Compositions and methods regarding guanidinium functionalized polycarbonates that provide potent antimicrobial activity against multidrug resistant (MDR) bacteria, including Klebsiella pneumoniae (K. pneumoniae) are provided. According to an embodiment, an antimicrobial guanidinium-functionalized polymer is provided that comprises a hydrophobic molecular backbone with cationic guanidinium moieties respectively bound to the hydrophobic molecular backbone via butyl spacer groups. The antimicrobial guanidinium-functionalized polymer self-assembles into a micelle structure with hydrophobic residuals of the antimicrobial guanidinium-functionalized polymer buried inside the micelle structure and the cationic guanidinium moieties exposed on an external surface of the micelle structure to target pathogens.