Abstract: The invention provides a novel surface polypeptide from Aeromonas hydrophila as well as fragments, variants and derivatives of this polypeptide. Also provided are polynucleotides encoding the polypeptide, fragments, variants and derivatives. Compositions containing the polypeptide and polynucleotides of the invention are also disclosed as well as methods useful in the treatment and prevention of bacterial infection in an animal, wherein said infection is caused by bacteria of a genus selected from the group consisting of Aeromonas, Vibrio and Edwardsiella, and in the diagnosis of bacterial infection in an animal, wherein said infection is caused by bacteria of a genus Aeromonas.