Abstract
Chronic co-infections involving Staphylococcus aureus (Sa) and Pseudomonas aeruginosa (Pa) are characterized by complex biochemical gradients and physical forces that drive severe disease and complicate clinical outcomes. While previous genetic analyses suggest that P. aeruginosa primarily directly antagonizes S. aureus growth through the production of bacteriotoxins, these studies often use well-mixed liquid cultures that lack the spatial complexity of an in vivo infection. To determine if these interactions differ in a spatial model of interspecies competition, this study used a colony biofilm experimental model to screen and characterize 42 transposon mutants putatively involved in interspecies dynamics between S. aureus USA300 and P. aeruginosa PAO1 and PA14. The results indicate that the majority of these mutations significantly altered competitive outcomes, predominantly favouring the overgrowth of S. aureus by P. aeruginosa. These shifts in dominance are attributable to generalized growth defects and the capacity of cells to adapt to the emergence of physiological gradients within the colony biofilm. The findings in this study demonstrate that S. aureus – P. aeruginosa interactions in structured environments cannot be solely attributed to direct mechanisms of antagonism, like the production of bacteriotoxins. Instead, the capacity for species to adapt to, and to colonize increasingly stressful, nutrient-limited microenvironments appears to be a primary driver of community structure.