How bacteria talk to each other

The wrong mental model

It is easy to picture bacteria as mindless specks, each one doing its own thing until it divides. On that picture, a bacterium either has a behaviour or it does not, and the number of neighbours around it makes no difference to anything.

That is not what happens. A bacterium can tell roughly how many of its own kind are nearby, and it withholds its most expensive behaviours until that number is high. Glowing, attacking a host, building a sticky film: these are held back while the cell is outnumbered, and switched on together once the crowd arrives.

How the sensing actually works

Every cell leaks the same small chemical, called a signal molecule (an autoinducer, in the literature), into its surroundings, and every cell has a receptor that senses how much of it is there. One bacterium alone in open water makes almost no difference to the concentration, because the molecule diffuses away as fast as it is produced. Pack thousands of cells into a small space and their molecules accumulate faster than they escape, so the level climbs.

The switch is sharp because the loop feeds itself. When the molecule binds its receptor, the receptor switches on a set of genes, and one of those genes makes more of the same molecule. Past the threshold, the concentration races upward, and the population flips more or less simultaneously rather than one cell at a time. That is why a culture goes from dark to bright within hours instead of brightening gradually.

It also means the cells never sense the crowd directly. They sense how much signal molecule is around them, and that amount stands in for a crowd. Destroy the molecule as fast as it is made and an enormous population will behave like a lonely one. That gap between the crowd and the reading is exactly what researchers are trying to exploit with drugs that jam the signal molecule rather than kill the bacterium.

A sea the size of Iceland, glowing at once

Sailors reported milky seas for centuries: a pale, steady glow to the horizon that lasts all night. In 2019 a satellite low-light sensor caught one south of Java spanning roughly 100,000 square kilometres, about the size of Iceland, visible across two moonless stretches between late July and early September. The brightness implies somewhere around a hundred billion trillion luminous bacteria. In 2022 a crew who had sailed through the patch published what they had seen from the deck, confirming the satellite detection.

That glow is a direct readout of density, because luminous marine bacteria emit no light at all below about 100 million cells per millilitre. A sea that stays evenly lit for weeks is a threshold that stays crossed over an area the size of a country.

The same reading, wired backwards

Sensing the crowd is one mechanism; what a species does with the answer is a separate question. Cholera senses it the same way and does the opposite with it. Zhu, Bassler, Mekalanos and colleagues showed in 2002 that a crowded population of Vibrio cholerae switches on a protein called HapR, and HapR presses down the genes for its toxin and for the sticky film that holds the cells to the gut wall.

So cholera attacks while it is still sparse, and once it is crowded it goes quiet and lets go, riding the fluid its toxin caused out to the next person. That also means jamming the signal molecule is no universal fix. In luminous bacteria, and in the lung pathogen Pseudomonas, blocking it would switch expensive behaviours off. In cholera it would leave the attack genes on. The sensing is the same everywhere; the wiring is species by species.