Provided herein are methods for treating planktonic bacteria or a biofilm. The methods include contacting the planktonic bacteria or biofilm with an effective amount of an isolated Citrobacter freundii colicin A polypeptide, wherein the polypeptide has an antibacterial activity against the planktonic bacteria or the biofilm. Methods are also provided herein for treating a subject that has a bacterial infection, for example caused by a biofilm. The methods can be used to treat a biofilm on a living or non-living surface. Also provided herein are Citrobacter freundii colicin A polypeptides that have an antibacterial activity against planktonic bacteria or surface attached bacteria, and nucleic acid sequences encoding the polypeptides. Medical devices comprising a surface having an antimicrobial effective amount of a Citrobacter freundii colicin A polypeptide, or a nucleic acid molecule encoding the polypeptide, are also disclosed.