A pharmaceutical composition for use in killing and/or inhibiting the growth and/or proliferation of a microorganism in a subject in need thereof, or for treating a subject afflicted with a microbial infection is disclosed. The composition comprises: (a) an effective amount of an isolated peptide, wherein the peptide comprises the arginine-rich carboxy-terminal region of hepatitis B virus core protein (HBc) and exhibits an antimicrobial activity; and (b) a pharmaceutically acceptable carrier. The peptide exhibits an activity against Gram-negative bacteria, Gram-positive bacteria, and/or fungi.