A vaccine strain of Pasteurella multocida obtained by mutagenesis.

A virulent avian septicemia strain of Pasteurella multocida (P. multocida) serotype 1 was mutagenized with N-methyl-N'-nitro-N-nitroso guanidine. A selection procedure was devised to isolate mutants with changed growth properties. Mutants were isolated that had either prolonged doubling time or temperature sensitivity for growth at 41 degrees C. A correlation was observed between growth properties in vitro and multiplication in vivo. Mutants defective in growth were avirulent. A positive correlation was observed between the rates of multiplication of mutant strains in vitro and their ability to protect against heterologous challenge.