Sessile growth reveals novel paradigms of Pseudomonas aeruginosa iron-regulated antimicrobial activity against Staphylococcus aureus

Pseudomonas aeruginosa is an opportunistic pathogen that is frequently associated with both acute and chronic infections, the latter of which are often polymicrobial. P. aeruginosa possesses a complex regulatory network that modulates nutrient acquisition and virulence, but our knowledge of these networks is largely based on studies with shaking cultures, which are not likely representative of conditions during infection. Here, we provide proteomic, metabolic, and genetic evidence that regulation by iron, a critical metallo-nutrient, is altered in static P. aeruginosa cultures. We identified type VI secretion as a target of iron regulation in P. aeruginosa in static but not shaking conditions, and we present evidence that this regulation occurs via PrrF sRNA-dependent production of 2-alkyl-4(1H)-quinolone metabolites. We further discovered that iron-regulated interactions between P. aeruginosa and a Gram-positive opportunistic pathogen, Staphylococcus aureus, are mediated by distinct factors in shaking versus static bacterial cultures. These results yield new bacterial iron regulation paradigms and highlight the need to re-define iron homeostasis in static microbial communities.