MICR3310 Lecture 5: Microbial Symbioses 8/29/24

What is an example of an association that microbes might have with animals?

-Skin
-Digestive tract
-Mouth

What is this?

-Both partners benefit
-One partner cannot survive without the other (obligatory)

Mutualism

What is this?

-Both partners benefit
-Both could also grow alone (not obligatory)
-Root nodule bacteria + plants

Cooperation

What is an example of Cooperation?

Root nodule bacteria + plants

What is this?

-Only one organism benefits
-Other one does not benefit, is not harmed

Commensalism

What is this?

-One has adverse effects on the other
-Soil microbes production of antibiotics

Amensalism

What is an example of Amensalism?

Soil microbes producing antibiotics

What is this?

-Predator hunts and kills prey
-Epibiotic predators attach to surface of prey (Vampirococcus)
-Endobiotic predators invade cytoplasm or periplasm, consume (Bdellovibrio)

Predation

What is an example of Predation?

-Epibiotic bacteria (Vampirococcus)
-Endobiotic bacteria (Bdellovibrio)

What is this?

-Attaches to surface of prey
-Secretes enzymes that result in cell lysis, release of cell contents
-EX: Vampirococcus

Epibiotic predator

What is this?

-Invade cytoplasm or periplasm
-Consumes contents of cell
-EX: Bdellovibrio, Daptobacter

Endobiotic predator

What is this?

-Parasite lives inside host
-Host is not killed, but may be harmed
-Includes: All infectious agents that cause illness

Parasitism

What is this?

-Organisms compete for a common resource
-Environments such as soil

Competition

What interaction is this?

-Two or more microbial groups living symbiotically
-Can be free-living or inside host
-Supported by crossfeeding/syntrophy

-Example: Gut bacteria

Consortia

What are some mechanisms that support consortia robustness?

-Cross-feeding
-Syntrophy
-Degrading substrate together

What is an example of a Consortia?

Gut bacteria

What is this?

-Inability to synthesize one or more essential nutrients
-Widespread in natural environment microbes

Auxotrophy

What interaction is this?

-Chlorochromatium
-Green sulfur bacteria (non-motile, phototrophic, brown or green)
-Motile-non-phototrophic bacteria

Microbe-microbe symbiosis (mutualisms)

What microbe involved in mutualism is this?

-Many epibionts surrounding a single motile organism
-Consortia classified by color/morphology/gas vesicles
-Niche: Stratified lakes

Chlorochromatium

What is the niche environment of Chlorochromatium and other mutualism consortia such as green sulfur bacteria and motile (non-phototrophic) bacteria?

-Stratified lakes
-Light penetrates
-H2S is present

Why do anaerobic microbial mutualisms like Chlorochromatium reposition themselves rapidly in environments such as stratified lakes?

-Require optimal light/H2S
-Must remain in sweet spot
-Stay away from O2

What is this?

-Epibiont: Green sulfur bacteria
-Strict anaerobe, requires light/H2S
-Transfers alpha-ketoglutarate from TCA cycle to its central partner Betaproteobacterium
-Example: Chlorobium chlorochromatii

Chlorochromatium aggregatum

Which species of Chlorochromatium aggregatum is this?

-Can be grown alone in lab
-Never has been found alone (free-living in environment)

Chlorobium chlorochromatii

What is the central partner organism that receives alpha-ketoglutarate from Chlorochromatium aggregatum?

Betaproteobacteria

What is the epibiont of Betaproteobacteria that transfers over nutrients like alpha-ketoglutarate?

Chlorochromatium aggregatum

What is this?

-Receives alpha-ketoglutarate from its epibiont Chlorochromatium aggregatum
-Only assimilates carbon in presence of light/sulfide

Betaproteobacteria

What is this?

-Required by Betaprotebacteria
-Synthesized as TCA cycle intermediate
-Supplied by epibiont Chlorochromatium aggregatum

Alpha-ketoglutarate

True or False: Betaproteobacteria can assimilate fixed carbon but only in the presence of both light and sulfide

True

Transcriptome/proteome analysis suggest that central organism partners are metabolically coupled. What does this mean?

-Exchange amino acids/intermediates
-Betaproteobacteria receives alpha-ketoglutarate from Chromochlromatium aggregatum

How do we know that central organism partners like Betaproteobacteria and Chlorochromatum aggregatum share periplasmic space?

