Volunteers will receive the test article three times daily following meals beginning 2 days prior to oral ETEC challenge (strain H10407). Test article will be administered for a total of 7 days. Monitoring procedures will assess volunteer safety, the primary endpoint (diarrhea), stool microbiology (H10407 excretion), and ETEC-specific immunology. All volunteers will receive antibiotic treatment (ciprofloxacin, trimethoprim-sulfamethoxazole or amoxicillin) starting 5 days after ETEC challenge or sooner based on pre-defined clinical criteria. Follow-up visits for 2 weeks post-discharge will monitor safety and immunological parameters.