Low Discrimination of Charged Silica Particles at T4 Phage Surfaces

Bacteriophages have a variety of interesting properties that can be exploited in biological assays, including high binding specificity for the identification of bioactive molecules, and particularly, their capacity to propagate via bacterial lysis. However, the development of phage-based assays and disinfection devices is hampered by ineffective methods to stabilize and orient the phage in active form. For example, T4 phage, which has an anionic head and cationic tail, binds targets using its tail fibers, hence the adsorption of their tails to a substrate during immobilization, limits the ability of T4 to bind targets. Previously, we demonstrated that T4 phage retains infectivity despite adsorption to large silica particles of anionic or cationic charge. In an effort to better understand this phenomenon, we have instead used silica nanoparticles – much smaller than the phage – to probe the locus of preferential adsorption. Silica nanoparticles (NPs) (10 and 30 nm) were synthesized from tetraethoxysilane (TEOS) using the Stober process. Charged particles were prepared by mixing appropriate silica dispersions with the cationic polyelectrolyte (PDADMAC), leading to both negative (zeta potential (ζ +10) silica surfaces. These particles were mixed with 1.5 × 1010 pfu/ mL T4 (model) phage and analyzed by transmission electron microscopy (TEM). Results show that regardless of the particle concentration, both positively and negatively charged 30 nm particles bind effectively to both head and tail, with a slight preference for the head. No difference between NP associations to head or tail was seen with 10 nm particles, possibly because of their lower charge surface density and smaller size, which can accommodate small features on the phage surface better. The high hydrophilicity of silica facilitates maintenance of phage hydration and, likely, phage mobility, such that orientation is less important for bacterial binding in this system.