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In 1928, Alexander Fleming returned from holiday to find that a stray mould had contaminated a Petri dish in his London laboratory, killing the bacteria around it. That chance observation, which led to the isolation of penicillin, inaugurated the antibiotic era and transformed medicine. Routine surgery, childbirth and cancer chemotherapy, all of which carry a high risk of bacterial infection, became vastly safer. Yet from the very beginning, Fleming himself warned that the miracle might be temporary. In his 1945 Nobel lecture, he cautioned that exposing microbes to doses too low to kill them could educate them to resist, a prediction that has proved grimly accurate.
Antibiotic resistance is not a human invention but an ancient evolutionary strategy. Bacteria have waged chemical warfare against one another for billions of years, and genes conferring resistance predate the clinical use of antibiotics by millennia. What medicine has done is to accelerate natural selection on a global scale. When a population of bacteria encounters an antibiotic, the susceptible cells die while the few that carry protective mutations survive and multiply. Because bacteria reproduce in minutes and can swap resistance genes across species through a process called horizontal gene transfer, a successful defence can spread through a hospital, a city or a continent with astonishing speed.
The principal driver of this acceleration is misuse. Surveys suggest that a substantial share of antibiotic prescriptions written in outpatient clinics are unnecessary, typically issued for viral illnesses such as colds, against which antibiotics are powerless. In many countries the drugs are sold over the counter without any prescription at all. Agriculture compounds the problem: in some regions, more antibiotics by weight are administered to healthy livestock, to promote growth and pre-empt disease in crowded conditions, than are used to treat sick humans. Resistant strains that emerge in animals can reach people through food, water and farm workers.
The consequences are no longer hypothetical. A landmark analysis published in 2022 estimated that bacterial antimicrobial resistance was directly responsible for 1.27 million deaths in 2019, more than either HIV or malaria, and was associated with nearly five million additional deaths. The World Health Organization now ranks resistance among the top ten threats to global public health. Infections that were once trivial, including common urinary tract infections and some forms of pneumonia, are becoming difficult or, in a small but growing number of cases, impossible to treat with existing drugs, and the routine procedures of modern medicine are quietly accumulating risk.
Paradoxically, the scientific pipeline that should rescue us is running dry, largely for commercial reasons. Antibiotics are taken briefly, cure cheaply and are deliberately held in reserve, which makes them unattractive investments compared with drugs for chronic conditions that patients take for life. Between 2017 and 2021, only a handful of genuinely novel antibiotics reached the market, and several small firms that did win approval promptly went bankrupt. Proposed remedies include so-called pull incentives, in which governments guarantee a reward for a successful drug regardless of sales, decoupling profit from the volume of pills sold.
In the meantime, researchers are exploring alternatives. Phage therapy, which deploys viruses that infect and destroy specific bacteria, has saved patients with otherwise untreatable infections, though each treatment must be tailored almost individually. Genome editing tools are being adapted to snip resistance genes out of bacterial populations, and monoclonal antibodies may one day replace some broad-spectrum drugs. Vaccines, too, have an underappreciated role: by preventing bacterial and viral illness in the first place, they reduce both the need for antibiotics and the occasions on which resistance can evolve.
Most experts argue that no single fix will suffice. Because resistance flows between people, animals and the environment, they advocate a One Health approach that coordinates surveillance and policy across all three domains. Hospital stewardship programmes, which audit prescribing and restrict the most powerful drugs, have measurably cut resistant infections where they are rigorously enforced. Diagnostic tests that distinguish bacterial from viral illness within an hour could remove much of the guesswork that leads to needless prescriptions, and several such tests are already nearing clinical use.
Whether these measures arrive quickly enough remains an open question. Resistance is a problem of cumulative neglect, and its solutions are equally cumulative: thousands of small decisions by doctors, farmers, regulators and patients. The lesson of the antibiotic era may ultimately be that drugs, like ecosystems, are a shared resource that can be exhausted. How societies learn to govern that resource will determine whether the twenty-first century keeps the inheritance Fleming stumbled upon, or squanders it.