Microbial Symbioses and Endosymbioses

Symbiosis

The close, long-term interaction between individuals of different species

changes in symbiotic relationships

cost or benefits can change over time or depending on the environment

symbiotic relationship acquired from ancestors

vertically acquired

symbiotic relationship acquired from environment

horizontally acquired

Why do bacteria engage in quorum sensing?

To regulate gene expression involved in virulence or mutualism

Describe what happens in quorum sensing.

small diffusible autoinducer molecules interact with receptors at a certain threshold concentration → coordinated gene expression

Gram negative autoinducer

generally N-acyl homoserine lactones

Gram positive autoinducer

generally peptides

Microbe Associated Molecular Patterns (MAMPs)

Essential, highly conserved microbial molecules that trigger innate immune responses in eukaryotes if non-beneficial

Why have Shigella lost their flagellum?

MAMP lost to evade the immune response.

Aliivibrio fischeri

Gram-negative gammaproteobacteria found in free living marine environments and in symbiosis with E. scolopes

A. fischeri genome

2 circular chromosomes

Euprymna scolopes

Hawaiian bobtail squid. Very small predator, lives in shallow waters off central Pacific islands

How is A. fischeri beneficial to E. scolopes?

Bioluminescence formed by A. fischeri prevents the squid casting shadow so they don’t alert prey/predators

How do A. fischeri establish the symbiotic relationship with E. scolopes?

Colonise squid’s specialised light organ by moving into pores then crypts. Infection established and division using host nutrients occurs

Why is only A. fischeri able to colonise E. scolopes?

• binding of A. fischeri to cilia → gene expression changes

• host mucus contains chitobiose and immunity factors which may inhibit others

What happens as a result of A. fischeri MAMPs?

Cell death and destruction of the ciliated surface used for colonisation. Epithelial swelling and increase in microvilli

Changes in host behaviour as the squid matures

Arrhythmic → completely nocturnal as diel/circadian pattern established.

What changes does establishing diel/circadian pattern result in? (squid/A. fischeri)

transcriptomes oscillate, host crypt epithelium restructured and bacterial metabolism changes

What happens to A. fischeri at dawn?

squid vents 90% of bacteria, seeding the environment with A. fischeri

key regulators of bioluminescence

autoinducer synthase LuxI and regulator LuxR

LuxI

produces specific AHL N-3-oxohexanoyl-homoserine lactone (3-oxo-C6-HSL)

AinS (A. fischeri)

produces C8 which is sensed by AinR → derepression of LitR → LuxR expression

LuxR

binds AHL at high concentration and directs transcription of target genes luxCDABEG

Nitrogen Fixation

oxygen sensitive process converting inert N2 into biologically accessible NH3

diazotrophy

ability to fix nitrogen to biologically relevant and useable compounds eg ammonia

chemical methods of diazotrophy

lightning and the Haber-Bosch process

biological diazotrophy

bacteria associated with leguminous plants eg Rhizobium leguminosarum and free living diazotrophs eg Azotobacter vinelandii and P. stutzeri

nitrogenase

catalyse nitrogen fixation. Contain an Fe-S cofactor and either Fe/molybdenum or Fe/vanadium mixed reactive centres

Rhizobial bacteria

alpha/betaproteobacteria free living in soil or in association with legume root nodules. Differentiate into bacteroids which fix nitrogen

Rhizobial genome

megasymbiotic plasmids

rhizosphere

region of soil around plant roots, affected by plant exudates

flavonoids

Polyphenolic compounds released by roots when nitrogen scarce. Diffuse across rhizobial membranes and act as signals.

What happens when flavonoids enter rhizobial cytoplasm?

They bind TF NodD which activates expression of nod genes. Nod gene products synthesise nod factors

What does perception of nod factors by plant LysM-RLKs cause?

calcium spikes, root hair deformed, infection thread forms and nodule organogenesis

critical steps in root nodule formation

• rhicadhesin mediated recognition and attachment

• root hair curling

• invasion of root hair by Rhizobia (like endocytosis)

• bacteria in infection thread grow towards root cell

• bacteroid state forms in plant root cells

• division → nodules

differences between free-living and symbiont Rhizobia

different cell morphology, DNA content, gene expression and metabolism

leghemoglobin provided by plant

maintains microoxic environment in the nodule

How does the root nodule-Rhizobia relationship benefit the bacteria?

plant provides dicarboxylates

nif genes

expressed for nitrogen fixation → nitrogenase production

specificity of symbiosis (plant root nodules)

signals eg EPS formation

aphids and Buchnera aphidicola

Buchnera synthesise amino acids for aphids

difference between free-living and symbiont Buchnera

extreme gene reduction of 160 to 800kbp compared with free-living bacteria genome of 2 to 10 Mbp. Only genes for host fitness present