Segmented filamentous bacteria - metabolism meets immunity

Authors: HEDBLOM, GRANT - University Of Minnesota; REILAND, HOLLY - University Of Minnesota; Sylte, Matthew; JOHNSON, TIM - University Of Minnesota; BAUMLER, DAVID - University Of Minnesota

Submitted to: Frontiers in Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/7/2018
Publication Date: 8/25/2018
Citation: Hedblom, G.A., Reiland, H.A., Sylte, M.J., Johnson, T.J., Baumler, D.J. 2018. Segmented filamentous bacteria - metabolism meets immunity. Frontiers in Microbiology. 9:1991. https://doi.org/10.3389/fmicb.2018.01991.
DOI: https://doi.org/10.3389/fmicb.2018.01991

Interpretive Summary: The presence of certain bacteria in the intestinal tract of animals are thought to benefit the development of their immune system, and enhance their ability to fight off infections. Segmented filamentous bacteria (SFB), initially identified in the 1970s in many animal species, was recently identified to enhance the development of the immune system in the intestinal tract of mice and prevent infection by pathogenic bacteria, but how these bacteria survive within the animal’s intestinal tract and their interaction with the intestinal cells are beginning to be understood. Segmented filamentous bacteria are thought to be part of the good bacteria in the intestinal tract of animals, and do not appear to cause disease in mammals. Bacteria such as SFB are unique amongst other good intestinal bacteria because they associate with intestinal cells without causing any harm to the cells. The close interaction of SFB and the intestinal cells allows SFB to receive nutrients from the animal, which is likely involved to satisfy SFBs strict metabolic and nutritional requirements for growth. The close interaction to intestinal cells benefits the animal’s health because molecules from SFB, called antigens, are transported inside the intestinal tract and help to develop the immune system and protect against some infections. Meat-producing turkeys have SFB in their intestinal tract, however some develop “Light Turkey Syndrome”. This is a disease where they weigh significantly less than healthy turkeys and are more susceptible to intestinal infections. Light turkey syndrome may be caused by the presence of fewer SFB in their intestinal tract. Very little is known about the SFB in the intestinal tract of meat-producing turkeys, and whether they impact the development of the immune system or their susceptibility to intestinal infections. Developing an understanding of whether SFB impacts intestinal immunity of meat-producing turkeys may offer solutions to enhance digestive and immune health.

Technical Abstract: Segmented filamentous bacteria (SFB) are a group of host-adapted, gut 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 life cycle initiated by epithelial binding, making it difficult to culture these bacteria in vitro. SFB intimately bind to the surface of absorptive intestinal epithelium without inducing an inflammatory response. Rather, their presence impacts the generation of innate and acquired immunity and 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) and stimulating the differentiation and maturation of Th17 cells. 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 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 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 will serve to demonstrate the importance of studying and further characterizing commensal, host-specific bacteria, as many animal studies have demonstrated their importance to host health.