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THE DIGESTIVE SYSTEM OF VERTEBRATES

TOPIC: Microbial production of nutrients


Bacterial fermentation of carbohydrates and metabolism of nitrogen in the hindgut
Figure 9.1.  Bacterial fermentation of carbohydrates (left) and metabolism of nitrogen (right) in the hindgut of mammals.  Most of the SCFA and ammonia are absorbed from the hindgut, but the microbial protein is lost in the feces of species that do not practice coprophagy. The hindgut bacteria of birds, reptiles and adult amphibians perform similar functions, except that the major waste product of protein metabolism is uric acid, rather than urea, and it enters the hindgut in the urine via the cloaca. (From Wrong & Vince 1984 and Stevens & Hume 1995.)  (From CD Chapter 9)


Table 9.1a.  (From CD Chapter 9)
Fermentation products of rumen bacteria
Function: C = cellulolytic, X = xylanolytic, D = dextrinolytic, P = pectinolytic, PR = proteolytic, GU = glycerol-utilizing, LU = lactate-utilizing, SS = major soluble sugar fermenting. Products: F = formate, A = acetate, E = ethanol, P = propionate, L = lactate, B = butyrate, S = succinate, H = hydrogen, C = carbon dioxide. (Modified by Allison 1984 from Hespell 1981) 


Table 9.1b. (From CD Chapter 9)
Fermentation products of rumen bacteria
Function: D = dextrinolytic, P = pectinolytic, PR = proteolytic, L = lipolytic, M = methanogenic, GU = glycerol-utilizing, LU = lactate-utilizing, SS = major soluble sugar fermenting, HU = hydrogen-utilizing. Products: F = formate, A = acetate, E = ethanol, P = propionate, L = lactate, B = butyrate, S = succinate, V = valerate, CP = caproate, H = hydrogen, C = carbon dioxide, M = methane. (Modified by Allison 1984 from Hespell 1981) 



Table 9.2.  (From CD Chapter 9)
Microbial counts in the foregut of herbivorous mammals and birds
(From Stevens & Hume 1995)


Table 9.3.  (From CD Chapter 9)
Microbial counts in the midgut of vertebrates
(From Stevens & Hume 1995)


Table 9.4.  (From CD Chapter 9)
Microbial counts in the hindgut of vertebrates
(From Stevens & Hume 1995)


Pathways of carbohydrate metabolism by bacteria in the rumen
Figure 9.2.  Pathways of carbohydrate metabolism by bacteria in the ruminant forestomach. (From Van Soest 1994.)  (From CD Chapter 9)


Rate of fermentation of alfalfa in the rumen
Figure 9.3. Rate of fermentation of alfalfa components in the rumen. (From Baldwin et al. 1977).  (From CD Chapter 9)


Rumen pH, and proportions of acetic, propionic, and lactic acid
Figure 9.4. Relationship between ruminal pH and the proportions of acetic, propionic, and lactic acid produced. (From Kaufmann et al. 1980.)  (From CD Chapter 9)


Composition of rumen gases in a dairy cow fed hay and grain
Figure 9.5.  Composition of rumen gases in a dairy cow on a ration of hay and grain (Washburn & Brody 1937.)  (From CD Chapter 9)


Table 9.5.  Short-chain fatty acids in the foregut of herbivorous birds and mammals. (From CD Chapter 9)
Short chain fatty acids in the foregut of birds and mammals
Dashes indicate absence of information. Contributions of SCFA to maintenance energy were estimated from the rate of SCFA production by in vitro isotope dilution or measurements of digesta flow. Total maintenance energy was either calculated as twice  the BMR or assumed to be equivalent to ad libitum digestible energy intake in captive, nonreproducing, adult animals. (From Stevens & Hume 1995)


