Lecture 15 - Microbial symbioses with microbes, plants, and animals

Question 1

what are lichens?

Answer 1

mutualistic relationship
between a fungus and an alga (or
cyanobacterium)

Question 2

what is Chlorochromatium aggregatum?

Answer 2

bacterial mutualistic relationship; consists of green sulfur bacteria and a flagellated rod-shaped bacterium

Question 3

true/false
The green sulfur bacteria are
obligate anaerobic phototrophs

Answer 3

true

Question 4

what is Methanotrophic consortia?

Answer 4

couple the
activities of two anaerobes to effectively
oxidizing methane to carbon dioxide in
anoxic marine sediments; Use “nanowires” for direct interspecies
electron transfer

Question 5

what does the mutualistic relationship between legumes and nitrogen-fixing bacteria lead to?

Answer 5

Infection of legume roots by nitrogen-fixing
bacteria leads to formation of root nodules that
fix nitrogen; leads to significant increases in
combined nitrogen in soil

Question 6

what is a cross-inoculation group?

Answer 6

a group of related legumes that can be infected by a particular species of rhizobia.

Question 7

what is nitrogenase and what is it inhibited by?

Answer 7

enzyme that fixes nitrogen, inhibited by high oxygen levels

Question 8

what is leghemoglobin?

Answer 8

an oxygen-binding protein in the nodule that binds the free oxygen; protects nitrogenase from free oxygen

Question 9

how does the leghemoglobin function?

Answer 9

the heme group of leghemoglobin cycles between the oxidized form (Fe3+) and reduced form (Fe2+) to supply enough oxygen for bacterial respiration while also keeping free oxygen within the nodule low

Question 10

what happens to the rhizobia after infection?

Answer 10

they divide rapidly within the nodule, change shape (bacteroids) and form a symbiosome within the nodule

Question 11

what is involved in attachement?

Answer 11

cell surface proteins of rhizobia and plants (rhicadhesin)

Question 11

what do the roots of leguminous plants secrete?

Answer 11

organic compounds that stimulate the growth of rhizosphere microbial communities

Question 12

what happens if there are suitable rhizobia in the soil?

Answer 12

they will form large populations and then attach to the root hairs

Question 13

what is an infection thread?

Answer 13

a cellulosic tube formed by the plant, induced by the bacterium, that spreads down the root hair

Question 14

true/false
plant cells divide to form a tumor like nodule that consists of plant cells filled with bacteroids

Answer 14

true

Question 15

what are the major organic compounds transported to the bacteroids and what are they used for? (2 paths)

Answer 15

malate, succinate, fumerate
used in citric acid cycle leading to the electron transport chain to generate ATP
pyruvate as electron donor

Question 16

what does nitrogenase convert?

Answer 16

nitrogen to ammonia, which is assimilated to glutamine and asparagine and transported through the plant

Question 17

what is mycorrhizae?

Answer 17

mutualisms between plant roots and fungi
plant transfers carbohydrates to fungi
fungi transfer inorganic nutrients

Question 18

what are the two classes of mycorrhizae?

Answer 18

endomycorrhizae and ectomycorrhizae

Question 19

what are the characteristic of ectomycorrhizae?

Answer 19

• fungus stays outside the plant roots
• found primarily in forest trees (boreal and temperate)
• some trees can form multiple myccorhizae associations with multiple fungi (allow exchange of nutrients and carbon between trees of the same and different species)

Question 20

what are characteristics of endomycorrhizae?

Answer 20

• fungal mycelium becomes deeply embedded with root tissues
• 5 classes, most common is arbuscular mycorrhizae
• more common than ectomycorrhizae
• cannot be cultured in pure culture

Question 21

explain arbuscular mycorrhizae root colonization

Answer 21

• plant roots release hormone strigolactone (stimulates growth of plant root system and fungal germination and mycelial branching)
• fungi produce oligosaccharide singaling molecules to initiate mycorrhizal state
• fungal mycelium forms attachement structure HP (hyphopodium), penetrates through epidermal cells and cells of outer cortex
• mycelium can spread intercellularly or intracellularly in the outer cortex

Question 22

how can microbial symbionts be acquired?

Answer 22

1. environmental reservoirs (horizontal transmission)
2. parents (herritable transmission) (obligate, lacking a free-living replicative stage)

Question 23

two categories based on host dependancy

Answer 23

1. primary symbionts (required for host reproduction)
2. secondary symbionts (not required for host reproduction)

Question 24

primary vs secondary symbionts

Answer 24

primary: restricted to bacteriome, reside in specialized cells in bacteriome called bacteriocytes
secondary: can invade different cells or live extracellularly in insect hemolymph, can co-reside or displace primary symbionts, must confer benefits to host

Question 25

wolbachia (secondary symbiont)

Answer 25

when affect males with wolbachia mate with unaffected females, progeny are not viable
when mosquitos are infected with it, transmission of disease is reduced

Question 26

what is a defense strategy used by insects to deter patogens and predators?

Answer 26

production of toxic and antimicrobial chemicals

Question 27

what is the defense chemical most often a product of?

Answer 27

microorganisms symbiotically associated with the insect

Question 28

what are the two digestive plans that herbivores have evolved?

Answer 28

1. hindgut fermentation: uses caecum and/or large intestine
2. foregut fermentation: fermentation chamber proceeds small intestine

Question 29

what are cellulomes?

Answer 29

protein scaffold containing multiple polysaccharide-degarding enzymes found on the surface of polysaccharide-degrading anaerobic bacteria and fungi

Question 30

what are the main fermentation products of ruminant microbes?

Answer 30

volatile fatty acids (used by animal as main energy source), methane, and carbon dioxide

Question 31

what is acidosis, how is it caused and how to prevent it?

Answer 31

the inflammation of rumen epithelium cause by pH<5.5, prevention includes gradual transition of diets or or mixture of grain and fibre diet
can cause hemorrhaging and death