AimsThis study aimed to investigate the fate of Bacillus clausii spores orally administered as lyophilized or liquid formulation to healthy volunteers. Methods and ResultsThe study was a randomized, open-label, cross-over trial in which two commercial probiotic formulations containing spores of four antibiotic-resistant B.clausii strains (OC, NR, SIN, T) were given as a single dose administration. Faecal B.clausii units of each strain were counted on selective media and extrapolated for the total weight of evacuated faeces. RAPD-PCR typing was used to confirm B.clausii identification. Bacillus clausii was found alive in faeces for up to 12days. In some volunteers, the recovered amount of OC, NR or SIN was higher than the number of administered spores. Bioequivalence among the two formulations was demonstrated. ConclusionsBacillus clausii spores survive transit through the human gastrointestinal tract. They can undergo germination, outgrowth and multiplication as vegetative forms. Bacillus clausii strains can have different ability to survive in the intestinal environment. Bacillus clausii spores administered as liquid suspension or lyophilized form behave similarly invivo. Significance and impact of the studyThis work contributes towards a better understanding of the behaviour of B.clausii spores as probiotics.