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MICR3310 > 5 - Microbial Symbioses > Flashcards

5 - Microbial Symbioses Flashcards

1
Q

Symbiosis

A

Stable association of one organism with another

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2
Q

Types of symbiosis

A
  • Mutualism
  • Cooperation
  • Commensalism
  • Amensalism
  • Predation
  • Parasitism
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3
Q

Mutualism

A

Both partners benefit but one partner cannot survive without the other

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4
Q

Cooperation

A

Both partners benefit but both can also grow alone

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5
Q

Commensalism

A

One benefits whilst the other is neither harmed nor helped

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6
Q

Amensalism

A

One has adverse effects on the other

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7
Q

Predation

A

Predator hunts and kills preyP

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8
Q

Parasitism

A
  • Parasite lives within host
  • Host is not killed but may be harmed
  • All infectious agents causing illness belong to this category
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9
Q

Competition

A

Organisms compete for a common resource

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10
Q

Consortia

A
  • Two or more microbial groups living symbiotically
  • May be free-living or within a host
  • Range of mechanisms support consortia robustness
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11
Q

Example of mechanism that support consortia robustness

A

Cross feeding (e.g. syntrophy)

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12
Q

Chlorochromatium

A
  • Microbial mutualisms found in freshwater
  • Consists of green sulfur bacteria PLUS motile, non-phototrophic bacteria
  • Consortia morphology depends on species composition
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13
Q

Green sulfur bacteria

A

Non-motile, phototrophic, brown or green

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14
Q

Chlorochromatium structure

A

Many epibionts surrounding a single, motile organism

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15
Q

Chlorochromatium location

A
  • Found in stratified lakes where light penetrates (for phototrophy) and H2S is present
  • Consortia reposition themselves rapidly to remain in optimal light and sulfur compounds and away from O2
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16
Q

Chlorochromatium aggregatum

A
  • Epibiont (organsim on outside): green sulfur bacteria
  • Strict anaerobes
  • Require light for anoxygenic photosynthesis
  • Require H2S (use reduced sulfur species as electron donors)
  • e.g. Chlorobium chlorochromatii
  • Consortia are about 5 µM long
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17
Q

Chlorochromatium aggregatum central partner organism

A
  • Betaproteobacteria
  • Requires alpha-ketoglutarate (TCA cycle intermediate, supplied by the epibiont
  • Only assimilates fixed carbon in presence of light and sulfide
  • Epibiont is active in these conditions: transfer nutrients to central partner
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18
Q

Metabolically coupled Chlorochromatium
aggregatum

A
  • Exchange amino acids and other intermediates
  • Protrusions visible in SEM may indicate sharing of periplasmic space
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19
Q

Legumes

A
  • Plants that bear their seeds in pods
  • E.g. soybeans, clover
  • Can grow without added nitrogen fertiliser
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20
Q

Legume-root nodule symbiosis

A
  • Legumes partner with symbionts (bacteria)
  • Collectively referred to as rhizobia (from Rhizobium genus)
  • Symbionts can live freely in soil or symbiotically with plants
  • Symbionts convert N2 to ammonia (NH3)
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21
Q

Cooperative relationship between plants and microbes

A
  • Symbiont supplies plant with fixed N
  • Plant supplies symbiont with fixed carbon
  • Both can grow alone, but not as well
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22
Q

Steps in root nodule formation

A
  1. Recognition of each other
  2. Secretion of Nod factors causing root curling
  3. Invasion of the root hair
  4. Movement of bacteria to root by way of infection thread
  5. Formation of bacteroids and development of N2 fixing state
  6. Continued cell division
    (both plant and microbes),
    forming mature root nodule
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23
Q

Colonisation by rhizobia

A
  • Signalling molecules from plants and rhizobia
    exchanged in rhizosphere
  • Plants release flavonoid inducer molecules that stimulate colonisation
  • Bind to bacterial protein NodD
  • When plant gets Nod factor signal, gene expression in root epidermal cells is altered, altering calcium levels and triggers root hairs to curl, trapping bacterial cells
  • Bacterium then induces formation of infection thread
24
Q

