Courtesy Katherine L. Knight and Ki-Jong Rhee © 2004 American Association of Immunologists

Left is a section of appendix following introduction of Bacteroides fragilis; right is a section of appendix following introduction of B. fragilis plus Bacillus Subtilis. B. fragilis alone results in no gut-associated lymphoid tissue (GALT) development while the combination shows a B lymphocyte follicle (red dots).

Commensal or nonpathogenic bacteria have established a mutually beneficial relationship with the host immune system that allows them to reside peacefully within the lumen of a healthy host's gut. Lumenal colonization by diverse commensal species is essential for host health, permitting processing of indigestible carbohydrates and preventing colonization by pathogenic bacteria. But it's a delicate balance. "How [does the gut] maintain the necessary bacterial concentration but not become inflamed?" asks Andrew Neish, a pathologist at Emory University School of Medicine in Atlanta.

The way that bacteria...

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