Antibiotic Resistance: How Bacteria Stop Responding to Treatment

Antibiotics are sometimes withheld for infections that are unlikely to need them. This lesson explains the biology behind that decision: what antibiotic resistance actually is, how it develops, and why it turns a normally treatable infection into a much harder disease course problem.

Bacteria become resistant, not people Antibiotic resistance means that bacteria are not adequately inhibited or killed by an antibiotic that would usually affect them. Your body does not become resistant to an antibiotic. A person may instead carry resistant bacteria without symptoms or develop an infection caused by them. Resistance usually relates to particular antibiotics or antibiotic groups. A bacterium can resist one medicine while remaining susceptible to another. Antibiotic resistance is one part of antimicrobial resistance. The wider term also includes resistance involving viruses, fungi and parasites. Resistance can be intrinsic or acquired Intrinsic resistance is a natural property of a bacterial species. The bacterium may lack the antibiotic target or prevent the medicine reaching it effectively. Acquired resistance develops when previously susceptible bacteria gain a resistance trait. This can happen through mutation or through receiving resistance genes from other bacteria. A mutation is a change in bacterial genetic material. Some mutations alter the target, entry or handling of an antibiotic. Horizontal gene transfer means that bacteria exchange genetic material outside ordinary reproduction. Mobile genetic material can sometimes carry resistance to several antibiotics. The antibiotic does not teach bacteria or make them decide to adapt. Genetic variation may already exist or arise during bacterial reproduction. Treatment then changes which bacteria survive and multiply. Antibiotic exposure creates selection pressure Selection pressure occurs when antibiotic exposure disadvantages susceptible bacteria more than resistant bacteria. Susceptible bacteria may die or stop multiplying. Resistant bacteria can then survive, reproduce and become a larger share of the population. Antibiotics can affect bacteria causing the infection and other bacteria living in the body. This wider effect can select resistant bacteria that were not causing the original illness. Necessary treatment can still create selection pressure, but it also provides an expected clinical benefit. Unnecessary use adds avoidable pressure without that benefit. Using an antibiotic for longer than needed can also increase exposure and adverse effects. Prescribers should therefore use the shortest effective course for the particular infection. Bacteria use several resistance mechanisms A resistance mechanism is the biological method that reduces an antibiotic's effect. Some bacteria produce enzymes that destroy or change the antibiotic. Others alter the structure that the medicine normally targets. Bacteria can reduce drug entry or pump the antibiotic back out. They may also use a different biological pathway that bypasses the blocked step. Different mechanisms can occur together. This can produce resistance to several antibiotics without making the bacterium resistant to every possible treatment. Resistance does not automatically make a bacterium more aggressive. The clinical danger often comes from delayed effective treatment or fewer safe treatment options. Resistant bacteria and genes can spread Resistant bacteria can spread between people. Some can spread while causing infection, while others are carried through colonisation without symptoms. Resistance genes can also move between bacteria. People, animals, food, water and the environment are connected parts of this wider problem. Infection prevention, sanitation, vaccination and appropriate antibiotic use can reduce infections and the need for antibiotics. They can also limit opportunities for resistant bacteria to spread. A laboratory result needs clinical interpretation Culture grows bacteria from a suitable sample so the laboratory can identify them. Not every infection needs or allows a useful culture. Culture and susceptibility testing estimates which antibiotics are likely to inhibit the cultured bacterium under laboratory conditions. A resistant result means that standard treatment exposure is unlikely to work reliably against that isolate. It does not mean that every antibiotic will fail. A susceptible result supports the likely effectiveness of an antibiotic. It cannot guarantee recovery because the body site, dose, absorption and source control also matter. The result can also represent colonisation or contamination rather than the cause of illness. Clinicians interpret it with the sample site, symptoms and examination findings. Choosing an antibiotic safely Antimicrobial stewardship means using antimicrobial medicines carefully to achieve the best outcome while preserving future effectiveness. The decision includes whether an antibiotic is needed, which medicine fits the likely bacteria and which dose, route and duration are appropriate. Local guidance uses current resistance data to support initial treatment before the exact organism is known. Previous cultures and recent antibiotic use can also change the choice. A narrow spectrum antibiotic targets a smaller range of bacteria. It is preferred when it is clinically suitable and likely to treat the infection. A broad spectrum antibiotic affects a wider range of bacteria. It may be necessary for a serious infection when treatment cannot safely wait for laboratory results. When results become available, the clinician reviews whether treatment should continue, change or narrow. The person's response and possible alternative diagnoses remain important. Take antibiotics exactly as directed Take an antibiotic exactly as directed by the prescriber or pharmacist. Do not shorten, extend or repeat the course without professional advice. Do not save antibiotics for another illness or share them with anyone. The medicine, dose and duration may be wrong for the next infection. Return unused or expired antibiotics to a pharmacy for safe disposal. Do not flush them away or place them in household waste. Failure to improve does not prove resistance. The diagnosis may be wrong, a complication may have developed or drainage and another procedure may be needed. A kidney infection example Acute pyelonephritis is a bacterial infection of one or both kidneys. NICE recommends obtaining a urine sample before antibiotics and sending it for culture and susceptibility testing. Treatment usually begins before the result because delaying effective therapy can be unsafe. The choice considers severity, previous results, recent antibiotics and local resistance data. When the result returns, the clinician reviews the antibiotic. They change it when resistance makes the current treatment unsuitable and use a narrow spectrum option where possible. Seek urgent medical advice if symptoms worsen or do not start improving within 48 hours. Reassessment considers resistance, sepsis, complications and other possible diagnoses. Call 999 or go to A&E if someone with suspected infection is confused, has slurred speech or breathes very fast. Emergency help is also needed for blue, pale or blotchy skin, collapse, or a rash that does not fade when pressed. This lesson explains antibiotic resistance and stewardship. It cannot diagnose a bacterial infection, interpret your laboratory result or select an antibiotic for you.

Antibiotics select among bacteria that differ in susceptibility. Resistance may be intrinsic or acquired through mutation or gene transfer. Safe prescribing balances prompt effective treatment with the narrowest suitable medicine and shortest effective duration.