by the mixing contractions of the rumen
(Ulyatt et al., 1986). Rumination is also a
potent stimulus for salivation. The con-
tinual addition of saliva to the fermenta-
tion helps neutralize acids generated by
fermentation. This is important because
acid inhibits fermentation.
The feeds consumed by ruminants to
be digested by the microbial ecosystem are
physically much larger than the micro-
organisms themselves. The micro-
organisms, therefore, must solubilize the
feeds by secreting enzymes to digest the
feed chemically into soluble components.
There are many different enzymes manu-
factured intracellularly by the micro-
organisms for excretion into the ruminal
environment in various forms. These
enzymes include cellulases, hemi-
cellulases, pectinases, amylases, proteases
and lipases. There are multiple enzymes
within each class that act concurrently in
hydrolysing the substrate into a soluble,
absorbable form. The enzymes may be
released altruistically into the environment
of the microorganism, i.e. whatever effect
the released enzyme has in making soluble
substrate available to the microorganisms
by catalysing hydrolysis of substrate, the
soluble products of hydrolysis are available
to any microorganism in the area. The
released enzyme may remain attached to
the exterior surface of the secreting micro-
organism similarly to the brush border
enzymes described previously. As the
microorganism comes in contact with sub-
strate, the solubilized products are gener-
ated in very close proximity to the cell that
manufactured the enzyme. Many micro-
organisms attach themselves to the ‘surface’
(keep in mind this is an inner surface of
the substrate to which the microorganism
gained access via a nick in the outer sur-
face, i.e. the substrates are digested from
the inside to the outside) of the substrate
by means of a glycoprotein matrix between
the cell wall of the substrate and the cell
wall of the microorganism. This is the pre-
dominant mode of digestion (Cheng et al.,
1991). Enzymes are secreted into the space
enclosed by the glycoprotein matrix. As
substrate is solubilized, the products are
captured by the glycoprotein matrix, mak-
ing these nutrients available only to the
microorganism that secreted the enzyme.
Some microorganisms secrete enzymes that
hydrolyse the structural components of
plant material but do not use the solubilized
products as an energy substrate. Like the
chitinase activity in poultry, this hydrolysis
is necessary to gain access to more desir-
able substrate. The solubilized structural
components, however, are used as substrate
by other species of microorganisms.
Not all microorganisms have the same
complement of enzymes in the same propor-
tions. Ruminal microorganisms can be
organized into 11 groups by substrate and
product preferences (Yokoyama and
Johnson, 1988), though there is considerable
overlap of species among the substrate/
product-grouping scheme.
1.Cellulolytic, e.g. Bacteroides succinogens,
Ruminococcus flavefaciens, Ruminococcus
albusand Butyrivibrio fibrisolvens.
2.Hemicellulolytic, e.g. Butyrivibrio
fibrisolvens, Bacteroides ruminicola and
Ruminococcussp.
3.Pectinolytic, e.g. Butyrivibrio fibrisolvens,
Bacteroides ruminicola, Lachnospira multi-
parus, Succinivibrio dextrinosolvens,
Treponema bryantii and Streptococcus
bovis.
4.Amylolytic, e.g. Bacteroides amylophilus,
Streptococcus bovis, Succinimonas amylo-
lyticaand Bacteroides ruminicola.
5. Ureolytic, e.g. Succinivibrio dextrino-
solvens, Selenomonas sp., Bacteroides
ruminicola, Ruminococcus bromii,
Butyrivibriosp. and Treponemasp.
6.Methanogens, e.g. Methanobrevibacter
ruminantium, Methanobacterium formi-
cicumand Methanomicrobium mobile.
7.Sugar-utilizing, e.g. Treponema bryantii,
Lactobacillus vitulinusand Lactobacillus
ruminus.
8.Acid-utilizing, e.g. Megasphera elsdenii
and Selenomonas ruminantium.
9.Proteolytic, e.g. Bacteroides amylophilus,
Bacteroides ruminicola, Butyrivibrio
fibrisolvensand Streptococcus bovis.
10.Ammonia-producing, e.g. Bacteroides
ruminicola, Megasphera elsdenii and
Selenomonas ruminantium.132 R.W. Russell and S.A. Gahr