A taste of our own medicine

In football, one team's offense is pitted against the other team's defense, and vice versa. Much the same happens in humanity's battle against bacteria, except that some of our adversaries seem to have cloned our defense to fight our offense.

Bacteria that parasitize other organisms must first break down some of their host's tissues before setting up shop. The bacterial tools of choice are protein-slicers called proteases. Animals retaliate with protease inhibitors. One of the gene families responsible for making protease inhibitors is present in all animals, but absent from all plants, and so biologists have regarded it as an innovation that probably appeared at the dawn of animal evolution.

Imagine the surprise of Aidan Budd of the European Molecular Biology Laboratory in Heidelberg, Germany, when he and his colleagues discovered homologous genes in certain bacterial species. Because those bacteria generally invade the body and, like many bacteria, can collectivize some of their genetic resources, the investigators think that a forerunner of at least one such bacterial species once stole the gene from an animal (an extremely rare event) and shared it with other parasitic bacteria.

What good is the ill-gotten gene to the bacteria? All animal immune systems make their own offensive proteases; perhaps the thieves can now turn the tables and render animal proteases null and void. ("Bacterial [[alpha].sub.2]-macroglobulins: Colonization factors acquired by horizontal gene transfer from the metazoan genome?" Genome Biology 5:R38, May 26, 2004; genomebiology.com/2004/5/6/R38)

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