Aqueous antimicrobial coating compositions are disclosed comprising at least one organosilane homopolymer, present as a distribution of polymer chain lengths, and optionally at least one amine. A method of preparing an antimicrobial coating comprises coating a surface with the aqueous antimicrobial coating composition and allowing the composition to dry into a film that exhibits residual antimicrobial efficacy against microorganisms even after mechanical abrasion of the coating. The organosilane homopolymer may comprise only 3-aminopropylsilanetriol homopolymer, mixtures of 3-aminopropylsilanetriol homopolymer, 3-chloropropylsilanetriol homopolymer and 3-(trihydroxysilyl)propyl dimethyloctadecyl ammonium chloride homopolymer, or any one of various unique organosilane homopolymers having multiple amine functionality.