target alteration: Changes in the structures recognized by an antibiotic reduce its ability to bind and act.
- Reduced entry: Changes in bacterial surface barriers or entry channels limit drug penetration, particularly in bacteria with an outer membrane.
- Active removal: Efflux pumps expel antibiotics, lowering their concentration inside the bacterium.
- Pathway bypass: Alternative cellular processes allow bacteria to avoid dependence on a process blocked by the antibiotic. (cdc.gov)
These defenses can occur together. Consequently, resistance is not necessarily restricted to one drug, and bacteria can carry genes conferring resistance to several antibiotic types. A mechanism affecting multiple drugs can further complicate interpretation of the organism’s susceptibility profile. (cdc.gov)
Genetic exchange and transmission
Horizontal gene transfer allows resistance determinants to move between bacteria rather than only passing from parent to offspring. Important routes include conjugation, involving direct transfer between bacterial cells; transformation, involving uptake of extracellular DNA; and transduction, mediated by bacteriophages. Resistance genes may be carried on plasmids or associated with transposable elements, which help redistribute genetic material within and between bacterial genomes. (cdc.gov)
The spread of resistance therefore involves both transmission of resistant bacteria and movement of resistance genes. Resistant organisms circulate in health-care facilities and communities, including through contact between people, animals, and food. International travel can also carry resistant organisms between locations. Exposure to a resistant bacterium does not require previous antibiotic treatment in the exposed person. (cdc.gov)
Measurement and related phenomena
Laboratories use antimicrobial susceptibility testing to characterize bacterial responses to particular drugs. One central measurement is the minimum inhibitory concentration (MIC), the lowest concentration preventing visible growth under specified laboratory conditions. Clinical breakpoints interpret laboratory measurements using information about bacterial susceptibility, drug exposure, and treatment outcomes. A breakpoint is therefore not simply an unchanging biological boundary. (fda.gov)
Under the European Committee on Antimicrobial Susceptibility Testing framework, “susceptible, standard dosing regimen” and “susceptible, increased exposure” are both susceptible categories. “Resistant” indicates a high likelihood of therapeutic failure even with increased exposure. Epidemiological cutoffs serve a different purpose: they distinguish organisms without phenotypically detectable acquired resistance mechanisms from those outside the wild-type distribution, rather than directly predicting treatment success. (eucast.org)
Resistance is distinct from antibiotic tolerance and antibiotic persistence. Resistance involves growth at higher antibiotic concentrations. Tolerance involves slower killing without a corresponding increase in MIC; persistence describes a surviving subpopulation with prolonged tolerance. These distinctions matter because bacterial survival during exposure does not always demonstrate resistance. Biofilm formation can also contribute to prolonged infection and survival during treatment. (nature.com)
Clinical and public-health significance
Antibiotic resistance narrows treatment options for infectious diseases and increases risks associated with medical procedures dependent on effective infection treatment. The World Health Organization reported that bacterial antimicrobial resistance was associated with more than 4.7 million deaths globally in 2021. “Associated with” is not equivalent to deaths caused specifically by resistance: it includes deaths occurring with resistant infections. (who.int)
WHO’s 2024 bacterial priority pathogens list groups antibiotic-resistant pathogens into critical, high, and medium priorities for research and public-health interventions. It includes resistant Gram-negative bacteria, rifampicin-resistant Mycobacterium tuberculosis, and resistant forms of pathogens such as Staphylococcus aureus, Salmonella, and Neisseria gonorrhoeae. These categories guide priorities rather than describing every infection’s individual severity. (who.int)
Surveillance and institutional responses
Surveillance tracks resistance patterns and supports responses to their spread. Antibiotic stewardship consists of organized efforts to improve antibiotic use; institutional programs include leadership support, prescribing interventions, monitoring, and feedback. Other established responses include infection prevention, vaccination, laboratory capacity, and research into new medicines. Access to appropriate diagnostics and effective existing treatments is also important: WHO identifies inadequate access, alongside antimicrobial misuse and overuse, as contributing to the resistance problem. (cdc.gov)