Download rumen microbiology-2012

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Animal Microbe Interactions
Most interactions between microbes and animals are beneficial. The mutualistic relationships of microbial and
animal population involve nutrient exchange and maintenance of a suitable habitat. These associations help
the animals to digest difficult components of their diet particularly cellulose.
Intestinal symbionts may be commensals or benefit the animal through vitamin production and protection
against pathogens. The endozoic algae of coral polyps and other invertebrates supply a major part of the
animal’s nutritional needs through their photosynthetic activity. Associations with chemoautotrophic bacteria
in deep-sea thermal vent environment allow invertebrates to live on geothermal energy independent of
photosynthetically produced organic carbon. In a less common mutualistic relationship, endosymbiotic
bacteria produce light for some marine invertebrates and fish.
Interaction Between Microbes and the Gut
Most warm-blooded animals contain extremely complex microbial communities within their gastrointestinal
tracts. In the human intestine, the strict anaerobes belonging to the genera Bacteroides, Fusobacterium,
Bifidobacterium and Eubacterium are found in large numbers.
In some animals such as pigs, the gastrointestinal tract microflora contribute to the nutrition of the animal by
fermenting carbohydrates. The other activities include amino acid degradation, supply of required vitamins by
the microorganisms, e.g. synthesis of vitamin K. The presence of gut microflora and the pre-emptive
colonisation, constitute an important barrier to attack by intestinal pathogens. In this way, the gut microflora
playa role as symbionts.
Interaction Between Microbes and the Rumen
Ruminants ate animals that ingest and digest cellulose-rich foods and include the herbivores like cow, sheep,
giraffe, deer, moose, antelope and goat. These animals do not produce the enzyme cellulase to digest the food,
instead they have an organ called the rumen where the cellulose is broken down to simpler compounds by the
action of rumen microflora and other accessory materials.
The rumen provides a relatively uniform and stable anaerobic environment that has a temperature of 30-40°C,
a
pH
of
5.5-7.0
and
a
continuous
supply
of
ingested
material.
There is a long residence time for the food in the rumen. The rumination process grinds the plant material and
provides an increased surface area for microbial attack. The animal's saliva also contributes to rendering the
ingested plant material susceptible to microbial attack. The movement of the ruminant stomach supplies
sufficient mixing for optimal microbial growth and metabolic activities.
The food in the rumen is mixed by the musculature of the walls of the rumen and is comminuted by chewing
when it is returned to the mouth. The liquid part is contributed by the water that is drunk by the animal and the
saliva. The saliva contains a buffering system, that makes available all the nutrients in simpler form, i.e. the
nutrients that can be digested by the animal is made simpler and is obtained both by the animal and by the
microbes in the rumen.
The overall fermentation that occurs within the rumen converts cellulose, starch and other ingested nutrients
to CO2, H2' CH4 and low-molecular weight organic acids such as acetic, propionic and butryric acids. The
organic acids are absorbed into the bloodstream of the animal, where they are oxidised aerobically to produce
energy. The fermentatively produced CO2 and methane by methanogenic bacteria within the rumen are
expelled and do not contribute to the nutrition of the animal.
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Microbes of the Rumen
The rumen harbours a great diversity of microorganisms. The bacterial population includes the cellulose
digestors, starch digestors, hemicellulose digestors, sugar fermentors, fatty acid utilisers, methanogenic
bacteria, proteolytic bacteria and lipolytic bacteria. These populations include Bacteroides, Ruminococcus,
Succinomonas, Methanobacterium, Butyrivibrio, Selenomonas, Succinivibrio, Streptococcus, Eubacterium
and Lactobacillus.
These bacterial populations produce acetate, the predominant acid
within the rumen. The bacteria also produce propionate, the only
fermentation acid that can be converted into carbohydrates by the
ruminant. Some nitrogen fixation activity also takes place in the
rumen.
Bacteria:
Bacteria make up about half of the living organisms inside of the
rumen. However, they do more than half of the work in the
rumen. The bacteria work together. Some breakdown certain
carbohydrates and proteins which are then used by others. Some require certain growth factors, such as Bvitamins, which are made by others. Some bacteria help to clean up the rumen of others’ end products, such as
hydrogen ions, which could otherwise accumulate and become toxic to other organisms. This is called “crossfeeding”.
Classification of Rumen Bacteria
Fiber–Digesting (or Cellulolytic) Bacteria:
The fiber-digesters are some of the “fussiest” bacteria in the rumen. They are very sensitive to acid. When a
cow has acidosis (pH<6.0), the rumen produces a lower proportion of acetate to propionate because the fiberdigesters who primarily make acetate are not working well. Also, high levels of rumen available fat (generally
over 5% of the diet) reduce the growth of the fiber-digesters. The exact reason for fat’s negative effect on the
fiber-digesters is not known. Some think that it reduces the microbe’s ability to move nutrients into and out of
its body. Others think that the fat coats fiber particles making it difficult for the fiber-digesting microbes to get
in to do their work.