NodD

A
  • Transcriptional regulator
  • Nod genes transcribed, which encode enzymes needed for production of Nod factor (bacterial signaling compound)
25
Q

Root nodule formation

A
  • Infection thread reaches cortex
  • Each bacterial cell endocytosed by a plant cell into a discrete unit
  • Form symbiosome and bacteroid
  • Low O2 levels in symbiosome trigger cascade that controls N2 fixation
  • Enzymes of N2 fixation (dinitrogenase) are O2 sensitive
26
Q

Symbiosome

A

Individual bacterium and surrounding plant-derived endocytic membrane

27
Q

Bacteroid

A

Within symbiosome, bacterial cell differentiates into a
N2-fixing form

28
Q

Leghaemoglobin

A
  • O2 binding proteins that tightly control levels of O2
  • Production is induced by interaction between plant
    and bacteria
29
Q

Examples of plant microbe interactions

A
  • Legume-root nodule symbiosis
  • Mycorrhizae
30
Q

Mycorrhizae

A
  • Mutualism between plant roots and fungi
  • Nutrients are transferred in both directions
31
Q

Plant benefits of mycorrhizae

A
  • Absorb nutrients from environment more efficiently due to greater surface are provided by mycelium
  • Have competitive advantage
32
Q

Fungus benefits of mycorrhizae

A

Provided with steady supply of organic nutrients

33
Q

Other benefits of mycorrhizae

A

Support plant diversity

34
Q

Two main types of mycorrhizae

A
  • Ectomycorrhizae
  • Endomycorrhizae
35
Q

Ectomycorrhizae

A
  • Fungal cells form extensive sheath around outside of the root
  • Only minor penetration
  • E.g. oaks
36
Q

Endomycorrhizae

A
  • Part of the fungus becomes deeply embedded in root tissue
  • Fungi much more diverse than ectomycorrhizae
  • Most form coral like formations (arbuscules)
  • Belong to fungal division Glomeromycota (obligate plant mutualists)
37
Q

Arbuscular mycorrhiza (AM) steps in colonisation

A
  1. Germination of a soil-borne AM spore
  2. Produces short germination mycelium
  3. Recognises host plant though reciprocal chemical signalling
  4. Fungus forms hyphopodium with root epidermal cells
  5. Hyphae extend, form branched or coiled structures (arbuscules may spread intercellularly or intracellularly)
38
Q

Fungal signalling factors (Myc factors)

A
  • Lipochitin oligosaccharides
  • Initiate formation of mycorrhizal state
  • Closely related to Nod factors (in rhizobia)
39
Q

Mycorrhizae nutrient exchange

A
  • Hyphae remain separate from plant protoplasm by plant cytoplasmic membrane (forms a region called apoplast)
  • Fungi collect t N and P from soil, convert to arginine and polyphosphate
  • These are translocated through hyphae to the plant
40
Q

Apoplast

A

Increases surface area of contact between plant and fungus

41
Q

Termite mutualisms with microbes

A
  • Termites gain nutrients from microbes
  • Microbes gain nutrients and a safe home from termites
  • Lower termites feed on wood
  • Wood contains cellulose + hemicellulose (made up of many glucose molecules, but combine with lignin to form lignocellulose which is difficult to digest)
  • Symbionts in hindgut digest majority of cellulose and hemicellulose (nitrogen-fixing gut bacteria provide organic nitrogen to termites)
  • Termites also produce cellulases in salivary glands or midgut (but not hindgut)
42
Q

Within termite hingut

A
  • Mostly anaerobic
  • Wood fibres fermented to acetate + other short-chain fatty acids (these are the primary carbon and energy sources for the termite)
  • Presence of H2 drives reduction of CO2
  • O2 consumed by bacteria and by methanogenic archaea that use acetate, lactate or H2 as electron donor
  • Termite gut favours acetogenesis, not methanogenesis
43
Q

