Abstract
Antibiotics can improve ruminant growth and efficiency by altering rumen fermentation via selective inhibition of microorganisms. However, antibiotic use is increasingly restricted due to concerns about the spread of antibiotic-resistance. Plant-based antimicrobials are alternatives to antibiotics in animal production. The hops plant (Humulus lupulus L.) produces a range of bioactive secondary metabolites, including antimicrobial prenylated phloroglucinols, which are commonly called alpha- and beta-acids. These latter compounds can be considered phyto-ionophores, phytochemicals with a similar antimicrobial mechanism of action to ionophore antibiotics (e.g., monensin, lasalocid). Like ionophores, the hop beta-acids inhibit rumen bacteria possessing a classical Gram-positive cell envelope. This selective inhibition causes several effects on rumen fermentation that are beneficial to finishing cattle, such as decreased proteolysis, ammonia production, acetate: propionate ratio, and methane production. This article reviews the effects of hops and hop secondary metabolites on rumen fermentation, including the physiological mechanisms on specific rumen microorganisms, and consequences for the ruminant host and ruminant production. Further, we propose that hop beta-acids are useful model natural products for ruminants because of (1) the ionophore-like mechanism of action and spectrum of activity and (2) the literature available on the plant due to its use in brewing.
Highlights
TO RUMEN FUNCTIONThe rumen is the distinguishing adaptation of the ruminant animal
The rumen epithelium absorbs certain nutrients, and it serves as habit for a dense community of phylogenetically diverse microorganisms [2, 3]
Some of the ammonia produced by HAB and other microorganisms is assimilated into microbial protein that can be digested by the host
Summary
The rumen is the distinguishing adaptation of the ruminant animal. This first chamber of the digestive tract is, in some respects, more like an intestine than a gastric stomach [1]. Protozoa and some Gram-negative bacteria, like M. elsdenii, can utilize amino acids and produce ammonia. A particular group of rumen bacteria are known for exceptional rates of ammonia production via fermentation (i.e., deamination of peptides and free amino acids). This guild of amino acid-fermenters is termed the hyper-ammoniaproducing bacteria, HAP or HAB [9]. Like the saccharolytic bacteria, which metabolize sugars only after other bacteria degrade the polymer, many of the HAB ferment amino acids only after other members of the microbial community depolymerize protein [10]. Some of the ammonia produced by HAB and other microorganisms is assimilated into microbial protein that can be digested by the host. Some methane production is necessary for rumen microbial ecology, but both products represent matter lost from the system
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