Abstract
Segmented filamentous bacteria (SFB) are a group of host-adapted, commensal organisms that attach to the ileal epithelium of vertebrate and invertebrate hosts. A genetic relative of the genus Clostridium, these morphologically unique bacteria display a replication and differentiation lifecycle initiated by epithelial tissue binding and filamentation. SFB intimately bind to the surface of absorptive intestinal epithelium without inducing an inflammatory response. Rather, their presence impacts the generation of innate and differentiation of acquired immunity, which impact the clearance of extracellular bacterial or fungal pathogens in the gastrointestinal and respiratory tracts. SFB have recently garnered attention due to their role in promoting adaptive and innate immunity in mice and rats through the differentiation and maturation of Th17 cells in the intestinal tract and production of immunoglobulin A (IgA). SFB are the first commensal bacteria identified that impact the maturation and development of Th17 cells in mice. Recently, microbiome studies have revealed the presence of Candidatus Arthromitus (occasionally designated as Candidatus Savagella), a proposed candidate species of SFB, in higher proportions in higher-performing flocks as compared to matched lower-performing flocks, suggesting that SFB may serve to establish a healthy gut and protect commercial turkeys from pathogens resulting in morbidity and decreased performance. In this review we seek to describe the life cycle, host specificity, and genetic capabilities of SFB, such as bacterial metabolism, and how these factors influence the host immunity and microbiome. Although the role of SFB to induce antigen-specific Th17 cells in poultry is unknown, they may play an important role in modulating the immune response in the intestinal tract to promote resistance against some infectious diseases and promote food-safety. This review demonstrates the importance of studying and further characterizing commensal, host-specific bacteria in food-producing animals and their importance to animal health.
Highlights
The distal gastrointestinal tract of all animals is colonized by a diverse array of bacterial, fungal, and protozoan species
The hostspecific binding mechanism employed by these bacteria to attach to ileal epithelium is similar to that of enteric pathogens
As demonstrated in mice and rat models, segmented filamentous bacteria (SFB) have been shown to stimulate the maturation of the host’s Th17 and immunoglobulin A (IgA) responses, improving the ability of the host to protect against invading pathogens
Summary
The distal gastrointestinal tract of all animals is colonized by a diverse array of bacterial, fungal, and protozoan species. Many vertebrate intestines (such as mice, rats, chickens, humans, and turkeys) harbor commensal organisms named segmented filamentous bacteria (SFB) that bind to the host intestinal epithelium. Segmented filamentous bacteria selectively attach to the ileal epithelium of the host species via the production of rounded, nipple-like projections called holdfasts.
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