HMGB1 acts as an agent of host defense at the gut mucosal barrier.

Mucosal barriers provide the first line of defense between internal body surfaces and microbial threats from the outside world. 1 In the colon, the barrier consists of two layers of mucus and a single layer of tightly interconnected epithelial cells supported by connective tissue and immune cells. 2 Microbes colonize the loose, outer layer of colonic mucus, but are essentially excluded from the tight, epithelial-associated layer by host defenses. 3 The amount and composition of the mucus is calibrated based on microbial signals and loss of even a single component of this mixture can destabilize microbial biogeography and increase the risk of disease. 4-7 However, the specific components of mucus, their molecular microbial targets, and how they work to contain the gut microbiota are still largely unknown. Here we show that high mobility group box 1 (HMGB1), the prototypical damage-associated molecular pattern molecule (DAMP), acts as an agent of host mucosal defense in the colon. HMGB1 in colonic mucus targets an evolutionarily conserved amino acid sequence found in bacterial adhesins, including the well-characterized Enterobacteriaceae adhesin FimH. HMGB1 aggregates bacteria and blocks adhesin-carbohydrate interactions, inhibiting invasion through colonic mucus and adhesion to host cells. Exposure to HMGB1 also suppresses bacterial expression of FimH. In ulcerative colitis, HMGB1 mucosal defense is compromised, leading to tissue-adherent bacteria expressing FimH. Our results demonstrate a new, physiologic role for extracellular HMGB1 that refines its functions as a DAMP to include direct, virulence limiting effects on bacteria. The amino acid sequence targeted by HMGB1 appears to be broadly utilized by bacterial adhesins, critical for virulence, and differentially expressed by bacteria in commensal versus pathogenic states. These characteristics suggest that this amino acid sequence is a novel microbial virulence determinant and could be used to develop new approaches to diagnosis and treatment of bacterial disease that precisely identify and target virulent microbes.

Evolutionary and ecological forces that shape the bacterial communities of the human gut.

Since microbes were first described in the mid-1600s, we have come to appreciate that they live all around and within us with both beneficial and detrimental effects on nearly every aspect of our lives. The human gastrointestinal tract is inhabited by a dynamic community of trillions of bacteria that constantly interact with each other and their human host. The acquisition of these bacteria is not stochastic but determined by circumstance (environment), host rules (genetics, immune state, mucus, etc), and dynamic self-selection among microbes to form stable, resilient communities that are in balance with the host. In this review, we will discuss how these factors lead to formation of the gut bacterial community and influence its interactions with the host. We will also address how gut bacteria contribute to disease and how they could potentially be targeted to prevent and treat a variety of human ailments.