In evading phages, timing is everything. There are any number of ways for a bacterium to defend itself, but the benefits of evading an attack have to be weighed against the metabolic costs or readiness, so it makes sense to set up defenses only when they're needed. Since phages are thought to be more abundant and diverse in densely-populated environments with many different bacterial species, the authors reasoned that it would make sense for a bacterium to detect these conditions and turn on antiphage defenses on an as-needed basis.
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