An 85-year-old woman with dementia is admitted to hospital with worsening confusion, new urinary incontinence and constipation. These symptoms suggest a urinary tract infection, but the doctor treating her has a dilemma because the symptoms also suggest her dementia may be worsening or she has simple constipation.

Sending a sample to a lab for analysis could confirm bacteria in the urine, but getting a result takes days, so the doctor decides to play it safe and prescribe antibiotics.

Scenes like that are repeated every day in hospitals around the world, and they are leading to excessive and often unnecessary antibiotic use.

The inevitable consequence is the evolution of resistant bacterial strains. Bacterial genes undergo continual mutation. When a colony of bacteria is exposed to an antibiotic, a mutation may eventually occur in a single member of the colony, making it immune to the antibiotic.