Some lactic acid bacteria, especially spp., possess adhesive properties enabling colonization of the human gastrointestinal tract. Two probiotic strains, WCSF1 and 299v, display highly different mannose-specific adhesion, with 299v being superior to WCFS1 based on a yeast agglutination assay. A straightforward correlation between the mannose adhesion capacity and domain composition of the mannose-specific adhesin (Msa) in the two strains has not been demonstrated previously. In this study, we analyzed the promoter regions upstream of the gene encoding a mannose-specific adhesin in these two strains. The promoter region was mapped by primer extension and DNA sequence analysis, and only a single nucleotide change was identified between the two strains. However, Northern blot analysis showed a stronger transcript band in 299v than in WCFS1 correlating with the different adhesion capacities. During the establishment of a high-throughput yeast agglutination assay, we isolated variants of WCFS1 that displayed a very strong mannose-specific adhesion phenotype. The region upstream of the gene in these variants showed an inversion of a 104-bp fragment located between two perfectly inverted repeats present in the untranslated leader region. The inversion disrupts a strong hairpin structure that otherwise most likely would terminate the transcript. In addition, the ribosome binding site upstream of the gene, which is also masked within this hairpin structure, becomes accessible upon inversion, thereby increasing the frequency of translation initiation in the variant strains. Furthermore, Northern blot analysis showed a higher abundance of the transcript in the variants than in the wild type, correlating with a strong-Msa phenotype. Probiotic strains possess adhesive properties enabling colonization of the human intestinal tract through interactions between molecules present on the probiotic bacteria and components of the epithelial surface. In , interaction is mediated through bacterial surface proteins like Msa, which binds to mannose residues present on the intestinal cells. Such interactions are believed to be important for the health-promoting effects of probiotics, including displacement of pathogens, immunomodulation, and protective effects on the intestinal barrier function. In this study, we have identified a new molecular switch controlling expression of the gene in strain WCFS1. Strains with increased expression could be valuable in the development and manufacture of improved probiotic products.