Symbiosis

Name the 3 major types of symbiosis

mutualism, commensalism, parasitism

Describe mutualism and an example of it

Both organisms benefit from the interaction

E. coli present in the intestine produce some vitamines beneficial for the host; in return, they use nutrients to sustain growth

Describe commensalism and an example of it

The microbe benefits from the interaction; no impact on host

bacteria present on the skin or in the gut are generally considered as commensals

Describe parasitism and an example of it

The microbe benefits from the interaction at the expense of the host 

some intracellular pathogens (Chlamydiae, apicomplexans, viruses) defined as obligate parasites

What are some issues associated with the definitions of different types of symbiosis?

• some species interact with other bacteria which allows balance in the microbiome so maybe more mutualistic than commensalitic?

• can sometimes shift to being either good or bad for host like opportunistic organisms

mainly linked to commensalism def

What does endosymbiosis refer to?

symbiont living intracellularly

What does ectosymbiosis refer to?

symbiont living on the surface of the host cells

Describe the 2 endosymbiotic and 3 ectosymbiotic bacteria with metamonads. Where do the latter live?

• 2 endo bacteria: degrades cellulose and replaces mitochondria

• 3 ecto bacteria: provide motility (short and long spirochetes + Bacteroides)

Describe symbiotic associations in Hemipteran insects

several bacterial species are found: both intracellular and extracellular that provide supplements to their diet that lacks amino acids and nutrients

What are specialised structures for bacteria in Hemipteran insects called? What does their aggregation form?

bacteriocytes than can aggregate to form organs called bacteriomes

How do symbiotic and aposymbiotic males of the bean bug riptortus pedestris differ phenotypically?

symbiotic are stronger and fatter - bc are well fed and get correct nutrients for muscle mass than aposymbiotic ones

Where are legionella found? What type of organism do they parasite and how?

fresh water

Amoebaes by modifying normal lysosome function avoiding their own death and allowing their multiplication

How do bacterial predators act on algae? Name an example of one of these bacteria

attacks Gram-negative bacteria, it invades the periplasms, feeds and digests host cell peptidoglycan - has to be careful not to digest its own peptidoglycan

B. bacteriovorus

Considering plants can’t use atmospheric N2 themselves, how do they procure it?

via a symbiotic interaction called nodulation with soil bacteria called Rhizobia

What is the aim of nodulation ?

symbiotic process between plants and bacteria allowing increased growth rate, yield and use of atmospheric N2

What are Rhizobia? Describe its structure

bacteria involved in plant nodulation found in soil, part of the rhizosphere

Alphaproteobacteria (Gram negatives) and complex genome with 5 to 10 MBP with several plasmids

What do plant roots produce to attract bacteria?

flavonoids

Describe the 6 basic steps of plant nodulation from bacterial attraction to N2 fixation

• bacterial attraction

• production of nod factors

• root curling formation of infection threads

• bacterial differentiation into bacteroids

• N2 fixation

What is the role of Nod genes in nodulation?

master regulators that make polysaccharide nod factors triggering root curling

How do nod factors trigger root curling?

through calcium influxes and cytoskeletal changes

In what type of plant are infection threads most prevalent?

legumes

What are infection threads in plants? How are they formed?

specialised tubular structures allowing nitrogen-fixing bacteria to enter and colonise root tissues, forming root nodules

root hair forms a sharp curl and bacteria bound to the root hair become trapped between appressed cell walls

What are 2 nodules found within a rhibozium that play a crucial role in nitrogen fixation? What do they contain and what distinguishes them?

determinate and indeterminate nodules

contain bacteroids, difference is mainly shape, spherical vs rod

How does a nodule allow N2 fixation?

Inside the nodules are cells filled with Rhizobium bacteria. These bacteria fix atmospheric nitrogen (N2) by converting it into ammonia compounds (NH4OH)

What distinguishes obligate symbionts, parasites, organelles and viruses??

• Organelles are usually conserved across a wide range of organisms and are essential

• Parasites are harmful for the organism they infect

• Symbionts are often seen as "beneficial" organisms that live in association (an)other organism(s)

How are most bacterial species studied and how did new techniques allow the discovery of thousands of new species?

in axenic cultures (independently of any other living organism)

Metagenomics = 1000s of new bacterial species that cannot be grown in lab conditions bc don’t grow independently, due to specific and unusual symbiotic relationships

Which new approach allowed to define new species and shook the concept of minimal genome?

metagenomics

What is common to candidatus species’ genomes? What explains this?

all small

gene size is the result of gene loss + don’t have independent genome origin

Candidatus ‘something’ means what?

organism never studied independently, always within another organism

What makes the mealybugs’ symbiotic state unusual? Describe it

relationship between mealybugs and moranella and tremblaya - nested symbiosis where bug has relationship with bacteria which itself has one with another bacteria

Why does the unusual mealybug symbiotic state explain why they can’t be studied in axenic cultures?

combines genes with all 3 organisms allowing biosynthetic pathways, so some amino acids made by bug NEEDS genes from the bacteria so they can't grow without the other

What underpins Moranella peptidolgycan synthesis?

mosaic biosynthetic pathway where the pathway depends upon genes in all three species involved in the mealybug symbiosis

Explain what “Most symbioses involve a crosstalk between microorganisms and their host“ means?

communication and interaction between different signaling pathways= coordinated cellular responses and the integration of multiple signals from the env.

In what way do symbiotic systems drive evolution?

affects genome evolution by facilitating:

• gene transfer from one genome to another

• the loss from one genome of genes that are present in both symbiotic partners

Name a few examples of symbiotic interactions

• metamonads live with 5 bacteria, 2 provide them with nutrients, 3 allow mvt

• hemipteran insects with bacteriocytes that contain bacteria that provide the insects with nutrients, form bacteriomes

• Legionella pneumophila, parasites within Amoeba by avoiding lysosome

• Bdellovibrio bacteriovorus, parasite that feeds on algae host

• plant nodulation: plant + root nodules containing rhizobia bacteria

• Mealybugs + moranella + tremblaya can’t live without one another

Describe a bacteriome

collection of all bacteria residing within a specific host or environment