The ecological and evolutionary roles of antibiotics have been usually inferred from their therapeutical activity. Since those compounds inhibit bacterial growth, it was thought that they should be produced by soil microorganisms to inhibit the growth of competitors in natural habitats. It has been shown however that antibiotics modulate transcription of bacteria in a dose dependent manner. Furthermore, each antibiotic triggers a specific response, and those responses may have adaptive values. From these observations, it has been suggested that antibiotics may have a role as signalling molecules besides inhibitors. The facts that a number of antimicrobials not used for therapy (like lantibiotics) are involved in quorum sensing and that classical quorum sensing autoinducers have antimicrobial activity further support this dual role for antibiotics.