Alginate exopolymer significantly modulates the viscoelastic properties and resilience of bacterial biofilms

Biofilms are viscoelastic gels with a cross-linked network of biopolymers forming an extracellular matrix that protects bacteria from most antimicrobial treatments. This study examines the physical role of the matrix in preventing recolonisation using a mucoid Pseudomonas aeruginosa (P. aeruginosa ΔmucA) and isogenic wild-type Pseudomonas aeruginosa PAO1. We investigated the recolonisation of pre-formed live biofilms and the residual matrix left behind after bacterial eradication with N-acetyl cysteine (NAC). P. aeruginosa ΔmucA, which overproduces alginate, prevented recolonisation through swelling and increased elastic modulus. In contrast, the wild-type P. aeruginosa biofilm matrix exhibited minimal swelling and decreased elasticity, suggesting crosslink breakage. These observations align with polymer physics theories where alginate's polyelectrolyte nature drives swelling through the Donnan effect, enhancing matrix stability. Meanwhile, the Psl-rich wild-type matrix limited swelling but showed reduced mechanical stability. This study underscores the critical role of matrix composition in biofilm mechanics, influencing bacterial protection regardless of viability.