Is it feasible to control pathogen infection by competitive binding of probiotics to the host?

Colonization of commensal and pathogenic bacteria begins with the adherence on several biological components, such as mucus, epithelial cells, and lamina propria of the gastrointestinal tract (GIT) in human and animal hosts.^1,2 Major driving forces behind to strengthen the binding at the interface of bacteria and hosts are protein-protein and protein-carbohydrate interactions, due mainly to distribution patterns of glycolipids, sugar chains, and proteins on the surface layers of the bacteria and the hosts.^3,4 In terms of proteins involved with the host-bacterial interaction, commensal and pathogenic bacteria largely share molecular mechanisms of the initial steps of adherence to the hosts, indicating the presence of common adhesion proteins on their surfaces, which can interact with specific receptors or soluble macromolecules in the hosts.² For example, both commensal and pathogenic bacteria have similar proteinaceous structures, called pili, fimbriae, and flagella, which are filamentous architectures composed by assembling of one or more subunit proteins that protrude on the bacterial cell surface.^5,6,7

Historically, such bacterial filaments have been well studied on pathogens who use different types of pilus, such as chaperone-usher pili,⁸ curli pili,⁹ and type IV pili¹⁰ to attach onto the hosts from the “safe” distance circumventing their defense system. In fact, the binding of pilus proteins to the host cell receptors elicits different responses, leading to cytoprotection and remodeling of the host cells,^11,12 inflammation,¹³ and internalization of the bacteria.¹⁴ The sortase-dependent pili, which had been previously dedicated to pathogenic Gram-positive bacteria, e.g., Actinomyces,¹⁵ Corynebacterium,^16,17 Clostridia,^16,17 Enterococci,¹⁸ and Streptococcus,¹⁹ have been recently found also in probiotic Lactobacillus rhamnosus GG and Bifidobacterium bifidum PRL2010, demonstrating that they were involved not only with adherence but also with favorable immune response of the host.^20,21

Nonpilus adhesins of several bacteria also play pivotal roles in adherence to the host and its immune response. In pathogenic bacteria, trimeric autotransporter adhesins²² and adhesins belonging to a protein family termed microbial surface components recognizing adhesive matrix molecules (MSCRAMMs)²³ were found as virulence factors in Gram-negative and -positive bacteria, respectively. MSCRAMMs specifically recognize extracellular matrix (ECM) glycoproteins and they possess conserved LPXTG or related motifs responsible for anchoring the cell wall by the action of sortase.²⁴ A unique structure of MSCRAMMs, which consists of two separate domains and a highly flexible linker, enables them to take open and closed apo forms and to capture the ligand molecule at the interspace of the two domains.^25,26 Irrespective of Gram-stainability and pathogenicity, several moonlighting proteins, which show multiple functions in different cellular localization,²⁷ have been characterized as universal nonpilus adhesins. In contrast to MSCRAMMs, moonlighting proteins are supposed to be anchorless surface proteins. The canonical functions of moonlighting proteins attribute to the essential cellular metabolic processes, such as glycolysis,²⁸ protein synthesis and quality control,²⁹ and redox homeostasis;³⁰ however, some of them act as adhesins with a wide variety of affinities for mucin, host cells, carbohydrates, ECM glycoproteins, and plasminogen/plasmin when they emerged on the bacterial cell surface, although the exhibition mechanism remains controversial.³¹

ECM in the lamina propria of the GIT is a complex of glycosaminoglycans and ECM glycoproteins, including collagen, fibronectin, laminin, elastin, thrombospondin, tenascin, and nidogen, secreted by epithelial and mesenchymal cells.^32,33 Cells are surrounded by ECM and hence embedded in the tissue, giving physical strength and flexibility, and furthermore, ECM modulates differentiation, migration, polarity, proliferation, and vitality of the cells, by stimulating cell signaling pathways via proteoglycan-cytoskeletal integrin-growth factor receptor axis.³⁴ Under physiologically normal conditions, the major part of ECM underlie the epithelial cells in the GIT, but the localized exposure of ECM to the intestinal lumen occurs transiently by cell shedding during the turnover.³⁵ Moreover, ECM seems to be extensively exposed by trauma and bacterial/viral infections,³⁶ providing a favorable target for pathogenic bacteria to adhere. In this context, commensal or probiotic bacteria capable of adhering ECM may compete with pathogens to colonize on the host surface, and hence prevent their invasion further deep into the tissue.³⁷ Indeed, commensal and pathogenic bacteria express several cell surface moonlighting proteins in common that bind to the ECM glycoproteins, e.g., enolase,^38,39 glyceraldehyde-3-phosphate dehydrogenase,^39,40 and pyruvate dehydrogenase,^41,42 supporting possibly the feasibility of so-called “competitive exclusion”.

In this issue of Virulence, Lehri et al.⁴³ demonstrated Lactobacillus fermentum 3872 competed against Campylobacter jejuni for binding type I collagen. They showed that cell numbers of Campylobacter strains which adhered to collagen I immobilized on ELISA plates were significantly decreased when L. fermentum 3872 was co-cultured in 5- to 10-times higher population over C. jejuni. Furthermore, by co-immunoprecipitation, mass spectrometry, and ELISA techniques, they revealed a unique collagen binding protein anchoring on the bacterial cell surface of L. fermentum 3872 was a major player for the competitive action, whereas flagellin was found for binding collagen I in C. jejuni. Collagen I is well known as a component of ECM of the lamina propria, but it is also likely to present in submucosa, which is a layer of tissue that connects mucosa and muscularis in the GIT of primates.⁴⁴ Calabi et al.⁴⁵ reported Clostridium diffcile adhered to collagen I via its surface layer proteins, and hence collagen I is potential to be a good target for competitive exclusion of pathogens that adhere to ECM and/or submucosa in the GIT. To ensure the results shown by Lehri et al.,⁴³ in vivo experiments should be performed in the future as was reported by Nishiyama et al.,⁴⁶ in which Lactobacillus gasseri SBT2055 reduced C. jejuni infection in chicken by competitive binding to the host cells and/or co-aggregation with the pathogen in mediation of cell-surface aggregation-promoting factors.

One of what makes it difficult to illustrate entire mechanism of host-bacterial interaction is a complexity of adhesins present on the bacterial cell surface in strain- and environment-dependent manners. Therefore, selection of suitable probiotics or the rational design of optimum conditions to achieve the ideal competitive exclusion seems too ambitious, to date. After all, such a step-by-step approach that accumulates individual data towards adhesins and natural antibacterial substances in probiotics as reported by Lehri et al.⁴³ should be a sound method to pursue suitable anti-pathogenic probiotics, leading to the reduction of antibiotics usage in combat with pathogens.

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.