Termite gut favouring acetogenesis, not methanogenesis

A
  • Acetogens more metabolically versatile: can use more substrates as electron donors
  • Acetogens colonize H2
    -rich gut centre better, methanogens confined
    to gut wall (get little H2
    there)
44
Q

Microbe-invertebrate interactions associated with hydrothermal vents

A
  • Gutless tube worms (lack mouth, gut, anus
  • Have organ called trophosome
  • Symbiont bacteria are chemoautotrophs (oxidise H2S, use O2 as TEA, synthesise organic molecules)
  • Mutualism (both benefit, worm cannot survive alone)
45
Q

Trophosome

A
  • Contains bacteriocytes
  • Hosts cells with large population of sulfur-oxidising bacteria
46
Q

Cooperation between Bobtail squid and Aliivibrio fischeri

A
  • Squid has large population of A. fischeri within light organ (at night, bacteria emit light that resembles moonlight, camouflage squid from predators in waters beneath)
  • Newly hatched squid do not have A.fischeri (bacterium present in surrounding water)
  • Squid almost empties light organ of A. fischeri everyday, then begins to grow new population
  • A.fischeri grows much faster in light organ than open water
47
Q

Steps in Cooperation between Bobtail squid and Aliivibrio fischeri

A
  1. Squid encounters bacterial peptidoglycan
  2. Triggers squid to secrete mucus from developing light organ (initiates colonisation, mucus makes Gram -ve bacteria aggregate)
  3. Within aggregate, A. fischeri outcompetes all other bacteria and establishes monoculture within 2 hours of squid hatching
  4. Maturation of light organ (A. fischeri migrate up ducts into light organ tissue, lose flagella, become non-motile, divide and grow)
48
Q

Mutualism between ruminants and microbes

A
  • Diet rich in cellulose
  • Mammals can’t degrade cellulose alone (lack enzymes)
  • Only microbes have the enzymes necessary
  • Two characteristics evolved to support a plant-based diet
49
Q

Ruminants

A

Cattle, sheep, goats, deer

50
Q

Enzymes that microbes use to degrade cellulose

A

Glycoside hydrolases and polysaccharide lyases

51
Q

Two characteristics evolved to support a plant-based diet

A
  • Enlarged, anoxic fermentation chamber (rumen), full of microbes
  • Extended retention time (20 – 50 hours)
52
Q

Rumen

A
  • Large digestive organ with conditions perfect for fermenting
  • high numbers of bacteria and archaea
  • Within rumen, cellulose degraded to glucose
  • Rumen bacteria tightly adhered to food particles (pass through digestive tract to acid stomach, are digested)
  • Amino acids and vitamins made by rumen bacteria become nutrients for the ruminant
53
Q

Degradation of cellulose to glucose

A
  • Glucose fermented to volatile fatty acids (VFAs) and absorbed through rumen wall into bloodstream
  • CO2 and CH4 are produced (released by belching)
  • Methanogens consume all H2 and reduce with CO2
    to CH4
  • Inter-species hydrogen transfer occurs (syntrophy)
54
Q

How many species found in rumen

A
  • 300-400
  • Change in diet can adversely affect rumen composition
55
Q

Example of changing diet causing adverse effects

A
  • Abrupt switch to grain diet
  • Encourages growth of Streptococcus bovis
  • S. bovis produce lactic acid
  • Lots produced (animal can’t absorb it all)
  • Stronger acid than VFAs, reduces pH of rumen
  • Rumen acidification (acidosis) can lead to death
56
Q

Epibiotic predators

A
  • Attach to surface of prey
  • e.g. Vampirococcus
  • Secrete enzymes that result in cell lysis, release of cell
    contents
57
Q

Endobiotic predators

A
  • Invade cells
  • e.g. Bdellovibrio
  • Invade cytoplasm or periplasm and consume contents

MICR3310 (28 decks)

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