Modeling reciprocal adaptation of Staphylococcus aureus and Pseudomonas aeruginosa co-isolates in artificial sputum medium

Co-infections by Staphylococcus aureus and Pseudomonas aeruginosa are frequent in the airways of patients with cystic fibrosis. These co-infections show higher antibiotic tolerance in vitro compared to mono-infections. In vitro models have been developed to study the interspecies interactions between P. aeruginosa and S. aureus. However, these model systems fail to incorporate clinical isolates with diverse phenotypes, do not reflect the nutritional environment of the CF airway mucus, and/or do not model the biofilm mode of growth observed in the CF airways. Here, we established a dual-species biofilm model grown in artificial sputum medium, where S. aureus was inoculated before P. aeruginosa to facilitate the maintenance of both species over time. It was successfully applied to ten pairs of clinical isolates exhibiting different phenotypes. Co-isolates from individual patients led to robust, stable co-cultures, supporting the theory of cross-adaptation in vivo. Investigation into the cross-adaptation of the VBB496 co-isolate pair revealed that both the P. aeruginosa and S. aureus isolates had reduced antagonism, in part due to reduced production of P. aeruginosa secondary metabolites as well as higher tolerance to those metabolites by S. aureus. Together, these results indicate that the two-species biofilm model system provides a useful tool for exploring interspecies interactions of P. aeruginosa and S. aureus in the context of CF airway infections.