Concentrations of short chain fatty acids in the gastrointestinal tract of mammals
Figure 9.6. Concentrations of VFA (SCFA) along the gastrointestinal tracts of mammalian carnivores, omnivores, and herbivores. Animals were fed a at 12 hour intervals.  Each value represents the mean (+/- SE) of 12 samples, consisting of three samples collected at two, four, eight, and 12 hours after a meal, from the oral (S1) and aboral (S2) segments of the stomach, three equal-length segments of the small intestine (SI1, SI2, SI3), the cecum (Ce), and two or three equal-length segments of the colon (C1, C2, C3).  (Modified from Argenzio et al. 1974b; Clemens et al. 1975a; Clemens & Stevens 1979; Clemens 1980.)  (From CD Chapter 9)


Table 9.6.  (From CD Chapter 9)
Short chain fatty acids in the midgut of vertebrates
Dashes indicate absence of information. Contributions of SCFA to maintenance energy were estimated from the rate of SCFA production by in vitro isotope dilution or measurements of digesta flow. Total maintenance energy was either calculated as twice  the BMR or assumed to be equivalent to ad libitum digestible energy intake in captive, nonreproducing, adult animals. (From Stevens & Hume 1995)


Table 9.7a.  (From CD Chapter 9)
Short cahin fatty acids in the hindgut of vertebrates
* Absorption from cecum (or ceca) alone.
Dashes indicate absence of information. Contributions of SCFA to maintenance energy were estimated from the rate of SCFA production by in vitro isotope dilution or measurements of digesta flow. Total maintenance energy was either calculated as twice the BMR or assumed to be equivalent to ad libitum digestible energy intake in captive, nonreproducing, and adult animals. (From Stevens & Hume 1995)


Table 9.7b.  (From CD Chapter 9)
Short chain fatty acids in the hindgut of vertebrates
* Absorption from cecum (or ceca) alone.
Dashes indicate absence of information. Contributions of SCFA to maintenance energy were estimated from the rate of SCFA production by in vitro isotope dilution or measurements of digesta flow. Total maintenance energy was either calculated as twice the BMR or assumed to be equivalent to ad libitum digestible energy intake in captive, nonreproducing, and adult animals. (From Stevens & Hume 1995)


Water and volatile fatty acids in the large intestines of ponies after feeding
Figure 9.7.  Volume, net transmucosal flux of water, and net appearance and disappearance of VFA (SCFA) in the large intestine of ponies, with time after feeding. All values, other than volume, are corrected for exchanges between segments that resulted from digesta flow. (Modified from Argenzio et al. 1974 a,b.)  (From CD Chapter 9)


Table 9.8.  (From CD Chapter 9)
Vitamin requirements for growth of rabbits, guinea pigs, and mice
(From NRC 1977, 1978)


Mechanisms proposed for the transport of short chain fatty acids across the forestomach and hindgut epithelium
Figure 9.8. Mechanisms proposed for the transport of  SCFA transport across gut epithelium. Hydrogen ions produced by hydration of the CO2 in the lumen or secreted by carrier-mediated Na+/H+ exchange in the lumen-facing membrane may protonate SCFA anions (Ac-) to their undissociated form (HAc), which passively diffuses across these membranes. The H+ and HCO3-  produced by carbonic anhydrase-catalyzed intercellular hydration of CO2 produces both H+ for carrier-mediated Na+/H+ exchange and HCO3- for  exchanged with SCFA- anions in the lumen.  SCFA may be transported across the basolateral membrane by either diffusion of the undissociated form or carrier-mediated exchange of SCFA- anions with blood HCO3-.  (Modifications and combinations of models from Stevens et al. 1969; 1986 and Titus & Ahearn 1992.)  (From CD Chapter 9)


Colonic water exchange, plasma renin activity, and aldosterone levels in ponies
Figure 9.9.  Relationship between colonic water exchange, plasma rennin activity, and aldosterone levels (+/- SE) in ponies fed a pelleted hay-grain diet at 12-hour intervals. (From Clarke et al. 1990a.) (From CD Chapter 9)


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