LAUSR

Interspecies interactions can drive the emergence of unexpected microbial phenotypes that are not observed when studying monocultures. The cystic fibrosis (CF) lung consists of a complex environment where particular microbes, living as polymicrobial biofilm-like communities, are associated with negative clinical outcomes for persons with CF (pwCF). However, the current lack of in vitro models integrating the microbial diversity observed in the CF airway hampers our understanding of why polymicrobial communities are recalcitrant to therapy in this disease. Here, integrating computational approaches informed by clinical data, we built a mixed community of clinical relevance to the CF lung composed of Pseudomonas aeruginosa, Staphylococcus aureus, Streptococcus sanguinis and Prevotella melaninogenica. We developed and validated this model biofilm community with multiple isolates of these four genera. When challenged with tobramycin, a front-line antimicrobial used to treat pwCF, the microorganisms in the polymicrobial community show altered sensitivity to this antibiotic compared to monospecies biofilms. We observed that wild-type P. aeruginosa is sensitized to tobramycin in a mixed community versus monoculture, and this observation holds across a range of community relative abundances. We also report that LasR loss-of-function, a variant frequently detected in the CF airway, induces tolerance of P. aeruginosa to tobramycin specifically in the mixed community. The molecular basis of this community-specific recalcitrance to tobramycin for the LasR mutant variant is the increased production of redox-active phenazines. Our data support the importance of studying clinically-relevant model polymicrobial biofilms to understand community-specific traits relevant to infections. SIGNIFICANCE STATEMENT The CF lung is colonized by biofilm-like microbial communities that exhibit both resistance and tolerance (collectively called “recalcitrance”) to antimicrobials used in the clinic. Here, we leveraged clinical data from pwCF to inform our understanding of communities exhibiting recalcitrance. We developed and validated an in vitro model that revealed novel, community-specific phenotypes relevant to the clinic. We used this model to explore the underlying mechanism associated with a community-specific emergent behavior. We posit that in vitro models of polymicrobial communities may help in developing new antimicrobial strategies to improve patient outcomes, and that the approach used here can be applied to other polymicrobial models.