Pseudomonas aeruginosa lasR is a keystone gene in polymicrobial cultures

The airways of people with cystic fibrosis (CF) are often co-infected by Pseudmonas aeruginosa and a variety of other co-habiting microbes; the infections are polymicrobial. P. aeruginosa isolates from the CF airways are also known to commonly acquire mutations in the quorum sensing regulator, lasR. The appearance of these lasR mutants is associated with a worsening clinical prognosis. In this work, we show that loss of lasR function has a significant impact on the stability of inter-species interactions in a polymicrobial ecosystem, and in particular on the dynamics of a common CF-associated fungus, Candida albicans. Titres of C. albicans were stable in the presence of wild type P. aeruginosa and Staphylococcus aureus. However, when wild type P. aeruginosa was replaced by a {Delta}lasR mutant, C. albicans titres progressively declined over time. This instability could be reversed by ectopic expression of a Type VI Secretion System effector cluster (tsi) in the {Delta}lasR mutant. We also noted that challenge of the polymicrobial cultures with a clinically-relevant combination of antibiotics (colistin and ciprofloxacin) led to a hyphal bloom of the fungus. This bloom was abolished in the {Delta}lasR mutant, but again, was restored by ectopic expression of the tsi cluster. Finally, we show that whereas wild type P. aeruginosa is relatively agnostic to the presence of other microbes, the {Delta}lasR mutant is not and undergoes substantial transcriptional reprogramming. Our data indicate that lasR has a large and previously unrecognised impact on inter-species interactions. We therefore propose that lasR is an ecological keystone gene.