This is a Phase 1, randomized, double-blind, placebo controlled, dose-escalation, inpatient study. Subjects will receive a single oral dose of WRSs2, WRSs3 or placebo. Five doses of each vaccine, ranging from 1x10^3 to 1x10^7 colony forming units (cfu) will be evaluated in this study. For each dose, a cohort of 18 subjects (n=18) will be randomized into 3 groups to receive WRSs2 (n=8), WRSs3 (n=8) and placebo (n=2). The three groups will receive approximately the same inoculum dose. Ingestion of the vaccine will be preceded by ingestion of a sodium bicarbonate buffer. Subjects will be enrolled in serial groups at escalating doses to enable the identification of the dose that minimizes reactogenicity while inducing a robust immune response while allowing for a head-to-head comparison of the 2 vaccine candidates. Dose escalation will proceed by approximately 1 log10 cfu levels based on the observations from the preceding group (up to a maximum dose of 1x10^7 cfu). During the 10-13 day inpatient monitoring period, subjects will be evaluated at least twice daily. This assessment will include asking about any symptoms they may be experiencing, with targeted questions for reactogenic symptoms twice each day. Additional assessments will include daily measurement of vital signs, daily-history directed physical examinations and the collection, and grading (for consistency) of each stool along and weighing of all loose or watery stools. Stool specimens or rectal swabs (if unable to produce a stool) will also be evaluated for grossly visible and occult blood and will be cultured for the presence of the vaccine strain. All volunteers will receive antibiotic therapy 8 days after inoculation (unless meeting criteria for early treatment). Subjects will be discharged after initiating antibiotic treatment and passing 2 consecutive stools (at least 6 hours apart) negative for S. sonnei using standard stool culture and identification techniques. Following evaluation and history-directed physical examination, subjects will be discharged, with follow-up on study days 14+/-1, 28+/-2 and 56+/-4 to provide additional specimens for safety checks or immunology monitoring. Subjects will be given an ice-pack and cooler for collection of a stool sample at home which is to be brought to the clinic on the day of the follow-up visit. An interim safety report will compile findings from the preceding group with the PI's interpretation and plan to go forward. The Data Safety Monitoring board (DSMB)/Safety Monitoring Committee (SMC) or its equivalent must concur with the plan before proceeding. The decision to advance to the next highest dose level will be based solely on the safety profile at the tested dose level and will be made independently for each vaccine candidate. The next inpatient group (at the next higher dose) will not be admitted until a minimum of 4 weeks have elapsed from the time of the preceding group's vaccination to ensure adequate time for safety monitoring. The primary objectives are: to evaluate the safety and clinical tolerance of WRSs2 and WRSs3 by monitoring the presence and severity of clinical signs and symptoms. The secondary objectives are: to evaluate the immune response in blood and stool following ingestion of WRSs2 and WRSs3; to assess frequency and duration of fecal shedding of WRSs2 and WRSs3 following ingestion; and to assess any secondary infectious spread of different vaccine strains (WRSs2 or WRSs3) to subjects.