Bacteria communicate by means of chemical signal molecules called autoinducers. This process, called quorum sensing, allows bacteria to count the members in the community and to alter gene expression synchronously across the population. Quorum-sensing-controlled processes are often crucial for successful bacterial--host relationships--both symbiotic and pathogenic. Most quorum-sensing autoinducers promote intraspecies communication, but one autoinducer, called AI-2, is produced and detected by a wide variety of bacteria and is proposed to allow interspecies communication. Here we show that some species of bacteria can manipulate AI-2 signalling and interfere with other species' ability to assess and respond correctly to changes in cell population density. AI-2 signalling, and the interference with it, could have important ramifications for eukaryotes in the maintenance of normal microflora and in protection from pathogenic bacteria.