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infection, even when challenged with comparatively low numbers of bacteria. The result is a triumph for the approach of mining bacterial metabolic diversity for applied uses. In this case, Dong et al. capitalized on the discovery that many bacteria can degrade, and so inactivate, AHLs. Earlier, the same group 5 had isolated hundreds of bacterial species from soil and screened them for the ability to degrade AHLs. From this collection, a bacterium of the genus Bacillus was identified as being especially promising. It was from this strain that aiiA was cloned. As well as their exciting observations of plants expressing this gene, the authors 1 also show how the AiiA protein inactivates AHL signals: it degrades the molecule's lactone ring, so acting as an AHL-lactonase (Fig. 1). Another soil bacterium has been identified that also degrades these signals, through an uncharacterized AHL-acylase activity 6. Clearly, there are different mechanisms for the biodegradation of AHLs, inviting comparisons to the way in which penicillins can be inactivated. Enzymes that degrade AHLs might be useful in controlling many other quorum-sensing bacterial populations, such as biofilms. These are dense populations of bacteria, attached to some surface, that grow to form complex, tissue-like structures; they are extremely resistant to antibiotics and have been implicated in many human diseases 7. For example, the formation of biofilms by Pseudomonas aeruginosa is common in the lungs of people with cystic fibro-sis. Here, quorum sensing regulates the expression of virulence factors and the development of biofilms 8. As aerosols of mucous-degrading enzymes and other chemicals are already used to treat lung disease in cystic fibrosis patients, it might prove fruitful to add AHL-degrading enzymes to the mix. But in the rush to develop products based on AHL-lactonases that will no doubt come, microbiologists should not lose sight of the significance of AHL-degrading enzymes to the physiology and ecology of the bacteria that produce them. There are questions to be answered, such as: do these enzymes have narrow or broad substrate specificity? And how are they regulated in the cell? Moreover, what is the effect of signal degradation on the cells involved, and in the environment at large? No doubt bacteria have a stake in disarming the quorum-sensing systems of other species. For example, Erwinia regulates its synthesis of carbapenem, an antibacterial compound, through AHL-dependent mechanisms. Presumably, carba-penem keeps other bacteria away from the nutrient cache accumulated during plant infection. …