Agriculture Reference
In-Depth Information
Structural Carbohydrate
Fermenters
Ruminococcus albus
Ruminococcus flavefaciens
Fibrobacter Succinogeneses
Butyrivibrio fibrisolvens
Eubacterium cellosolvens
Nonstructural Carbohydrate
Fermenters
Bacteriodes ruminicola
Bacteriodes amylophilus
Selenomonas ruminantium
Streptococcus bovis
Succinomonas amyloytica
Eubacteriumlimosum
Megasphaera elsdenii
Hemicellulose
Cellulose
Starch, Sugar
Pectin
Fructosans
Xylose
Glucose
Fructose
Pyruvate
Pectinolytic Species
Succinivibrio dextrinosolvens
Lachnospira multiparus
Formate
Acetate
Oxalacetate
Lactate
Organic Acid Fermenters
Megasphaera elsdenii
Veilonella alcalescens
H
2
CO
2
Succinate
Acrylate
Proteolytic Species
Peptostreptococci sp.
Clostridia sp.
CH4
Acetate
CO
2
Butyrate
Propionate
Hydrogen Utilizers
Methanobacterium ruminantium
Vibrio Succinogenes
Lipolytic Species
Anaerovibrio lipolytica
Figure 8.2
Carbohydrate metabolism in the
rumen.
Major Rumen Protozoa
Holotrichs
Isotricha
Dasytricha
Entodiniomorphs
Entodinia
Epidinium
Ophryoscolex
Cellulolytic bacteria generally have low metabolic rates
and higher generation intervals (18 hours). Fermentation
is slow and may require B vitamins, minerals, NH
3
, CO
2
,
branch chain fatty acids, and a proper pH (6.2-6.8). This
pH requirement matches the pH in the rumen of ruminants
fed forage-based diets. Small amounts of iso-acids are
required as growth factors for cellulolytic bacteria when
grown in culture. These iso-acids are isobutyrate, isovala-
rate, and 2-methylbutyrate that arise from deamination of
branched chain amino acids of valine, leucine, and isoleu-
cine, respectively. However, diet supplementation with
iso-acids is seldom needed because these acids are avail-
able due to crossfeeding from other bacteria and regularly
available from protein catabolism. The typical acetate :
propionate : butyrate
Figure 8.1
Major rumen microorganisms,
bacteria and protozoa (adapted from Van Soest,
1994).
further anaerobically to phosphoenolpyruvate (PEP) and
pyruvate. Acetate and to a lesser extent butyrate and
formate (as a transient intermediate) originate from
pyruvate. Via β-OH butyrate, pyruvate yields butyrate.
Oxaloacetate and succinate serve as transient intermediates
leading to propionate while lactate and acrylate also yield
propionate. Propionate production via oxaloacetate to suc-
cinate is the most common pathway used by rumen bacte-
ria for producing propionate although production via
acrylate is a highly effi cient pathway favored when animals
consume a high grain diet.
Megasphaera elsdinii
, a sec-
ondary fermenter, may be the main organism responsible
for this pathway. Methane, generated when acetate and
butyrate are formed, and propionate are products of sec-
ondary fermenters, and their production is important in
optimizing fermentation. Secondary methanogenic bacte-
ria convert formate to methane (Figure 8.2).
ratio
generated
from
fermentation
of cellulose fermentation is 70 : 15 : 10.
F
ERMENTATION
OF
S
TARCH
Primary amylolytic bacteria hydrolyze α1,4 and α 1,6
linkages in starch (amylose and amylopectin) to form
maltose, isomaltose, and glucose and converting them to
VFA, metabolic acids (lactic acid), and formate as a
transient intermediate. Although substrates and bacterial
species differ, the fermentation pathways generally
are similar to those described for fi ber fermentation.
Amylolytic bacteria have higher metabolic rates and lower
generation intervals (15 minutes to 4 hours). Fermentation
is rapid and requires NH
3
, amino acids, and a proper pH
(5.5-6.6). This pH optimum matches the ruminal pH of
ruminants fed diets rich in grain. The typical acetate :
