A bacterium hit with a dose of ampicillin behaves like a runner pushed to the limit: first it gasps out acid, then falls out of rhythm. Scientists measured this breakdown with sensors thinner than a human hair: they detect pH changes right at the bacterial surface.

Ampicillin doesn't just break the bacterial wall. It sends metabolism into overdrive: the bacterium first pumps out organic acids, acidifying its surroundings, and a few minutes later starts releasing alkali. This reversal isn't random: it's a precise sign that metabolic pathways inside have gotten tangled. Formic acid matters most: the more of it released, the faster the bacterium dies.

The cause of death isn't oxidative stress, as previously thought, but an energy surge. After receiving the antibiotic, the bacterium produces more ATP: its fuel. This excess energy kills it, much like overheating kills an engine. Strains with the strongest ATP spike and the largest formic acid release died fastest. Now doctors could quickly test whether an antibiotic will work on a specific bacterium, simply by listening to its "breathing" with a sensor.