We previously discovered that covalently coating glass and plastic slides with certain long poly(vinyl-N-alkylpyridinium) chains enables the resultant surfaces to kill a variety of airborne and waterborne bacteria on contact. In the present study, these findings have been extended to an unrelated polymer class, polyethylenimines (PEIs). Alkylated PEIs attached to flat macroscopic surfaces and to those of nanoparticles make these materials highly bactericidal toward both Gram-positive and Gram-negative pathogenic bacteria. Systematic chemical modifications of the immobilized PEI conducted herein shed light on the relationship between the structure of the polymer and the antibacterial efficiency of the resultant coating.