Visible protrusions in SEM

Which plant-microbe interaction is this?

-Cooperative relationship between plants and microbes
-Symbiont supplies plant with fixed nitrogen
-Plant supplies symbiont with fixed carbon
-Both can grow alone, but grow better together

Legume-root nodule symbiosis

What is this?

-Soybeans, clover, alfafa
-Plants with seeds in pods
-Can grow without nitrogen fertilizer

Legumes

What is this?

-Symbionts that partner with legumes
-Converts N2 to ammonia (NH3)
-Allorhizobium, Azorhizobium, Bradizhobium, Mesorhizobium, Sinorhizobium, Rhizobium

Rhizobia

True or False: Both legumes like sobyean root nodules and Rhizobium symbionts can live separately without each other

True

What are these?

1. Recognition of each other (plant/bacterium)
2. Secretion of Nod factors, 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 devision (plant/microbe) forming mature root nodule

Steps in root nodule formation

What is this?

-Signalling molecules from plants/rhizobia exchanged in rhizosphere
-Infection thread formation is induced by bacterium

Colonization

What is the first step that stimulates Rhizobia colonization?

Plants release flavonoid inducer molecules

What is this signal molecule released by plants to stimulate Rhizobia colonization?

Flavonoid inducer molecules

What bacterial protein do flavonoid signal molecules bind to?

NodD

What type of bacterial protein is NodD?

Transcriptional regulator

What genes are transcribed as a result of flavonoid signal molecules binding to NodD?

nod genes

What is the purpose of nod genes?

Encode enzymes for Nod factor

What type of compound is Nod factor?

Bacteria signalling compound

What happens when a plant gets the signal for Nod factor?

Root epidermal cells gene expression is altered

How are root epidermal cells altered to be different after Nod factor signal is produced?

-Altered calcium levels
-Root hairs curl
-Bacterium induces infection thread to form

Why do plant root hairs begin to curl after receiving Nod factor?

Traps bacterial cells

What happens as a result of the Nod factor signal once calcium levels have been altered/root hairs have curled?

Bacterium induces formation of infection thread

What happens after the infection thread grows into the cortex?

-Bacteria cells get endocysed by plant cell
-Bacteria differentiates into bacteroid from inside of symbiosome

What is this?

-Individual bacterium surrounded by plant-derived endocytic membrane
-Formed when infection thread reaches cortex

Symbiosome

What induces the endocytosis and formation of symbiosomes from bacteria/plant cells?

Infection thread grows into plant cortex

What is this?

-N2-fixing form of bacteria
-Forms inside of symbiosome plant-derived membrane
-Possibly irreversible

Bacteroid

True or False: Once a bacteria cell has differentiated into a bacteroid from inside of the symbiosome, it cannot go back

True

What triggers the signal cascade that controls N2 fixation in symbiosomes?

Low oxygen levels

What activity happens inside of the bacteroid once it detects low oxygen?

-Increased respiration
-Increased nitrogen fixation

What is this?

-Enzyme for N2 fixation
-Sensitive to oxygen, requires anaerobic levels

Dinitrogenase

What oxygen-binding protein controls the O2 levels in the root-nodules?

Leghemoglobin

What induces the production of oxygen-binding Leghemoglobin?

Interaction between plant and bacteria

What type of interaction is this?

-Nutrients are transferred in both directions between plant roots and fungi

Mutualism (Mycorrhizae)

How does the plant benefit from Mycorrhizae mutualism?

-Absorbs nutrients from environment more efficiently
-Mycorrhizae support plant diversity

How does Mycorrhizae fungi help the plant to absorb nutrients more efficiently?

Mycelium increase surface area

How does the fungus benefit from Mycorrhizae mutualism?

Steady supply of organic nutrients

True or False: There is a positive correlation between abundance/diversity of mycorrhizae and plant diversity

True

What type of Mycorrhizae fungi is this?

-Fungal cells form extensive sheath outside of root
-Only minor penetration
-Forest trees, conifers, oaks

Ectomycorrhizae

What are some Northern hemisphere examples of Ectomycorrhizae?

Forest trees, conifers, oaks

What type of fungi are Ectomycorrhizae?

