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A pot of water sits on the stove. The temperature drops: plus one degree, zero, minus one — the water stays liquid. But at some point, a light breath is enough to make the whole mass instantly freeze into ice. Physicists call such jumps first-order phase transitions. Similar switches happen in living communities — only the role of temperature is played by other forces.

In the new work, scientists rigorously showed that microbial populations undergo genuine first-order phase transitions. As an example, they took bacteria carrying plasmids — small circular DNA molecules that float inside the cell separately from the main chromosome. A plasmid can confer antibiotic resistance or the ability to feed on a new substance, but copying and maintaining it costs the bacterium resources. If the benefit doesn't outweigh the cost, the plasmid becomes a burden.

The authors derived a strict lower bound: if the average number of plasmids per cell drops below a certain threshold, the entire population loses them in a jump. Above the threshold, plasmids persist; below it, they vanish almost instantly. No smooth transition. Such an abrupt breakdown explains why experiments with microbes often show an "all-or-nothing" pattern: either everyone has the trait, or no one does.

The link to cell lineages adds depth. Each bacterium divides, and its descendants inherit the plasmids. If, due to random fluctuations, a branch's plasmid count falls below the threshold, that branch is doomed, and soon the whole population follows. Thus individual histories combine into a collective tipping point. The practical takeaway is simple: biotechnologists who insert useful genes into bacteria can calculate in advance how many copies to maintain so that the culture doesn't "freeze" into an undesirable state.