The present invention relates to development of recombinant vectors and an attenuated Pasteurella multocida P52 vaccine strain by targeted gene knock-out approach. The wild type Pasteurella multocida P52 vaccine strain is a highly virulent organism used to prepare killed vaccine in India. The developed attenuated strain is non-virulent to mammals and maintains immunogenic properties. The developed strain can be used as live vaccine against Haemorrhagic Septicaemia. Recombinant vectors were constructed using suicide plasmid which can not replicate in Pasteurella multocida. Pasteurella multocida P52 was transformed with recombinant vector by electroporation. Kanamycin resistance determinant marker was incorporated in the genome of Pasteurella multocida P52 during homologous recombinantion with the recombinant vector. The developed attenuated P. multocida P52 strain is an auxotroph which can not grow in absence of aromatic amino acids. As an immunogen, the attenuated strain exhibits a remarkable activity with enhanced immune response. In addition, the attenuated strain is stable mutant. The organism can be used as freeze dried vaccine with longer shelf life. As live vaccine, the attenuated strain of Pasteurella multocida confers stronger immunity and requires low dose (less number of live organisms) for immunization. This vaccine composition is very safe and can be administered through a variety of routes.
