Contact-dependent growth inhibition (CDI) systems have been implicated in the killing of susceptible competing bacteria on cell–cell contact. In Burkholderia spp., this system involves two-partner secretion proteins that are encoded by the bcpAIOP locus: a bacterial cell delivers a toxic effector domain that is derived from the carboxyl terminus of the exoprotein BcpA (BcpA-CT) to a neighbouring cell, which is protected from CDI if it produces the immunity protein, BcpI. Garcia et al. show that CDI systems also have a role in cell-to-cell communication and cooperation between bacteria that produce the toxin–antidote protein pair. The delivery of catalytically active BcpA-CT results in changes in gene expression in immune recipient cells that affect community behaviour, such as the formation of biofilms. The authors termed this form of interbacterial communication contact-dependent signalling (CDS). The findings of this study suggest that the bcpAIOP locus has a dual function: antagonism of non-self cells as well as cell-to-cell communication between Burkholderia species that express the same bcpA–bcpI allele.