Densely grafted star- and comb-shaped molecular brushes composed of poly[2-(dimethylamino)ethyl methacrylate] were prepared via atom transfer radical polymerization (ATRP) using either cyclotriphosphazenes or polyphosphazenes as initiators. The grafting conditions were optimized in order to obtain well controlled brush structures. The kinetics of the reactions were first-order with respect to the monomer concentration in both cyclotriphosphazene and polyphosphazene systems. The resultant star- and comb-shaped brushes were quaternized with iodomethane, iodobutane, iodoheptane, iododecane and iodododecane, and the products were characterized by NMR, gel permeation chromatography (GPC) and differential scanning calorimetry (DSC). Electrospinning of the quaternized brushes gave rise to microfibers with diameters in range of 700 nm to 1.1 μm, evidenced by scanning electron microscopy (SEM). The antibacterial activity of the quaternized brush species in both aqueous solution and as fibrous solids against Escherichia coli (E. coli) has been evaluated. In aqueous solution, star-shaped brushes quaternized with iodoheptane showed the best antibacterial effect, with a minimum inhibitory concentration (MIC) as low as 250 μg mL⁻¹. In the fibrous solid state, more than 99% of E. coli were killed within 2 h after contacting 100 mg of microfibers electrospun from the star-shaped brush polymers quaternized with either iododecane or iodododecane.