Background. M01ZH09 (Salmonella enterica serovar Typhi [Ty2 aroC- ssaV-] ZH9) is a live oral-dose typhoid vaccine candidate. M01ZH09 was rationally modified with 2 independently attenuating mutations, including a novel mutation in Salmonella pathogenicity island (SPI)-2. We demonstrate that M01ZH09, in a single oral dose, is well tolerated and prompts broad immune responses, regardless of whether prevaccination with a bicarbonate buffer is given. Methods. Thirty-two healthy adult subjects were randomized and given 5 ⊗ 109 cfu of M01ZH09, with (presentation 1) or without (presentation 2) prevaccination with a bicarbonate butter. Immunogenicity data included Salmonella Typhi lipopolysaccharide (LPS)-specific immunoglobulin (Ig) A antibody-secreting cells (enzyme-linked immunospot [ELISPOT] assay), IgG serologic responses to Salmonella Typhi LPS, lymphocyte proliferation, and interferon (IFN)-γ production. Results. The vaccine was well tolerated; adverse events after vaccination were mild. No fever or prolonged vaccine shedding occurred. Immunogenicity data demonstrated that 88% and 93% of subjects who received presentation 1 and presentation 2, respectively, had a positive response by ELISPOT assay; 81% of subjects in both groups underwent IgG seroconversion on day 14. Both groups had similar cellular immune responses to presentation 1 and presentation 2; lymphocyte proliferation to Salmonella Typhi flagellin occurred in 63% and 67% of subjects, respectively, and 69% and 73% of subjects, respectively, had an increase in IFN-γ production. Conclusion. The oral typhoid vaccine M01ZH09 is well tolerated and highly immunogenic in a single oral dose, with and without prevaccination with a bicarbonate buffer. Field studies to demonstrate protective efficacy are planned. [ABSTRACT FROM AUTHOR]