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Antimicrobial peptides (AMPs) have been considered as an alternative to small molecule antibiotics since they are difficult to develop antimicrobial resistance. Hyperbranched polylysine (HPL), an AMP mimics, has gained attention due to its broad-spectrum antibacterial activities, but it also suffers from high toxicity. Here we report a facile strategy to engineer the toxicity of HPL by copolymerizing lysine (K) with a hydrophobic amino acid, e.g., alanine (A), tryptophan (W) or phenylalanine (F), to afford hyperbranched random copolymers. These copolymers have comparable antibacterial activities to HPL while their cytotoxicities and in vivo toxicities are lowered when the type and content of hydrophobic amino acid and the size of copolymers are optimized. The G. mellonella infection model demonstrates that the copolymers are effective against the S. aureus infection in vivo. The copolymers kill the bacteria through the disruption of cell membranes and the bacteria do not develop resistance to the copolymers. [ABSTRACT FROM AUTHOR]