Many commensal bacteria produce antibiotics to gain the upper hand over their competitors in a shared environment. Lian-Hui Zhang and colleagues now reveal a new mechanism for this, in which some bacteria inhibit quorum-sensing signalling by other species to reduce their virulence.

Quorum sensing using N-acyl homoserine lactones (AHLs) is widely used in communication between related bacteria to coordinate gene expression. Other species produce AHL-degrading enzymes, indicating that these could be used to compete against bacteria that rely on AHL signalling. However, until now, no evidence for this intriguing theory has been found.

Bacillus thuringiensis, which is widely used in the biocontrol of insect pests, expresses the AHL-degrading enzyme AHL-lactonase. Zhang and colleagues tested whether this species could interfere with the growth of the plant pathogen Erwinia carotovora, which signals using AHLs. Co-culture of the two species blocked the accumulation of E. carotovora AHLs, although this had no effect on the growth of the pathogen. To test whether inhibition of AHL signalling affected E. carotovora pathogenesis, potato slices were co-inoculated with both species. The soft-rot symptoms caused by E. carotovora were strongly reduced, indicating that B. thuringiensis suppresses the virulence of the pathogen.

To confirm this, the authors inoculated potato slices with E. carotovora transformed with a plasmid expressing green fluorescent protein (GFP). When slices were pre-treated with a B. thuringiensis suspension, the number of GFP-expressing bacteria was only slightly reduced in comparison with control slices that were pre-treated with water only. However, a more marked effect was seen on virulence. E. carotovora spread substantially on control slices and caused soft-rot symptoms. But, after pre-treatment with B. thuringiensis, the pathogen was confined to the site of inoculation and failed to cause infection.

To verify that AHL-lactonase is required for the suppression of E. carotovora virulence, Zhang and co-workers made B. thuringiensis mutants that do not produce this enzyme. These bacteria were unable to prevent the soft-rot symptoms caused by E. carotovora.

This study demonstrates a novel mechanism for antagonistic bacterial interactions, which the authors term 'signal interference'. It also has exciting implications for biocontrol, as B. thuringiensis and other species that express AHL-degrading enzymes might well prove useful in the control of bacterial plant pathogens.