Construction and characterization of virG (icsA)-deleted Escherichia coli K12 Shigella flexneri hybrid vaccine strains

Human challenge studies with EcSf2a-2, an aroD deletion-attenuated Escherichia coli K12-Shigella flexneri hybrid vaccine expressing S. flexneri 2a somatic antigen and the invasive phenotype indicated that, at doses of 2 x 10(9) bacteria, EcSf2a-2 was immunogenic but also reactogenic and therefore not sufficiently attenuated. Two factors that may contribute to the residual reactogenicity are the spontaneous appearance of plaque-positive variants in the E. coli K12 recipient and the presence of the arg locus encoding enterotoxin or cytotoxin, transferred from S. flexneri 2a into the E. coli recipient. EcSf2a-3 was derived from EcSf2a-2 by introducing a deletion in the virG gene, whose expression is required for plaque formation and keratoconjunctivitis in guinea pigs. EcSf2a-5 contains the same deletion in the E. coli-S. flexneri hybrid strain, 7921, but does not contain the arg locus. Lack of virG expression in these hybrid strains did not affect the immune response to LPS or the development of protective immunity in the guinea pig model.