Stable Salmonella live vaccine strains with two or more attenuating mutations and any desired level of attenuation.

Mutants optimally attenuated for highly susceptible hosts and protecting after a single oral vaccination are often overattenuated for host species being less susceptible. Therefore, to select vaccine strains optimally attenuated for the particular host species it is essential that a range of mutants with graded levels of attenuation be provided so as to permit lesser susceptibility to be compensated for by a correspondingly lower level of attenuation. This, while guaranteeing the stability through two-marker or multi-marker attenuation, can be suitably accomplished by slightly to moderately virulence-reducing mutations. Aspartic acid auxotrophy and, in particular, 'metabolic drift' mutations, possibly by additionally incorporating antiepidemic markers, are adopted for the mouse model to demonstrate stepwise production of S. typhimurium and S. typhi vaccine candidate strains with graded attenuation or any level of attenuation desirable. It is emphasized that this basic approach is relevant to practice.