-Fungal cells form extensive sheath outside of root
-Only minor penetration

What type of Mycorrhizae fungi is this?

-Part of fungus becomes deeply embedded in root tissue
-Most of them form arbuscules
-Belongs to Glomeromycota, cannot culture alone in lab

Endomycorrhizae

What is this?

-Tree/coral-like formations
-Endomycorrhizae form them
-Can spread intracellularly or extracellular

Arbuscules

What triggers the formation of arbuscules and other hyphae such as branched/coiled structures?

-Mycelium recognizes host plant's reciprocal chemical signals
-Fungus forms hyphopodium with root epidermal cells

What does the fungus form with root epidermal cells before the hyphae?

Hyphopodium

What type of fungi are Endomycorrhizae?

-Part of fungus becomes deeply embedded in root rissue
-Most diverse fungi
-Most form arbuscules

True or False: Ectomycorrhizae is more diverse than Endomycorrhizae

False

What are the two main types of Mycorrhizae fungi?

1) Ectomycorrhizae
2) Endomycorrhizae

Which Mycorrhizae is the most diverse?

Endomycorrhizae

What are the steps in Arbuscular Mycorrhiza colonization?

1) Germination of soil-born spore
2) Produces short germination mycleium
3) Recognizes host plant through reciprocal chemical signalling
4) Fungi forms hyphopodium with root epidermal cells
5) Hyphae extend, form branched/coiled structures like arbuscules (may spread intercellularly or extracellularly)

True or False: The relationship between host plants and mycorrhizae evolved 400 million years ago

True

What is an important evolutionary step in plants inhabiting dry lands?

Mycorrhizae root colonization

What are the fungal signalling factors are used for mycorrhizae?

Lipochitin oligosaccharides

What is this?

-Initiates formation of the mycorrhizal state
-Closely related to rhizobia Nod factors
-Used as Myc factors

Lipochitin oligosaccharides

What type of fungal signalling factors are lipochitin oligosaccharides?

Myc factors

Which evolved first, legume-root nodule symbiosis (Nod factors) or mycorrhizae (Myc factors)?

Myc factors

True or False: Hyphae are not separated from the plant protoplasm

False

What is this?

-Region of plant cytoplasmic membrane
-Separates hyphae from plant protoplasm
-Translocates N/P in the form of arginine and polyphosphate from the hyphae to the plant
-Increases surface area between plant and fungus

Apoplast

What do fungi convert nitrogen/phosphorus to after collecting them from the soil?

-Arginine
-Polyphosphate

How are arginine and polyphosphate transferred from the fungus hyphae to the plant?

Translocation via apoplast

What does the apoplast do?

-Translocates arginine/polyphosphate from hyphae to plant
-Increases surface area of contact between plant and fungus

Which parts of plant/fungi does the plant cytoplasmic membrane containing apoplast separate?

-Hyphae
-Plant protoplasm

What type of mycorrhizae is dominant in inland/coastal areas?

AM (Arbuscular Mycorrhizae)

What type of mycorrhizae is dominant in southwest forests?

EM (Ectomycorrhizae)

What type of interaction is this?

-Insects gain nutrients from microbes
-Microbes gain nutrients/home from insects

Termite mutualism with microbes

What microbes do the lower termites consume after feeding on wood?

Bacteria, archaea, protists

What fibers does wood contain?

-Cellulose
-Hemicellulose

True or False: Cellulose and hemicellulose contain very few glucose molecules

False

What does cellulose combine with to form lignocellulose, a form that is even more difficult to digest?

Lignin

True or False: Only protists can diagnose lignocellulose, the product of lignin + cellulose

True

Other than protists digesting cellulose, what other benefits do microbes give to termites?

-Nitrogen fixing gut bacteria
-Provides organic nitrogen

True or False: Termites have cellulase in their saliva and midgut, but not their hindgut

True

What happens inside of the termite's anaerobic hindgut?

Wood fibers are fermented

What are the products of anaerobic wood fermentation in the termite's hindgut that also serve as carbon/energy sources?

-Acetate
-Short chain fatty acids

What are the products of CO2 reduction within the termite's hindgut that require H2?

-Acetate

-Methane

What happens in the presence of O2 inside the termite's gut?

-O2 is consumed
-Uses acetate, lactate, or H2 as electron donor