Protection of adult rabbits and monkeys from lethal shigellosis by oral immunization with a thymine-requiring and temperature-sensitive mutant of Shigella flexneri Y.

As an approach to the development of attenuated strains, thymine-requiring (Thy-) and temperature-sensitive (Ts-) single and double mutants of Shigella flexneri Y have been isolated by u.v. mutagenesis and selection. The mutants retained the 140 MDa plasmid, were Sereny-negative and avirulent in 'conditioned' rabbits and in monkeys. Rabbits and monkeys immunized orally with two doses of the mutants were solidly protected against disease and lethality when challenged with the homologous virulent strain. The immunized animals had higher levels of antibodies reactive to the lipopolysaccharide of the wild type strain and also a shorter duration of shedding of the challenge bacteria. It was concluded that Thy- and Ts- mutants of Shigella spp. are highly attenuated and immunogenic to be considered for further studies towards the development of vaccine candidates.