L5, L6 - The Social Life of Bacteria

Question 1

What is special about the bacteria Vibrio fischeri, and what animal is it found in?

Answer 1

The squid Eprymna scolopes contains the symbiotic bacteria Vibrio fischeri.

This bacteria is bioluminescent, and is used by the quid to disguise its shadow in the moonlight (it hunts at night)

Question 2

What concentration of bacteria is needed for bioluminescence?

Answer 2

Bioluminescence only occurs when bacteria are at high density (~10^10cells/ml) in the light organ.

Question 3

How is cell density measured by bacteria?

Answer 3

Quorum sensing

Question 4

What operon controls Quorum Sensing?

Answer 4

The Lux Operon

Question 5

Describe how the operon controls quorum sensing

Answer 5

1) LuxI protein catalyses the synthesis of an acyl homoserine lactone (AHL), which diffuses out of cells
2) Bacteria also take up AHL. When enough bacteria produce AHL, concentration rises indie cells and AHL binds to LuxR protein
3) LuxR with AHL bound increases transcription of bioluminescence genes in the Lux Operon

Question 6

How does LuxR effect the Lux Operon?

Answer 6

With AHL bound, LuxR binds to the lux box in the DNA next to the lox operon promoter. This increases the binding of RNAP to the promoter

The LuxI gene itself is in the Lux Operon. Activation of LuxI transcription increases AHL synthesis; the system increases its own activity - positive feedback mechanism.

Question 7

When is Quorum sensing useful to pathogenic bacteria?

Answer 7

Pathogenic bacteria can use QS to produce degradative enzymes only when density of bacteria is high

Question 8

What can activator proteins like LuxR code for in pathogenic bacteria?

Answer 8

Cell wall degrading enzymes (in plant pathogens)

Proteases (animal pathogens)

Enzymes for antibiotic synthesis (antibiotics kill competitors)