Lecture 6 - Exam 2: Interactions Among Microbial Populations

Question 1

What are positive versus negative interactions?

Answer 1

Positive: leads to increase growth rate and population size
Negative: reduced population size and slower growth rate

Question 2

Interactions among microbial populations has to deal with what?

Answer 2

Population densities. When population density is low, there tends to be more positive interactions. When there is a high population density, then we get negative interactions (competing for resources)

Question 3

Describe the effect of interaction of neutralism, commensalism, synergism (protocooperation), symbiosis (mutualism), competition, amensalism (antagonism), and predation & parasitism.

Answer 3

Neutralism: 0 & 0
Commensalism: 0 & +
Synergism: + & +
Symbiosis: + & +
Competition: - & -
Amensalism: 0 & -
Predation & Parasitism: + & -

Question 4

Synergism (mutualism): The relationship between the two populations is __________. They have to have ______ partners.

Answer 4

Obligate ; fixed

Question 5

What is the difference between predation and parasitism?

Answer 5

Parasitism contact time with its host is longer and can keep the host alive (or it can die). The parasite is usually smaller than its host. Predation contact time is shorter (lion eats rabbit). The size of the predator is usually larger than its prey.

Question 6

If we have 0 for one population and + for the other is that considered a positive, neutral, or negative interaction?
What about if it is 0 and -?

Answer 6

Positive ; Negative

Question 7

Describe neutralism.

Answer 7

Lack of interaction between populations. Can be separated by time or space. Ex: microbes at the top of the soil versus microbes in the lower soil profile.

Question 8

Describe Commensalism.

Answer 8

One population benefits from the presence/activity of a second population but the other remains unaffected. Ex: Photosynthetic organism produces oxygen and that benefits aerobes in the soil.

Question 9

Describe synergism.

Answer 9

(Protocooperation, syntrophy) Association of mutual benefit to both species, but without that cooperation being obligatory. Don’t have to be fixed partners in a relationship. Ex: The decomposition of organic material is important for the soil. We have one population breaking down one compound and produce another one, and then we have another population breaking down a product produced by the first population. Both populations can work together to completely break down the substrate.

Question 10

Describe symbiosis (mutualism).

Answer 10

An obligatory relationship between two populations that benefits both populations. Ex: The relationship between the two populations are obligatory, Mycorrhizae (symbiosis between plants and fungi)

Question 11

What are some symbiosis organism examples?

Answer 11

Mycorrhizae, Legume-rhizobium symbiosis, Actinorhizal symbiosis (involved actinobacteria and woody plants, together they fix nitrogen), Lichens, Leafcutter ants (bring back leaves to their nest but don’t eat them, they feed the fungus.

Question 12

What are the three types of lichens and how are they classified?

Answer 12

Crustose, foliose, fruticose. They are based on their attachment to the substrate.

Question 13

Describe each of three types: Crustose, Foliose, and Fruticose

Answer 13

Crustose: Firmly attached to their substrate
Foliose: Not as firmly attached, they sit up more on the rock
Fruticose: Fully standing up and develop short stalks… least attached to substrate.

Question 14

Lichens are what type of indicators?

Answer 14

Lichens are sensitive ecological indicators… Sensitive to air pollution in cities… if tombstones are not covered in lichens then can be a sign of air pollution.

Question 15

Describe competition.

Answer 15

A negative relationship between two populations in which both populations are adversely affected with respect to their survival and growth.

Question 16

Describe amensalism (antagonism).

Answer 16

Growth of one population is restricted by presence of the second population, but the latter is not affected. Involves the chemical compounds (antibiotics, acids ethanol, bacteriocins).

Question 17

Describe Predation and parasitism.

Answer 17

Direct attack and feeding of one population on the other.
In microbial world, these two things are kind “blurry.”

Question 18

What are some examples of predation and parasitism?

Answer 18

Protozoa (the main predators in the soil) consume 100-1000 bacteria per hour.
Nematodes prey on bacteria, protozoa, fungi and smaller nematodes.
Nematode trapping fungi. They set up traps for moving animals. They then send in hyphae to digest the contents of the nematodes and small fauna.

Question 19

Nematodes can catch fungi and there are two types. What are they?

