Genetically attenuated bacterial vaccines with multiple mutations of the same phenotype

A biologically pure culture of an avirulent bacterial strain useful as a vaccine, wherein said avirulent strain is derived from a virulent bacterial strain by introducing into said virulent strain at least two mutations of the same phenotype which render said virulent strain avirulent, while permitting said avirulent strain to retain immunogenicity;

• wherein said phenotype is a temperature-sensitive phenotype which suppresses growth at the normal body temperature of a host for said virulent strain.