The depletion mechanism can actuate bacterial aggregation by self-produced exopolysaccharides and determine species distribution and composition in bacterial aggregates

Abstract

Bacteria causing chronic infections are often found in cell aggregates suspended in polymer secretions, and aggregation may be a factor in infection persistence. One aggregation mechanism, called depletion aggregation, is driven by physical forces between bacteria and polymers. Here we investigated whether the depletion mechanism can actuate the aggregating effects of P. aeruginosa exopolysaccharides for suspended (i.e. not surface attached) bacteria, and how depletion affects bacterial inter-species interactions. We found cells overexpressing the exopolysaccharides Pel and Psl, but not alginate remained aggregated after depletion-mediating conditions were reversed. In co-culture, depletion aggregation had contrasting effects on P. aeruginosas interactions with coccus- and rod-shaped bacteria. Depletion caused S. aureus (cocci) and P. aeruginosa (rods) to segregate from each other, S. aureus to resist secreted P. aeruginosa antimicrobial factors, and the species to co-exist. In contrast, depletion aggregation caused P. aeruginosa and Burkholderia sp. to intermix, enhancing type VI secretion inhibition of Burkholderia by P. aeruginosa, leading to P. aeruginosa dominance. These results show that in addition to being a primary cause of aggregation in polymer-rich suspensions, physical forces inherent to the depletion mechanism can actuate the aggregating effects of self-produced exopolysaccharides and determine species distribution and composition of bacterial communities.