Answer 19

Sticky knobs and sticky net. The fungi here don’t have a septum in their hyphae and they produce these gooey materials and nematodes passing by get stuck in them.
The other type is nonconstricting and constricting rings. It is a more complex trapping mechanism. Fungi with septum in their hyphae. Have three cells forming rings. Some of these rings are nonconstricting and stay in fixed shape. The constricting rings can sense coming through, and they close shut.

Question 20

Bacterial predators include:

Answer 20

(more difficult to study due to their small size)
Agromyces ramosus, Myxobacteria, Bdellovibrio sp.

Question 21

Describe Agromyces ramosus.

Answer 21

Catalase-negative actinobacteria and uses manganese complex to remove the reactive oxygen species.
Is non-obligate predator of G+ and G- bacteria, meaning it uses organic material as carbon source and attacks both types of bacteria.
Method of predation: Prey cells produce growth initiation factor and this organism can sense the GIF-Mg2+ complex and attack them.

Question 22

Describe Myxobacteria.

Answer 22

Gram -, have gliding motility, non-obligate predator of G= and G-, excrete a lot of hydrolytic enzymes that help them digest food and G+ and G- cells, and have group predation (wolfpack strategy)

Question 23

Describe Bdellovibrio sp.

Answer 23

(bdellovibrio means leech) Tiny vibroid, G-, obligate anaerobe, highly motile (can swim at a speed of 160um/sec), obligate predator for G- bacteria.

Question 24

Describe the life cycle of Bdellovibrio bacteriovorus

Answer 24

Start as free swimming Bdellovibrio. They will attach to prey (5-10min), and will release an enzyme to lyse the cell wall and rotate to enter the host cell. They will then enter the periplasmic space (15 min) and will modify shape of the host (from rod to spherical) called a Bdelloplast. It will then use the contents of the host cells to become long filaments. These filaments will then divide to become more smaller pieces and then flagella is synthesized and then release hydrolytic enzymes and let themselves out of the host cell. THAT IS HOST DEPENDENT
HOST INDEPENDENT:
Once the the bdellovibrio exist the host cell, they will have an attack phase and free swimming. Then, they will have a nutrient medium and grow in biofilms or filamentous cells in liquid and then will become free swimming bdellovibrio again.

Question 25

What are the predatory strategies of Myxobacteria and Bdellovibrio?

Answer 25

Myxobacteria: Wolfpack
Bdellovibrio: Periplasmic, epibiotic (they attach to host cell but do not enter… suck the host cell content out)

Question 26

What are the types of parasites?

Answer 26

Chytrids: parasites of algae
Viruses

Question 27

When sampling soil, what are some of the most important considerations?

Answer 27

Taking representative samples, soil transport and storage, and soil sterilization.

Question 28

When taking representative samples, what do we have to remember to do?

Answer 28

Have random samples; composite samples. Consider the number of samples you need, dependent on the magnitude of variability at the study site. We desire the highest precision possible. If the soil is uniform then can get away with fewer samples. The best sampling plan should provide the greatest precision at a fixed cost or a specified precision at the lowest cost.
Other considerations when sampling: Depth (0-20/25cm), Association with plants, soil moisture, time of the year or day, and the management practice of the site (how long after the field has been tilled, if pesticides were applied etc.)

Question 29

When transporting soil and storing the samples, what do we have to keep in mind?

Answer 29

Sample should be transported to the lab on ice ASAP (lower the temperature the slower the change), avoid air drying (results in death of cells), sieve and store in a refrigerator or freezer (remove stones and larger roots, sieving can also change soil properties), analyze as soon as possible.

Question 30

When we sterilize the soil, what are some of the techniques we can use?

Answer 30

Heat and pressure: 121 C and 15 psi. We use intermittent sterilization. The drawbacks of using an autoclave is that we destroy amino acids, proteins, and vitamins. We modify the chemical properties of the soil. We can change the surface charge and destroy aggregate structure and release some metal irons (Mn24). Autoclaving is one way to sterilize the soil but it is not the best way. Gamma radiation is the best way. We use reactive source 60Co or 137Cs. We need to hit the sample at high dosage and at a high rate. After, we always need to check the soil sterilization.

Question 31

What is also listed under soil sterilization, but is NOT a way to sterilize soil?

Answer 31

Chemical inhibitors which temporally slows down growth of microbes. There are gaseous and nongaseous chemical inhibitors.

Question 32

What are the four types of parameters that we typically measure when looking at soil microorganisms?

Answer 32

Numbers, biomass, activity, and diversity. We can analyze any of those parameters by sampling