Spatial distribution of bacteria and extracellular polymeric substances impacts nanoparticle penetration in biofilms

Abstract

Extracellular polymeric substances (EPS) in bacterial biofilms complicate treatment by inactivating drugs and slowing down diffusion. Through enhanced penetration and resistance to degradation in bacterial biofilms, nanoparticle (NP) carriers can help improve biofilm treatment. However, the way in which biofilm architecture influences the diffusive properties and penetration of NPs in biofilms is still poorly understood. In this work, we combined single particle tracking (SPT) and confocal laser scanning microscopy (CLSM) in Salmonella biofilms with simulations of a Brownian dynamics model to quantify how macro- (spatial organization of the bacteria) and micro- (EPS dependent) structure of the biofilm affects NP penetration. In CLSM images we observed immobilization of NPs in the EPS, which allows shielding of bacteria from the NPs, an effect that was more pronounced in dispersed biofilms, grown in nutrient-rich conditions, than in compacted biofilms, grown in nutrient-poor conditions. SPT experiments revealed anomalous diffusion, with an increased probability for small displacements near clusters of bacteria. Simulations of a Brownian dynamics model revealed that EPS reinforces shielding by affecting the pore structure of the biofilm. Finally, in virtual biofilms with varying spatial distribution of bacteria, we found that even for the same number of bacteria, dispersed biofilm structures provide more shielding than biofilms organized in dense, compacted clusters, even when accounting for decreased NP diffusivity.