Ch 27 Positive Interactions (Mutualism, Cooperation, Commensalism)

What is symbiosis?

a stable association between two organisms, regardless of benefit, neutrality, or harm

What cellular organelle originated from ancient bacterial symbiosis?

mitochondria

What is a facultative interaction?

a relationship where the microbe has alternative lifestyles

• this interaction is NOT required for survival

What is an obligatory interaction?

a relationship where the microbe cannot survive without its partner

What organisms exemplify obligatory interactions?

viruses

• requires host cells for replication

What are obligate intracellular symbionts?

bacteria that cannot survive outside eukaryotic host cells

What is mutualism?

an obligatory relationship where both partnerrs benefit and depend on each other

What is cooperation (in microbes)?

a non-obligatory interaction where both partners benefit

Mutualism =

obligatory

Cooperation = 

not obligatory

What is antagonism?

a relationship where one organism negatively impacts another

What is syntrophy?

an interaction where one species benefits from the metabolic products of another

Why are mutualistic associations common between microbes and insects?

many insects consume nutrient-poor plant sap/animal fluids that lack essential vitamins and amino acids

What is Buchnera aphidicola?

a γ-proteobacterial endosymbiont that lives inside aphid cells

Where is B. aphidicola located within the aphid?

inside specialized host cells called bacteriocytes

Is B. aphidicola able to live independently of its host?

no

• its an obligate symbiont

What does the obligate nature of the aphid–Buchnera relationship indicate?

coevolution

• the two organisms have evolved together

How do aphids and B. aphidicola share biosynthetic pathways?

some steps in amino acid synthesis occur only in the aphid and others only in the symbiont

What is lignocellulose?

a combination of cellulose, hemicellulose, and lignin

• the main structural component of wood

What nutritional challenges do termites face from a wood-only diet?

• breaking down long polysaccharides

• obtaining organic nitrogen

What organisms help termites degrade lignocellulose?

mutualistic protists living in the termite gut

Do termites produce any enzymes for cellulose digestion?

yes

• complete degradation requires their protist symbionts (Trichonympha sp)

What solves the termite’s nitrogen limitation problem?

nitrogen-fixing bacteria in the termite gut (Elusimicrobium)

Which protist hosts an additional bacterial endosymbiont, Elusimicrobium?

trichonympha species

What does Elusimicrobium provide to Trichonympha?

conversion of glutamine into other amino acids and nitrogenous compounds

Lignocellulose is broken down by

trichonympha sp (protist)

In cooperation,

both organisms can grow independently outside the partnership

What bacterium–nematode pair exemplifies cooperative symbiosis?

xenorhabus nematophila (bacterium) and steinernema carpocapsae (nematode)

Where do juvenile S. carpocapsae nematodes harbor X. nematophila?

in their gut

How do juvenile S. carpocapsae mature?

they must find and infect an insect

What happens when the nematode S. carpocapsae consumes insect blood (hemolymph)?

X. nematophila is excreted in the nematodes feces and become free-living

What does X. nematophila do once excreted?

it replicates and uses a type III secretion system to kill the insect with secreted enzymes

After killing the insect, what do X. nematophila produce next?

compounds that protect the insect cadaver from degradation by other microbes and from ant predation

What additional role do X. nematophila play in the nematode life cycle?

they produce molecular signals that trigger nematode development to adulthood

X. nematophila =

bacterium

S. carpocapsae =

nematode

What is commensalism?

a symbiotic relationship where one organism benefits and the other is unaffected

How does commensalism differ from mutualism?

only one partner benefits in commensalism

Is commensalism usually facultative or obligatory?

facultative

How is Staphylococcus epidermidis on human skin an example of commensalism?

it gains nutrients and shelter while the host is typically unaffected

How do some gut microbes exhibit commensalism?

by consuming host mucus glycans or leftover nutrients without affecting host health

Commensalism is often

syntrophic (cross-feeding)

• modification of environment

An example of commensalism is nitrification. What is the first and second step?

NH3 → NO2 → NO3

• nitrosomonas (first step)

• nitrobacter (second step)