Symbiosis

Question 1

what is mutualism? give an example

Answer 1

• both organisms benefit from the interaction

e. g. E.coli present in human intestine
- they produce vitamin B12 which is beneficial for host
- host provides them with nutrients for growth

Question 2

what is commensalism? give an example

Answer 2

• the microbe benefits from the interaction, and there is no impact on the host
  e. g. bacteria present on the skin or in the gut for habitats

Question 3

what is parasitism? give an example

Answer 3

• the microbe benefits from the interaction at the expense of the host
  e. g. some intracellular pathogens (Chlamydiae, apicomplexans) are obligate parasites

Question 4

what is ectosymbiosis?

Answer 4

• symbiont living on surface of host cells

Question 5

what is endosymbiosis?

Answer 5

• symbiont living intracellularly

Question 6

what are metamonads (Mixotricha paradoxa)?

Answer 6

• ectosymbionts found on species of bacteria
• lives in symbiosis with other bacteria
• 1 endosymbiotic bacteria degrade cellulose to replace mitochondria that have been lost
• 3 ectosymbiotic bacteria provide motility through short and long spirochetes (anchored on surface) and Bacteroides possess 4 non-functional flagella

Question 7

what are the symbiotic associations in hemipetran insects?

Answer 7

• several bacterial endosymbionts are found in insects
• include intracellular and extracellular bacteria
• bacteria are found in specialised structures called bacteriocytes that can aggregate to form bacteriomes

Question 8

what is an example of an obligate parasitic intracellular bacteria?

Answer 8

Legionella pneumophila:

• when bacteria are engulfed by phagocytes, they get trapped in vesicles which form early endosomes
• found in freshwater
• virulent factors allow it to resist and neutralise the endosytic vacuole
• disables phagocytes and begins replicating
• parasite of amoebas
• can replicate with alveolar macrophages

Question 9

what is an example of a parasitic bacterial predator?

Answer 9

Bdellovibrio bacteriovorus

• attacks gram -ve bacteria
• recognises gram -ve surface, attaches and penetrates the periplasm
• it reseals the outer membrane and specific enzymes degrade the host peptidoglycan layer, but not its own peptidoglycan
• it feeds on cytoplasmic content and elongates into bdelloplast
• it then divides and releases progeny

Question 10

what is plant nodulation?

Answer 10

• plants cannot fix N2, so they can only use ammonia and nitrates
• Rhizobia bacterium induce the formation of root nodules that allow plants to fix atmospheric N2
• this enables an increased growth rate and yield of the plant

Question 11

what are examples of rhizobia?

Answer 11

Alphaproteobacteria (gram negative)

• soil dwelling bacteria
• complex genome (5-10mbp and several plasmids)

Question 12

what are legumes?

Answer 12

Fabaceae:

• beans
• peas
• peanuts

Question 13

what is the general nodulation process?

Answer 13

1. bacterial attraction
   - activation of nodulation genes
2. production of Nod factors
3. root curling
4. formation of injection threads
5. bacterial differentiation into bacteroids and nodule formation
6. N2 fixation

Question 14

what happens in bacterial attraction during the nodulation process?

Answer 14

1. production of root flavonoids to attract bacteria
2. lectins act as plant surface receptor and flavonoids as chemoattractants
3. bacteria adhere to root hairs via lectins
4. bacteria produce surface proteins with calcium
5. as bacteria pile up, they form biofilm aggregates to increase chance of triggering nodulation
6. nodulation genes are activated

Question 15

what happens during the production of Nod factors in nodulation?

Answer 15

• nod factors are short oligosaccharides between 4 and 6 residues
• they are heavily modified, containing fatty acids at non-reducing end, acetylation and sulphation

1. nod factors are secreted, initiating nodulaton

Question 16

what happens in root curling during the nodulation process?

Answer 16

• nod factors bind to receptors and trigger root curling

- root hairs curl and entrap the Rhizobia

Question 17

what happens during the formation of injection threads in nodulation?

Answer 17

1. bacteria can multiply and progress within the root structure
2. bacteria move and spread towards cortical cells via injection threads

Question 18

how do bacteria differentiate to bacteroids in nodulation?

Answer 18

1. bacteria bud off the infection threads and invade the cortical cells in symbiosomes
   - acidic environment
2. NCR peptides induce the differentiation of rhizobium into bacteroids
3. nodule is formed

Question 19

what are the two fates of rhizobia differentiating to bacteroids?

Answer 19

1. indeterminate nodules: nodules which keep growing via active meristem and bacteria eventually die
2. determinate nodules: longer due to active meristem, and bacteria remain viable and can resume growth if extracted

Question 20

how are nodules perfect for N2 fixation?

Answer 20

• nodule has low oxygen content which is optimal for N2 fixation

Question 21

how is nodulation a mutualistic symbiosis?

Answer 21

• Bacteria gain carbohydrates and amino acids from plant (if determinate fate)
• plant can now use N2

Question 22

what distinguishes obligate symbionts, parasites, organelles?

Answer 22

organelles: conserved across a wide range of organisms and are essential
parasites: harmful to the organism that it infects
symbionts: seen as beneficial organisms that live in association with other organisms

viruses can be identified based of their capsid

Question 23

how have new species microorganisms been identified?

Answer 23

• most bacterial species have been studied in anexic structures (independent from other organisms)
• metagenomics enabled to identify thousands of species that can’t be grown in labs

Question 24

give examples of some unusual symbiotic organisms:

Answer 24

1 = Candidatus Suclcia muelleru Candidatus Sulcia muelleri = 245 kpb (bacteriodetes)
2 = Candidatus Zinderia insecticola = 208 kbp (beta proteobacteria)
3 = Candidatus Carsonella rudii = 160 kbp (gamma proteobacteria)
4 = Candidatus Hodgkinia cicadicola = 144 kbp
5 = Candidatus Tremblaya princeps = 139 kb (alpha proteobacteria)

Question 25

what is the origin of small genomes in symbionts?

Answer 25

• phylogenetic studies indicate that symbionts do not have independent origin
• genome size is as a result of gene loss
• evolution favours plastic genome and loss of genetic material via shared metabolism between host and bacteria

Question 26

how are mealybugs model symbionts?

Answer 26

• nested symbiosis: endosymbiont inside an endosymbiont

- these insects have bacteriocytes which host the bacteria inside them (tremlaya and moranella)

Question 27

how is a mosaic biosynthetic pathway involved in amino acid biosynthesis?

Answer 27

• The two types of bacteria have a reduced genome

- Produce amino acids mosaically where the partners chip in to produce the needed amino acids

Question 28

how is a mosaic biosynthetic pathway involved in Moranella peptidoglycan biosynthesis?

Answer 28

• Tremblaya and Moranella lack equipment to from peptidoglycan
• The host insect encodes for peptidoglycan genes which are used by the bacteria