An intracellular bacterial infection in a plant or animal is treated by administration to a plant cell or animal cell of a particle to which an infectious bacteriophage is covalently attached, wherein the particle is internalised by the cell. Particles with phage attached and compositions comprising the particles are provided. A formulation, for treatment of a bacterial infection, comprises bacteriophage, liquid carrier and adhesive, which dries so that the adhesive adheres the bacteriophage to a surface, one such formulation comprising liquid carrier: 85%-99.98% by weight; bacteriophage: 0.01%-5% by weight; and adhesive: 0.01%-10% by weight.