Pili in Probiotic Bacteria

The ability to adhere to intestinal epithelial tissue and mucosal surfaces is a key criterion in selecting probiotics. Adhesion is considered to be a prerequisite for successful colonization and survival in the gastrointestinal tract to provide persistent beneficial effects to the host. Bacteria express a multitude of surface components that mediate adherence. Pili or fimbriae are surface adhesive components implicated in initiating bacterial adhesion and mediating interaction with the host. These nonflagellar proteinaceous fiber appendages were identified and explored over several decades in pathogenic bacteria, and many distinct types are known. However, the presence of pili in probiotics and/or commensalic bacteria has only recently been recognized. Thus knowledge about pili in probiotics is relatively limited, but structural and functional data have begun to emerge. Availability of these data in the future would enable us to understand the pili-mediated adhesion strategies of probiotics. This knowledge could be utilized to develop antiadhesion-based therapies against bacterial infections as well as probiotic designs for beneficial effects. This chapter will briefly summarize the current knowledge of pili in probiotics with emphasis on members of lactobacilli and bifidobacteria

Crystal structure of the atypically adhesive SpaB basal pilus subunit : Mechanistic insights about its incorporation in lactobacillar SpaCBA pili

To successfully colonize a host or environment, certain genera and species of Gram-positive bacteria have evolved to utilize the so-called sortase-dependent pilus, a long multi-subunit and non-flagellar surface adhesin. One example of this is Lactobacillus rhamnosus GG, a gut-adapted probiotic strain that produces SpaCBA pili. These structures are covalent hetero-oligomers built from three types of pilin subunit, each with a specific location and function (i.e., backbone SpaA for length, tip SpaC for adhesion, and basal SpaB for anchoring). Functionally, the SpaCBA pilus exhibits a promiscuous affinity for components on intestinal surfaces (e.g., mucus, collagen, and epithelial cells), which is largely attributed to the SpaC subunit. Then again, the basal SpaB pilin, in addition to acting as the terminal subunit during pilus assembly, displays an out of character mucoadhesive function. To address the structural basis of this unusual dual functionality, we reveal the 2.39 A resolution crystal structure of SpaB. SpaB consists of one immunoglobulin-like CnaB domain and contains a putative intermolecular isopeptide bond-linking lysine and internal isopeptide bond-asparagine in an FPKN pilin motif within the C-terminal end. Remarkably, we found that a C-terminal stretch of positively charged lysine and arginine residues likely accounts for the atypical mucoadhesiveness of SpaB. Although harboring an autocatalytic triad of residues for a potential internal isopeptide interaction, the SpaB crystal structure lacked the visible electron density for intact bond formation, yet its presence was subsequently confirmed by mass spectral analysis. Finally, we propose a structural model that captures the exclusive basal positioning of SpaB in the SpaCBA pilus.Peer reviewe