Mixed Populations and Co-Infection: Pseudomonas aeruginosa and Staphylococcus aureus.

The human pathogens Pseudomonas aeruginosa and Staphylococcus aureus are frequently co-isolated from chronic wounds or cystic fibrosis patient airways. Clinical studies analysing the impact of co-infection on patient clinical outcomes lead to contradictory results. However, laboratory approaches suggest that the two pathogens co-colonize the same infection niches and form a mixed-species biofilm, therefore favouring their resistance to antibiotics and immune response. In parallel, many recent studies have focused on the different interactions between the two bacterial species. It has long been recognized that P. aeruginosa usually outcompetes S. aureus, and the molecular mechanisms involved in this state of bacterial competition are now well understood. However, several recent studies show that interactions between P. aeruginosa and S. aureus can be diverse and evolve over time. Thus, many CF isolates of P. aeruginosa and S. aureus can coexist and develop cooperative behaviours. In this chapter, we will provide an overview of the current knowledge on the mixed populations of P. aeruginosa and S. aureus, from their mechanisms of establishment to their impacts on bacterial physiology and clinical outcomes.