Germs gone wild

Abstract

The normally harmless behavior of bacteria in the intestinal tract is maintained by community structure and the integrity of host defenses. When either or both of these are compromised, a few disgruntled outcasts can cause a riot, taking down the whole neighborhood (pages 799-806).

Figure 1. Epithelial integrity and microbial composition affects intestinal homeostasis and the innate immune response

(a) Pathobionts are normally present at low density in microbial community dominated by symbionts; (b) antibiotic mediated depletion of symbionts leads to expansion of antibiotic resistant pathobionts, however intact epithelial barrier prevents their invasion; (c) disruption of epithelial integrity can lead to a transient invasion of symbionts that are rapidly cleared by the immune system; (d) disruption of epithelial integrity in the context of antibiotic induced dysbiosis leads to invasion of pathobionts and subsequent septicemia. E. coli O21:H+ carries a variant of virulence gene cluster known as ETT2_sepsis, which may promote flagellin secretion through the type 3 secretion system (T3SS). Flagellin secreted into the host macrophages activates the NAIP5–NLRC4–IL-1β pathway ultimately leading to multiple organ damage caused by systemic effects of IL-1β.