L22: symbiosis

name the 3 major types of symbiosis + describe the characteristics of them

mutualism: both organisms benefit

commensalism: microbe benefits with no impact on the host

parasitism/predation/competition: microbe benefits at expense of host

name an example of a mutualistic relationship

E. coli in the intestine: produce vitamins beneficial for host and receive nutrients in return to sustain growth

name an example of a parasitic relationship

some obligate IC pathogens: chlamydia / apicomplexans

what is the issue with the definition of commensalism

most commensalistic organisms are in a balance of being harmful or beneficial

what are endosymbionts

symbionts living in the host cell intracellularly

what are ectosymbionts

symbionts living on the surface of a host cell

recall one example of an endosymbiotic or ectosymbiotic relationship

metamonads in certain species of termite gut

bacterial species in hemipteran insects

describe the system of metamonads in certain species of termite guts

2 endosymbiotic bacterium degrade cellulose + replace mitochondria

3 ectosymbiotic bacteria provide motility through short + long spirochetes + bacterioids

recall an example of an intracellular bacteria parasite

legionella pneumophila in freshwater: parasite of amoebae

recall an example of a bacterial predator parasite

bdellovibrio bacterivorous: attacks gram -ve bacteria + invades periplasm to feed on host cell

why is plant nodulation important

plants are usually unable to use atmospheric N₂ (use NH₄⁺ or NO₃ ) so some legumes form symbiotic interactions with rhizobia in the soil to fix N₂

what are rhizobia

alphaproteobacteria (gram -ve), soil dwelling bacteria (part of rhizosphere).

complex genome 5-10Mbp with several plasmids

recall the 6 stages of plant nodulation

1. attraction of bacteria through production of root exsudates / flavonoids (bacteria bind to root surface lectins)

2. production of Nod factors (short oligosaccharides)

3. root hairs curl + entrap rhizobium

4. formation of infection threads

5. bacterial differentiation into bacterioids induced by NCR peptides

6. N₂ fixation

what are the 2 fates of rhizobium differentiation

if viable: determinate nodules

if dead: indeterminate nodules

name an example of a complex + unusual symbiosis

candidatus organisms are well categorised but uncultured (IDd through metagenetic) + likely to be very small and endpsymbionts

mealybugs: example of nested symbiosis

why do symbionts often have small genomes

no independent origin → genome size is due to gene loss over time

describe an example of nested symbiosis

mealybugs: tremblaya inside mealybugs + moranella inside tremblaya