One genetic transformation turns friendly bacteria into assassins

By Tina Hesman Saey

Web edition: July 6, 2012
Print edition: July 28, 2012; Vol.182 #2 (p. 18)

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In nematode worms, friendly colonies of bacteria (green) can turn into toxic killers (red) by the flip of a genetic switch.

Credit: Courtesy of Alexander Martin

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.

The transformation between mild-mannered and killer forms depends on the orientation of a piece of DNA called the madswitch promoter, Harvard University researchers report in the July 6 Science. In the “on” direction, madswitch turns on genes needed for the bacteria to live inside worms and damps down production of substances that help kill insects. Bacteria with the promoter in the “off” direction are the killer type.