Interaction between Staphylococcus aureus and Pseudomonas aeruginosa is beneficial for colonisation and pathogenicity in a mixed biofilm

Debate regarding the co-existence of Staphylococcus aureus and Pseudomonas aeruginosa in wounds remains contentious, with the dominant hypothesis describing a situation akin to niche partitioning, whereby both microorganisms are present but occupy distinct regions of the wound without interacting. In contrast, we hypothesised that these microorganisms do interact during early co-colonisation in a manner beneficial to both bacteria. We assessed competitive interaction between S. aureus and P. aeruginosa in biofilm cultured for 24-72 h and bacterial aggregates analogous to those observed in early (<24 h) biofilm formation, and interaction with human keratinocytes. We observed that S. aureus predominated in biofilm and non-attached bacterial aggregates, acting as a pioneer for the attachment of P. aeruginosa. We report for the first time that S. aureus mediates a significant (P < 0.05) increase in the attachment of P. aeruginosa to human keratinocytes, and that P. aeruginosa promotes an invasive phenotype in S. aureus. We show that co-infected keratinocytes exhibit an intermediate inflammatory response concurrent with impaired wound closure that is in keeping with a sustained proinflammatory response which allows for persistent microbial colonisation. These studies demonstrate that, contrary to the dominant hypothesis, interactions between S. aureus and P. aeruginosa may be an important factor for both colonisation and pathogenicity in the chronic infected wound.