Competitive interactions mediated by released chemicals (e.g., toxins) are prominent in multispecies communities, but the effects of these chemicals at subinhibitory concentrations on susceptible bacteria are poorly understood. Although
Pseudomonas aeruginosaand species of the Burkholderia cepaciacomplex (Bcc) can exist together as a coinfection in cystic fibrosis airways, P. aeruginosatoxins can kill Bcc species in vitro. Consequently, these bacteria become an ideal in vitromodel system to study the impact of sublethal levels of toxins on the biology of typical susceptible bacteria, such as the Bcc, when exposed to P. aeruginosatoxins. Using P. aeruginosaspent medium as a source of toxins, we showed that a small window of subinhibitory concentrations modulated the colony morphotype and swarming motility of some but not all tested Bcc strains, for which rhamnolipids were identified as the active molecule. Using a random transposon mutagenesis approach, we identified several genes required by the Bcc to respond to low concentrations of rhamnolipids and consequently affect the ability of this microbe to change its morphotype and swarm over surfaces. Among those genes identified were those coding for type IVb-Tad pili, which are often required for virulence in various bacterial pathogens. Our study demonstrates that manipulating chemical gradients in vitrocan lead to the identification of bacterial behaviors relevant to polymicrobial infections. IMPORTANCEInterspecies interactions can have profound effects on the development and outcomes of polymicrobial infections. Consequently, improving the molecular understanding of these interactions could provide us with new insights on the possible long-term consequences of these chronic infections. In this study, we show that P. aeruginosa-derived rhamnolipids, which participate in Bcc killing at high concentrations, can also trigger biological responses in Burkholderiaspp. at low concentrations. The modulation of potential virulence phenotypes in the Bcc by P. aeruginosasuggests that these interactions contribute to pathogenesis and disease severity in the context of polymicrobial infections.