12-14. ruminant/horse/avian digestive phys

microbiota

• refers to the community of microogranisms (microbes) in a particular body site or environment

• not referring to the genomes present

microbiome

collection of the organisms in the microbiota including the genes encoded in their genomes → implies functional potential of the microbiota

what are functions of the microbiome?

• stimulate immune system

• decrease pathogen colonization

• regulate energy metabolism

• produce vitamins (more important in ruminants)

• increase energy harvest from diet

how can a host influence their microbiome?

• colonization at birth

• host genetics, immune system

• environment

• diet (change substrates provided to microbes)

cellulases

• enzymes that hydrolyze beta 1,4 linkages (cellulose)

• microbes have these enzymes; mammals do not

what are some advantages of being a ruminant?

• ability to subsist on low quality diets

• ability to digest cellulose and other components of plant structural carbohydrates

• unique ways of meeting protein needs

  • microbial lysis provides high value protein to host

  • recycle their “waste” (urea) for microbial use

• get vitamins B and K without having to eat them

  • made by microbes and are available to host

what are some requirements of the ruminant lifestyle?

• requires large time commitment to chewing and ingesting food at regular intervals

• nurture rumen population and environment

• have metabolic pathways that efficiently utilize volatile fatty acids (VFAs)

  • less reliance on glucose (microbes consume it first)

ventral layer (liquid zone)

• fluid and fine, small particulate ingesta

• provides the material that eventually flows out of the rumen/reticulum into the omasum

slurry zone

• gradation of particle size from highly fibrous, longer pieces to shorter, denser ones

• semi-digested, semi-solid layer

• some of this material is ruminated (regurgitated) as the cud bolus

• located above ventral layer; below mat layer

mat zone

• floating layer of fibrous plant material; contains the most recently ingested food

• high densities of microbes

• fluid in the mat layer contains salivary constituents and fermentative end products that are gradually being absorbed across the ruminal epithelium

• located above the slurry zone

how do the proportions of VFAs produced vary between high fiber and high starch (concentrate) diets?

• high fiber diet (↑ roughage)

  • less VFAs produced overall

  • produce more methane

  • greater % of acetate; lower % of propionate

• high starch diet (↑ concentrates)

  • more VFAs produced

  • produce more lactic acid

  • lower % of acetate; higher % of propionate

**note: acetate is always produced in the greatest quantities

how is dietary protein digested in ruminants?

• some moves to abomasum/small intestine for digestion & absorption

• bacterial proteases break dietary proteins down into peptides

• peptides are absorbed into bacteria where peptidases break them down further into amino acids

• amino acids have two fates:

  • used by microbes for growth

  • deamination

what are the products of amino acid deamination?

• VFAs → can be used by host animal for energy/gluconeogenesis

• ATP

• NH₃ → converted to urea in the liver

what are the possible fates for urea?

• excreted by kidneys

• “nitrogen recycling/urea nitrogen salvage”

  • urea circulating in blood diffuses back to rumen where it is broken down by urease (bacterial cell walls) and reused

• some is taken up by salivary glands, secreted into saliva, and re-enters rumen after swallowing

how is microbial protein digested in ruminants?

• lysozymes secreted by the abomasal epithelial cells hydrolyze bacterial cell walls → optimized for low pH

• releases amino acids & short peptides which can be absorbed in the small intestine

lipid digestion in ruminants

• dietary triglycerides and phospholipids are hydrolyzed by microbes into fatty acids and glycerol

  • glycerol can be fermented into VFAs and used by animal

  • rumen microbes hydrogenate unsaturated fatty acids into saturated fatty acids → one way to dispose of H⁺ in rumen

• medium, long-chain fatty acids are absorbed in the small intestine

why do rumen microbes hydrogenate unsaturated fatty acids?

unsaturated fatty acids are toxic to microbes → saturated fatty acids are less toxic

how are volatile fatty acids absorbed in the rumen?

