Lecture 10: Microbial Interactions

Question 1

What is ectosymbiosis/endosymbiosis? Give examples.

Answer 1

• Microorganisms living on the outside (ecto) and the inside (endo) of another organism
• Ex. Thiotrix species: sulfur-usuing bacterium; attached to surface of mayfly larva (which contains a parasitic bacterium)

Question 2

What are the 2 types of intrapopulation interactions?

Answer 2

1. Positive: growth rate increases
   - Cooperation: low to moderate cell densities
2. Negative: growth rate decreases
   - Competition: high cell densities

Question 3

Define bacterial cooperation (synthrophy).

Answer 3

Larger numbers of bacteria can encourage growth and survival

Question 4

Give an example of cooperation.

Answer 4

Desulfovibrio and Chromatium
- Organic matter and sulfate needed by Desulfovibrio are produced by the Chromatium
- Chromatium goes through photosynthesis and oxidizes sulfide to sulfate

Question 5

What do cell exudates release?

Answer 5

• Nutrients/carbon
• Exoenzymes
• Other macromolecules

Question 6

How do bacteria modify their environment?

Answer 6

1. Cell exudates
2. Modify physico-chemical environment
   - Metabolic redox, pH changes
   - Solubilize minerals/nutrients in soil/rock
   - EPS production

Question 7

Dictyostelium

Question 8

Myxobacteria (“social bacteria”)

Question 9

What is commensalism?

Answer 9

One species benefits, one is neither harmed nor helped (unidirectional)
- Symbiont can survive without host
- Ex. Nitrosomonas oxidizes ammonium to nitrite, Nitrobacter oxidizes nitrite to nitrate

Question 10

What is mutualism?

Answer 10

Both organisms benefit
- Obligatory relationship (+/+)
- Ex. Lichens

Question 11

What are negative interactions?

Answer 11

• Compeition: resource limitation
• Amensalism: production of inhibitory compounds
• Parasitism: one species benefits while the other is harmed

Question 12

Compare and contrast parasitism vs. predation. Use examples.

Answer 12

Not always a clear difference when it comes to microbes
- Parasitism: bacteria living in algae or protozoa
- Predation: protozoa engulfing bacteria

Question 13

What is facilitiation?

Answer 13

+/+ or 0/+
- One species has positive interaction on another species without direct and intimate contact
- May or may not benefit the individual
- NOT mutualism or commensalism because they are not closely associated with each other