Understanding the subdiffusive motion of bacteriophages in a mucus layer*

Mucosal surfaces are the entry points of pathogens into the animal body and act as the primary sites of defence. Experiments show that viruses called bacteriophages undergo subdiffusive motion within mucus due to their adherence to the mucus network. Understanding this behaviour is important for infection prevention and therapy. Experiments have demonstrated that the motion of a T4 phage exhibit a subdiffusive motion due to the attractive interaction of proteins on the phage to the glycans on the mucin network. This gives it greater residence time and increases its efficiency in infecting its bacterial host. While motion of nano-particles in a polymer network has been studied, the motion of a sticky entity in a network has not been reported. We model the phage as a dendrimer with "sticky" ends that can selectively bind with sites on a network of associative polymers. A multi-particle Brownian Dynamics simulation algorithm based on the GPU-accelerated Python package HOOMD-Blue has been employed. Hydrodynamic and excluded volume interactions are taken into account. The effect of polymer concentration, number of stickers and sticker strength on the dynamics of dendrimers has been analysed. The sticky dendrimers exhibit subdiffusion even when their size is smaller than the "mesh size" of the network, while the non-sticky dendrimers remain diffusive. The origin of subdiffusive behaviour is shown to be related to the time scale of sticker binding, the number of stickers and the radius of gyration of the dendrimer. The diffusion coefficient decreases for both the sticky and non-sticky dendrimers with increasing concentration. This trend is analogous to the observed behaviour of the adherent and non-adherent phages in varying mucin concentrations.