| Summary: | In the lungs of patients with cystic fibrosis (CF), the main pathogen <i>Pseudomonas aeruginosa</i> is often co-isolated with other microbes, likely engaging in inter-species interactions. In the case of chronic co-infections, this cohabitation can last for a long time and evolve over time, potentially contributing to the clinical outcome. Interactions involving the emerging pathogens <i>Achromobacter</i> spp. have only rarely been studied, reporting inhibition of <i>P. aeruginosa</i> biofilm formation. To evaluate the possible evolution of such interplay, we assessed the ability of <i>Achromobacter</i> spp. isolates to affect the biofilm formation of co-isolated <i>P. aeruginosa</i> strains during long-term chronic co-infections. We observed both competition and cohabitation. An <i>Achromobacter</i> sp. isolate secreted exoproducts interfering with the adhesion ability of a co-isolated <i>P. aeruginosa</i> strain and affected its biofilm formation. Conversely, a clonal <i>Achromobacter</i> sp. strain later isolated from the same patient, as well as two longitudinal strains from another patient, did not show similar competitive behavior against its <i>P. aeruginosa</i> co-isolates. Genetic variants supporting the higher virulence of the competitive <i>Achromobacter</i> sp. isolate were found in its genome. Our results confirm that both inter-species competition and cohabitation are represented during chronic co-infections in CF airways, and evolution of these interplays can happen even at the late stages of chronic infection.
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