Glow-in-the-dark bacteria living in nematode worms flip a genetic switch to change from peaceful cohabitants into killers.
The M-form (M for mutualism) of Photorhabdus luminescens bacteria make friendly colonies inside nematodes. But the microbes switch to the deadly toxin-producing P-form (P for pathogenic) when their hosts are ready to eat an insect from the inside out. Worms vomit up the bacteria into insects, and the bacterial toxins kill and help digest the feast.
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