1. Na⁺-coupled transport

   • SMCT (monocarboxylate) transporter can transfer ionized form in with Na⁺

2. passive diffusion

   • add H⁺ to neutralize charge → can now diffuse across cell membrane

**note: Na⁺, Cl^-, and H₂O are absorbed with VFAs

how is acetate utilized by the ruminant?

• used for energy (ATP) throughout the body

  • converted to acetyl CoA → citric acid cycle

• produced in greatest amounts

how is propionate utilized by the ruminant?

• gluconeogenesis in liver or oxidized for energy (ATP)

  • gluconeogenesis important for milk production in lactating cows

how is butyrate utilized by the ruminant?

preferred fuel source for rumen epithelial cells (critical for rumen growth / development)

what is the function of primary contractions in the rumen?

• mixing and particle separation

  • shuttles well-digested feed to omasum

• starts in reticulum

• most frequent type of contraction in rumen/reticulum

what are the functions of secondary contractions in the rumen?

• enhances mixing of rumen contents

• moves gas to cardia for eructation

• starts from dorsal sac, not reticulum

what is the function of the cranial pillar in eructation?

cranial pillar holds back fluid/feed from entering the esophagus and prevents it from getting aspirated

bloat

• distention of the rumen/reticulum caused by accumulated gas

• results from prevention of eructation → gas trapped in stomach

what are the functions of the omasum?

• some fermentation; remaining VFA absorption

• filters out large particles for further breakdown; allows only fine particles to move into the abomasum

what are the functions of the abomasum?

• secretes HCl, water, pepsin, lysozyme, chymosin (rennin) in neonates

• regulates flow into the duodenum (gastric emptying)

  • stimulated by antral/pyloric distention

  • inhibited by duodenal distention, low duodenal pH

• severe distention of the abomasum inhibits primary waves in rumen

what controls forestomach motility in ruminants?

• vago-vagal reflexes: afferent and efferents via vagus nerve

  • motility center in medulla

• motility is stimulated by fibrous feeds but inhibited by high concentrate diets

• motility also inhibited by:

  • severe distension

  • pH < 5

reticuluar groove

• groove formed during suckling of neonates → allows milk to bypass the rumen/reticulum and go to abomasum

• reflex initiated by tactile stimulation of the pharynx

how is rumen/microbial population development stimulated?

• microbiota established during birth

• microbial contaminants in milk

• ingestion of contaminated feed, water

• contact with feces, cud from adults

• VFAs stimulate mucosal growth in forestomach, especially butyrate

• solid feed (bulk) stimulates muscular growth and development of reflexes

what are the motility patterns in the cecum & colon of horses?

• segmentation (mixing): promotes fermentation and short chain fatty acid absorption

• propulsive peristalsis: moves ingesta distally

• retropulsive peristalsis: moves ingesta orally; increases time for microbial fermentation and absorption of VFAs, water, and electrolytes

why do horses need to have higher quality dietary protein?

microbial protein in the hindgut is largely unavailable to the host, so equine diets must have more digestible protein than bovids

how is hindgut pH maintained (hingut fermenters)?

• pancreatic and ileal secretions provide HCO₃^- and phosphate buffers to maintain pH at optimal levels during carbohydrate fermentation

• additional HCO₃^- is secreted by hindgut epithelial cells (cecum, colon)

what is the function of the avian crop?

• storage of food

• similar to fundus of monogastric stomach

what is the function of the proventriculus?

• glandular portion of stomach → secretes acid, pepsin

• propels food into the ventriculus

• analogous to the body of the monogastric stomach

what are the functions of the ventriculus (gizzard)?

• maceration (mechanical breakdown), storage, propulsion of food into small intestine

• analogous to antrum of monogastric stomach

how does the ventriculus differ in carnivores vs granivores?

• carnivores: thin-walled, distensible; less need for mixing and grinding

• granivores: highly muscular, non-distensible; high need for grinding of food