SpaCBA pili and the molecular mechanisms of adhesion. LGG is very good mucus adhering Lactobacillus strain compared to other probiotic strains such as L. casei Shirota and L. johnsonii LJ1 and the closely related strain L. rhamnosus Lc705. Radioactively labeled bacteria were allowed to adhere to isolated human intestinal mucus. The adhesion ratio (%) was determined by comparing radioactivity of bacteria added to the radioactivity of bound bacteria after washing (A). Data were published before . Presence of SpaCBA pili LGG cells based on a TEM image of LGG labeled with SpaA antiserum and 10 nm protein A gold particles  (B) and on a AFM image of LGG in air  (C). The predicted model of the pili shows a pilus backbone formed by the major subunit SpaA, as shown in the schematic figure. The minor subunit SpaC is present on the tip and decorates the pilus over the length at ratio 1:2 with SpaA. The Spa B minor pilin serves as a molecular switch for pilus termination and is bound to the peptidoglycan layer. However, it is suggested that leaky activity of the pilin-specific sortase can include SpaB decorations on the pilus (D). Adapted from . The SpaC pilin is thought to serve as a major adhesin of LGG. It can interact with other SpaC molecules, inducing homophilic adhesion, and with intestinal epithelial cells or their extracellular matrix, in heterophilic adhesion. The exact adhesion sites however remain unkown. Pili can have molecular spring properties which makes them capable to withstand shearing stress. Moreover, the SpaC pilin decorated over the pilus length provide a molecular zipper mechanism that can facilitate a close interaction between the host and the bacteria or bacteria with each other (E). Adapted from .