Production of a biofilm polymer can benefit bacteria when grown in co-culture under low iron conditions*

Biofilm formation is associated with resistance to antibiotics and the immune system. The opportunistic human pathogen Pseudomonas aeruginosa forms biofilm infections in lungs, wounds, and on medical devices. However, its ability to form biofilms originated in this bacterium’s native environment, primarily plants and soil. Such environments are polymicrobial and resource-limited. The P. aeruginosa extracellular polysaccharide Psl can bind iron and, for the strain PAO1, is also the dominant “glue” holding together multicellular aggregates and biofilms. Here, we quantify early biofilm growth using time-lapse confocal microscopy. We find that aggregates of P. aeruginosa have a growth advantage over single cells of P. aeruginosa in the presence of Staphylococcus aureus in low-iron environments. This growth advantage is linked to aggregates' high Psl content and to the production of an active factor by S. aureus. We posit that this may have been linked to the evolutionary development of the strong biofilm-forming tendencies of P. aeruginosa.