Abstract
Ruminants are characterized by their unique mode of digesting cellulose-rich plant material in their forestomach, the rumen, which is densely populated by diverse microorganisms that are crucial for the breakdown of plant material. Among ruminal microbial communities, the microorganisms in the rumen fluid or attached to feed particles have attracted considerable research interest. However, comparatively less is known about the microorganisms attached to the rumen epithelium. Generally, the tissue lining the gastrointestinal tract serves the dual role of absorbing nutrients while preventing the infiltration of unwanted compounds and molecules as well as microorganisms. The rumen epithelium fulfills critical physiological functions for the ruminant host in energy absorption, metabolism, and nutrient transport. Essential host metabolites, such as short-chain fatty acids, ammonia, urea, and minerals, are exchanged across the rumen wall, thereby exposing the rumen epithelial microbiota to these nutrients. The integrity of the gastrointestinal barrier is central to animal health and productivity. The integrity of the rumen epithelium can be compromised by high ruminal microbial fermentation activity resulting in decreased rumen pH or by stress conditions such as heat stress or feed restriction. It is important to keep in mind that feeding strategies in cattle have changed over the last decades in favor of energy- and nutrient-rich concentrates instead of fiber-rich forages. These dietary shifts support high milk yields and growth rates but raised concerns regarding a possibly compromised rumen function. This paper will provide an overview of the composition of rumen epithelial microbial communities under physiological and disease conditions and will provide insights into the knowledge about the function and in situ activity of rumen epithelial microorganisms and their relevance for animal health and production. Given that an impaired intestinal barrier will negatively affect economically significant phenotypes, a better understanding of rumen wall microbiota is urgently needed.
Highlights
Ruminants are important for humans in that they produce milk and meat as major protein sources for human nutrition
Because ruminants lack the digestive enzymes for cellulose degradation, ruminants rely on their GI-tract microbiota to convert otherwise indigestible plant material into fermentation products that can be utilized by the ruminant host animal
Because GI-tract epithelial surfaces are covered by microorganisms, these epithelial microbial communities are likely intricately involved in metabolic processes occurring at or across the GI-tract epithelium
Summary
Ruminants are important for humans in that they produce milk and meat as major protein sources for human nutrition. The release of LPS can be a result of lysed bacterial cells or of rapid growth of gram-negative bacteria in the rumen or at the rumen epithelium These metabolic alterations have the potential to damage the rumen epithelium, thereby decreasing its barrier function (Plaizier et al, 2008; Zebeli and Metzler-Zebeli, 2012; Aschenbach et al, 2019). Liver abscesses are generally regarded to be sequelae to ruminal acidosis in cattle fed diets high in readily fermentable carbohydrates and low in forage Such acidotic conditions can lead to a reduction of the rumen epithelial barrier function, which can allow pathogenic bacteria to enter the systemic circulation and, when reaching the liver, result in liver abscesses. The upper layer of the stratum granulosum forms a network of tight junctions that is critical for rumen epithelial barrier function (Aschenbach et al, 2019)
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