Interactions of Pseudomonas aeruginosa and Staphylococcus aureus in dual-species biofilms

Chronic wound infections have a profound effect on the morbidity and mortality of a large patient population and cost billions of dollars in direct medical costs annually in the United States. Biofilms are communities of microbes that reside in a polysaccharide shell and contribute greatly to the severity and antibacterial resistance of wound infections. Two of the most prominent and notoriously resistant bacterial species found together are Pseudomonas aeruginosa and Staphylococcus aureus.  However, few studies have examined the interactions of these two species in the context of wound infections.  Our goal was to create an in vitro model to examine the interspecies interactions between P. aeruginosa and S. aureus.  We hypothesized that the two species would work synergistically to promote biofilm formation.  We examined the growth of P. aeruginosa and S. aureus co-cultures in a flask, to represent free-living planktonic cell growth, and observed that P. aeruginosa quickly dominated the culture, presumably killing off all the S. aureus within 8 hours.  However, when the two species were grown together in conditions to simulate the wound environment, we observed a rapid formation of biofilms that remained dual-species for several days.  Interestingly, we also observed that P. aeruginosa required the presence of S. aureus to form a biofilm, and these interactions were dependent of the cell-to-cell signaling systems termed ‘quorum sensing’.  Our data suggest that synergistic interactions between P. aeruginosa and S. aureus in wounds may contribute to biofilm formation and thus healing delays and/or antibiotic tolerance.