Ruminal microbes, microbial products, and systemic inflammation

Abstract Text:

The ruminal ecosystem is inhabited by complex communities of microbes that include bacteria, archaea, protozoa, fungi and viruses.  The immune system of the animal has evolved to maintain tolerance to innocuous gut commensals and induce protective responses to pathogens.  Besides fermentative role, ruminal microbes do have the potential to influence the overall health of the host because of their ability to induce systemic inflammation.  The ruminal epithelium-vascular interphase allows absorption of fermentation products and also serves as a selective barrier to prevent translocation and systemic dissemination of bacteria, bacterial toxins, and immunogenic factors.  Ruminal dysbiosis that increases ruminal acidity and osmolarity may increase permeability and even induce a breach in the integrity of the epithelial and vascular endothelial barriers, thus facilitating entry of bacteria or bacterial antigens into the portal vein.  A classic example is the delivery of ruminal bacterium, Fusobacterium necrophorum, into the liver to cause abscesses, which is facilitated by ruminal damage induced by excessive accumulation of lactic acid or VFA.  Bacteria that manage to exit or bypass the liver can cause systemic inflammation in other organs, such as lungs, heart, joints, hoof, etc.   Shifts in microbial populations associated with dysbiosis result in increased concentrations of potentially toxic and inflammatory substances, which include endotoxic lipopolysaccharide (LPS), biogenic amines, ethanol, etc.  A bacterial product that has received a lot of interest is LPS, a component of all Gram negative bacteria.  The entry of LPS into the systemic circulation, either from the rumen or the lower gut, could trigger release of proinflammatory cytokines, reactive oxygen and nitrogen intermediates, and bioactive lipids.  The inflammatory response to the presence of ruminal LPS in the blood is evidenced by increase in acute phase proteins, such as haptoglobin and LPS binding protein.  Biogenic amines generated in the rumen that could lead to inflammation include histamine, tyramine, and ethanolamine.  Histamine that is absorbed from the rumen or produced endogenously in tissues during inflammation plays a key role in the development of laminitis.  Ethanolamine derived from bacterial phospholipids has the potential to enhance growth and virulence of certain gut pathogens.  In conclusion, ruminal microbes and their products generate many complex interactions with the host immune system, and dysbiosis has the potential to induce systemic inflammation.  Although inflammation is a protective reaction, the persistence of inflammatory mediators could have negative consequences for the host.

Keywords: Ruminal microbes, dysbiosis, inflammation