Bacterial species: Ruminococcus flavefacians, Ruminococcus albus, Bacteriodes succinogenes, Butyrivibrio
fibrisolvens
Growth Requirements: Cellulose, Hemicellulose, Pectin
Many also Require: Ammonia, Isoacids (Branched-Chain Amino Acids), Starch, B-vitamins - Fermentation
Products: Acetate, Butyrate, Hydrogen (H2), Carbon Dioxide (CO2)
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Rumen pH Requirement: High pH (above 6.0)
Fat Tolerance: Low
Susceptibility to Ionophores (Bovatec and Rumensin): Some are susceptible
Reproduction Speed: Slow
Starch and Sugar-Digesting (or Amylolytic) Bacteria:
Starch and sugar-digesters make up a significant part of the rumen’s bacterial population. Generally, highproducing dairy cows are fed diets containing more than 30% starches and sugars, so these bacteria are greatly
needed. Even if a cow is on an all-straw diet, the fiber-digesters still never account for more than 25% of the
rumen bacterial population. Starch and sugar-digesters are still present, cross-feeding off of the fiberdigesters’ byproducts.
Bacterial species:
Bacteriodes ruminocola, Bacteriodes amylophilus, Selenomonas ruminantium, Streptococcus bovis,
Succinomonas amylolytica
Growth Requirements:
Sugar, Starch, Peptides, Amino Acids
Many Also Require: Ammonia, B-vitamins
Fermentation Products:
Propionate, Butyrate, Acetate, Lactate, Hydrogen (H2), Carbon Dioxide (CO2)
Rumen pH Requirement: Tolerate a lower (more acidic) pH (5.7)
Fat Tolerance:
Higher than fiber digesters
Susceptibility to Ionophors (Bovatec and Rumensin):
Most aren’t susceptible
Reproduction Speed:
Faster than fiber digesters
Streptococcus bovis, “The Rumen Weed”
Streptococcus bovisis present only when large amounts of starch or sugars are fed and pH is low. It produces
lactic acid, a stronger acid than many of the other VFA’s produced in the rumen. When conditions are
favorable for Streptococcus bovis, it will grow explosively (doubling every 13 minutes). This type of growth
produces rumen acidosis. Streptococcus bovis is controlled by ionophores. This is one of the major reasons
for the favorable growth responses seen by the addition of ionophores to the diets of feedlot cattle.
Lactate-Using Bacteria:
Includes: Megasphaera elsdenii, “The Rumen Maid”
As mentioned above, some bacteria, such as Streptococcus bovis, produce a strong acid called lactic acid.
Megasphaera elsdenii uses lactic acid to grow. This helps to clean up the rumen a bit and raise rumen pH,
aiding the growth of the acid-intolerant fiber-digesters.
Hydrogen-Using (or Methane) Bacteria:
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Under normal rumen conditions, hydrogen (H2) does not accumulate in the rumen because it’s used by
hydrogen-using bacteria, such as Methanobacterium ruminantium.
Growth Requirements: Carbon dioxide and hydrogen
Fermentation products: Methane
The methane bacteria commonly produce methane in this way:
4H2 + CO2
---------> CH4 + 2H2O
Protozoa:
The next abundant population of microflora is the protozoans. Most are ciliates, but some flagellates such as
Diplodinium, Sarcodina, etc. are also present. They digest cellulose and starch, some ferment dissolved
carbohydrates. Some are predators on bacterial populations. The proteins of the protozoan are in turn digested
by the ruminant's enzymes. The rumen protozoa store large amounts of carbohydrates which the ruminants
digests along with the proteins of the protozoan biomass. The protozoans are digested readily than the bacteria
because
the
latter
have
resistant
cell
walls
and
high
nucleic
acid
contents.
As much as 50% of the microbial mass in the rumen can be made up of protozoa. However, their role, as
compared to the rumen bacteria, is not as significant. The protozoa are actually predators to the bacteria in the
rumen --- they eat the bacteria for dinner! Protozoa are about 40 times the size of rumen bacteria.
The rumen protozoa produce fermentation end-products similar those made by the bacteria, particularly
acetate, butyrate, and hydrogen. Rumen methane bacteria actually attach and live on the surface of rumen
protozoa for immediate access to hydrogen.
Rumen protozoa eat large amounts of starch at one time and can store it in their bodies. This may help to
slow down the production of acids that lower rumen pH, benefiting the rumen.
Rumen protozoa multiply very slowly in the rumen --- over 15-24 hours – as opposed to the bacteria that may
take as little as 13 minutes to multiply. For this reason, the rumen protozoa hide out in the slower moving
fiber mat of the rumen so that they aren’t washed out before they have a chance to multiply. Low roughage
diets reduce the retention of fiber in the rumen and may decrease the number of protozoa in a cow’s rumen.
Rumen Fungi:
Fungi are known to exist in the rumen (up to 8% of the total mass) but they are poorly understood. They
attach to feed particles and they reproduce very slowly. They may help out the fiber-digesting bacteria by
doing some of the initial work of splitting fibrous material apart and making it more accessible for the
bacteria. Higher numbers of fungi have been found in the rumens of cows fed very poorly digestible subtropical forages.
