Lecture 37

Question 1

Describe the role of microbes in ruminants.

Answer 1

Ruminants (cattle, sheep, goat, giraffe, etc.) consume plant material that is metabolized mainly by microbes in the rumen. This is because most nutrients are complex polymers (cellulose mainly, starch, etc.) that must be broken down before they can be metabolized.

Question 2

What are the 6 stages of rumen metabolism?

Answer 2

1. Degradation of cellulose and other complex carbohydrates
2. Fermentation of sugars and acids
3. Absorption of organic acids by rumen tissue
4. Methanogenesis using fermentation products H2 and CO2
5. Predation by protozoa (ciliates) on bacteria
6. Digestion of proteins and microbes in the lower GI tract

Question 3

What is the formula for the overall rumen metabolism? State what the major start- and end-products are.

Answer 3

Cellulose -> carbon (incl. organic acids) and energy

C6H12O6 -> acetate + proprionate + butyrate + CO2 + CH4 + H2O + heat

Question 4

What are the 3 main products absorbed by the ruminant? Name what they are.

Answer 4

1. Organic acids: acetate, proprionate, butyrate (aka volatile fatty acids)
2. Gasses (CO2 60-70%, CH4 30-40%)
3. Heat

Question 5

Describe the function of each of the organic acids absorbed by the ruminant.

Answer 5

Acetate: gets oxidized via the TCA cycle for energy
Proprionate: it is the only gluconeogenic (biomass-building) carbon source supplied to the animal
Butyrate: gets oxidized via the TCA cycle for energy

Question 6

What are the 4 main members of the rumen microbial community?

Answer 6

1. Bacteria
2. Archaea
3. Protozoa
4. Fungi

Question 7

Describe the function of rumen bacteria.

Answer 7

They are a diverse community that allow metabolic adaptation to diet. They degrade materials like cellulose, starch, hemicellulose, proteins, lipids, etc. and ferment sugars and organic and fatty acids.

Question 8

Describe the function of rumen archaea.

Answer 8

They are primarily methanogens. Some are symbionts of rumen protozoa.

Question 9

Describe the function of rumen protozoa.

Answer 9

Include ciliates. They degrade plant material and ferment sugars. They also prey on rumen bacteria and archaea and store carbohydrates that are readily digested in the lower GI tract.

Question 10

Describe the function of rumen fungi.

Answer 10

There are 2 types: saprophytic fungi and parasitic fungi.
Saprophytic fungi degrade plant material and ferment sugars.
Parasitic fungi parasitize ciliates.

Question 11

What is the estimated amount of bacteria, archaea, and protozoa present in the rumen gut?

Answer 11

Bacteria: 5.17 x 10^11
Archaea: 3.17 x 10^9
Protozoa: 4.02 x 10^7

Question 12

Name 6 benefits of the rumen association to microbes.

Answer 12

1. Abundant supply of food and other necessary compounds.
2. Saliva rich in PO4-, an excellent buffer for the acids being produced, and a constant pH
3. Even temperature
4. Re-chewing and movement in rumen mixes contents
5. Absorption of toxic waste products by ruminant tissues
6. Low [O2] perfect for anaerobes

Question 13

A. tumefaciens produces […]

Answer 13

Crown gall tumours, an agronomically important disease that affects most dicot plants.

Question 14

What are the 5 steps of the induction of virulent A. tumefaciens?

Answer 14

1. Initiation
2. Quorum sensing
3. Two-component regulatory system
4. Products of the other vir genes and T-DNA
5. Plant transformation

Question 15

Explain what happens in the initiation step of A. tumefaciens infection.

Answer 15

Wound of plant tissue causes release of water, sugars, and phenolic compounds, namely acetosyringone (AS).

Question 16

Explain what happens in the quorum sensing step of A. tumefaciens infection.

Answer 16

A minimum number of cell is needed at a wound site to induce tumour formation. This is determined by an N-acyl-homoserine lactone autoinducer signal. This autoinducer is encoded by traI and traR, which are homologous to luxI and luxR.

Question 17

Explain what happens in the two-component regulatory system of A. tumefaciens infection.

Answer 17

It has a two-component regulatory system consisting of VirA (sensor) and virG (regulator). VirA will be activated by AS and sugars, which themselves are activated by protein ChvE. VirA will phosphorylate and then phosphorylate VirG. VirG will then trigger the expression of the other vir genes.

Question 18

Explain what happens in the products of the other vir genes and T-DNA step of A. tumefaciens infection.

Answer 18

The vir genes will trigger the tranformation of the plant genome with T-DNA, which will enter via a pilus connecting the two cells. T-DNA integrates into the plant genome. opc and ops genes encoded by T-DNA are expressed in the plant tumour.

Question 19

Explain what happens in the plant transformation step of A. tumefaciens infection.

Answer 19

The activated onc genes disrupt the normal balance of plant hormones. ops genes will encode opine synthesis enzymes. The transformed plant cells therefore make nutrients specifically for the Agrobacterium that induced the tumour.

Question 20

How do Agrobacterium metabolize opines?

Answer 20

Through opine catabolism. They have opc genes on their Ti plasmid that encodes enzymes for this.

Question 21

Name 2 uses of Agrobacterium.

Answer 21

Can be used to transfer genes into plants for study/experimentation or for genetic engineering of crops.

Question 22

What is the role of microbes in the atmopshere?

Answer 22

There are estimated to be around 10^19 microbial cells in the clouds. They have a role in ice nucleation, precipitation, and movement of plant pathogens. However, little is known about them and they are very hard to study.