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The high diversity of microbial population in the rumen depends on the diet of the ruminant. When there is a
sudden change in the diet, there is an upset of the rumen fermentation system resulting in excessive
production of methane that can distend the rumen, sometimes to the extent that it compresses the lungs thus
suffocating the animal. The fungal population is comparatively low and only some genera of yeasts are
present. The food consumed by the ruminant goes to the rumen where there is great microbial diversity and
this diversity helps in the digestion of the food consumed. The rumen microbes digest the food for the
ruminant and in turn the ruminant provides shelter and food to the microbes.
Establishment of the rumen microbial population:At birth, rumen has no bacteria.
Normal pattern of establishment
Appear
Peak
Microorganisms
5-8 hours
4 days
E. coli, Clostridium welchii, Streptococcus bovis.
½ week
3 weeks
Lactobacilli
½ week
5 weeks
Lactic acid-utilizing bacteria
½ week
6 weeks
Amylolytic bacteria ,Prevotella-wk 6
1 week
6-10 weeks Cellulolytic and Methanogenic bacteria Butyrvibrio-wk 1Ruminococcus-wk 3,
Fibrobacter-wk 1
1 week
12 weeks
Proteolytic bacteria
3 weeks
5-9 weeks
Protozoa
9-13 weeks Normal population
Factors affecting establishment of population
– Presence of organisms
• Normally population is established through animal-to-animal contact
• Bacteria may establish without contact with mature ruminants
– Establishment of protozoa requires contact with mature ruminants
– Favorable environment
• Substrates and intermediates
• Increased rumen pH
• Digesta turnover
• Altering the rumen population
– Diet
• High forage > High pH, cellulose, hemicellulose, sugars
> High cellulolytic and hemicellulolytic bacteria
> High methanogens
> High protozoa
• High concentrate> Low pH, high starch
> Low cellulolytic and hemicellulolytic bacteria
> High amylolytic bacteria
> Low methanogens
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> Low protozoa, primarily oligotrichs
– Buffers
• Same as high forage
– Antibiotics
• Ionophores
– Microbial inoculants
– Ionophore effects on the rumen microbial population
– Ionophores
• Monensin
• Lasalocid
• Laidlomycin
– Actions
• Create pores in membranes of gram + bacteria
– Allows potassium to exit and hydrogen to enter cells
• Bacteria affected
Inhibits
Effects
Ruminococcus albus
Decreased acetate, formate and
Ruminococcus flavefaciens methane
Butyrvibrio fibrisolvens
Streptococci
Decreased lactate
Lactobacilli
Increases
Fibrobacter succinogenes
Increased propionate
Prevotella ruminicola
Selenomonas ruminantium
– Net results of feeding ionophores
• Increased propionate
• Reduced protein degradation
• Reduced deamination
• Reduced methane production
• Reduced lactate production
Ruminant Symbiosis - The ruminants are a group of herbivorous mammals like cattle, sheep, goats, camels,
giraffes etc. Ruminant gut provides a more obvious example of mutualism. They use plant cellulose as the
major carbohydrate source of their diet. However, their normal gut can not digest cellulose. Their digestive
tract
contains
no
less
than
four
successive
stomachs.
They have developed a special region for cellulose digestion. This region is the rumen, that is essentially vast
incubation chamber teeming with bacteria and protozoa. In cow, this resembles a large fermentation vat, about
100 litres, into which masticated plant materials enter for further digestion by large number of anaerobic
bacteria and protozoa.
The mutualistic microbes hydrolyse cellulose and other complex plant polysaccharides to their component
monosaccharides which are then fermented to simple fatty acids (acetic, propionic, butyric) and gases
(methane, carbon dioxide). The fatty acids are absorbed through the wall of rumen into the bloodstream for
use as carbon and energy source.
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The gases are passed out of rumen at frequent intervals. The microbial population of the rumen grows
rapidly. These microbial cells pass out of the rumen along with undigested plant material into the stomach.
These cells are destroyed and digested in the stomach through proteases (the rumen produces no digestive
enzymes), in normal way to provide essential amino acids and vitamins etc. required for the growth of animal
The cellulose digesting bacteria of rumen are all strict anaerobes. The species include Bacteroids
succinogenes, Ruminococcus flavofaciens, R. albus and Botryovibrio fibrisolvens.
The great bulk of bacterial population, however, is noncellulolytic. Many of the rumen bacteria including
some of the cellulolytic species are able of digesting starch, proteins and lipids.
Only lignin of the ingested plant matter escapes digestion. The products of digestion of polysaccharides,
proteins and lipids are fermented by the rumen bacteria. During these processes, hydrogen gas combines with
CO2 to form methane by Methanobacterium ruminantium.
****
* An ionophore is a lipid-soluble molecule usually synthesized by microorganisms to transport ions across
the lipid bilayer of the cell membrane. There are two broad classifications of ionophores.
1. Small molecules (mobile ion carriers) that bind to a particular ion, shielding its charge from the
surrounding environment, and thus facilitating its crossing of the hydrophobic interior of the lipid
membrane.
2. Channel formers that introduce a hydrophilic pore into the membrane, allowing ions to pass through
while avoiding contact with the membrane's hydrophobic interior.