Bacteria- Quorum sensing

Question 1

What communication and signalling processes do bacteria undertake

Answer 1

1. Bacteria undertake a range of complex processes dependant on a sufficient population size.
2. Virulence factor production
3. Biofilm formation
4. Bioluminescence
5. Antibiotic production
6. Many are ineffective/detrimental if too few cells present

Question 2

Describe bacterial communication

Answer 2

1. For bacteria communication is mediated by production and sensing of chemical signals.
2. The amount of signal molecule present is dependant on the number of cells
3. So signal intensity provides information on population density
4. Low cell density= low signal intensity= trait not expressed
5. High cell density= high signal intensity= trait expressed
6. Bacteria use this to regulate gene expression.
7. Referred to as Quorum Sensing (QS)

Question 3

Describe the Vibrio fischeri and the bobtail squid example of quorum sensing

Answer 3

1. V. fischeri is a Gram-negative marine bacterium that forms a stable symbiotic relationship with the bobtail squid.
2. The relationship with the squid involves colonisation by the bacteria of a specialised crypt, the light organ.
3. Immature squid are colonised by free-living V. fischeri present in the environment.
4. Once in the crypt, the bacteria are supplied with ample nutrients and can grow to high cell densities.
5. Benefit to the squid is production bioluminescence
6. Camouflage from predators
7. Forage in upper surface levels of water at night- would produce clear shadow and silhouette

Question 4

What is Quorum sensing

Answer 4

1. A molecular system to monitor population density

Question 5

Describe Bioluminescence in squid light organs

Answer 5

1. Bioluminescence in squid light organs one of the first systems described.
2. LuxI - Produces chemical signalling molecules
3. LucR - Receptor that can detect the signal molecules as they diffuse back into cell
4. Regulate gene expression of genes involved in production of light
5. Bacteria colonise light organs and produce autoinducer molecules (AIs).
6. Signals when density of the population is sufficient for light production to be beneficial- when high population density
7. Population is said to be quorate
8. Many QS genes in bacteria referred to as lux because of work on this system

Question 6

What are the signalling molecules in QS system called

Answer 6

1. Referred to as Autoinducers

2. Distinct types of AI in Gram negative and Gram positive QS systems.

Question 7

What AI is used in G-ve

Answer 7

1. G-ve use Acyl-homoserine lactones
2. a lot of conservation between
3. Variation in chain length
4. Like different dialects in same language

Question 8

What AI is used in G+ve

Answer 8

1. G+ve utilise oligo peptides
2. Very different to G+ve
3. Some “universal” AIs (AI-2) that work in a wide range of G-ve and G+ve species.
4. Furanosyl borate diester

Question 9

What are the 3 key principles All QS systems depend on

Answer 9

1. Members of the community generate the AIs
2. AIs are detected by receptors on membrane or in cytoplasm
3. AI detection leads to further AI production (as well as potential activation of QS regulated traits)

Question 10

What happens to number of AIs as population increases

Answer 10

1. Number of signalling molecules stays same per cell but as population increases more cells produce AIs
2. Then as population increases ratio of molecules to cells increases
3. Feedforward system
4. Until target gene expression is activated

Question 11

Give example of G-ve quorum sensing

Answer 11

1. Ai Synthase
2. Detection by two component system
3. Histadine kinase in membrane of bacterial cell
4. And response regulator controls gene expression
5. Feedforward onto AI synthase
6. And upregulation of genes under control of quorum sensing system

Question 12

Describe two component system of quorum sensing circuits and give example

Answer 12

1. Quorum sensing circuits: Two component systems
2. Very common mechanism for sensing external environment and relaying signals into the cell.
3. Many variations but canonical system is the two component sensor kinase (SK) system
4. SK recognises signal (e.g. AHLs)
5. SK internal kinase domain is phosphorylated at His residue
6. Interacts with response regulator and transfers ~P
7. Response regulator now active and can modulate gene expression

Question 13

Describe QS control of virulence in Pseudomonas aeruginosa

Answer 13

1. An important opportunistic G-ve pathogen
2. Particularly problematic in CF lung and burn wound infections
3. In both cases virulence regulated by QS.
4. Hierarchy of three QS “circuits” involved in this – which interact with each other
5. Las IR + rhl IR circuit have LUX IR type system
6. Las IR at top of hierarchy
7. Governs expression of components in other systems- rhi IR and PQS
8. Also controls virulence factors
9. PQS circuit HAS NON lux IR

Question 14

Describe the Las system in QS control of virulence in Pseudomonas aeruginosa

Answer 14

1. Activation upregulates production of virulence factors
2. Las also feedbacks onto las circuit itself- feedforward
3. And upregulation of PQS and rhl

Question 15

Describe the Rhi system in QS control of virulence in Pseudomonas aeruginosa

Answer 15

1. Upregulates virulence factors production when threshold reached
2. Feedforward itself
3. Oppression of PQS – dampen response

Question 16

Describe the PQS system in QS control of virulence in Pseudomonas aeruginosa

Answer 16

1. Upregulates rhl IR circuit

2. And virulence factors

Question 17

What are biofilms

Answer 17

1. Biofilms are surface-associated bacterial communities.
2. Cells embedded in a dense polymer matrix (slime!).
3. A basic survival strategy for bacteria
4. Biofilm associated cells more resistant to antibiotics, immune clearance, predation, other environmental factors

Question 18

Can we exploit QS to control bacteria?

Answer 18

1. The Las and Rhl systems have been evaluated as targets for new anti-virulence or anti-biofilm agents.
2. So called “QS interfering” drugs.
3. Key principle is to disrupt bacterial communication and coordination of gene expression.
4. E.g development of LasR or RhlR inhibitors.

Question 19

How can pyocyanin production be supressed

Answer 19

1. Mutants with receptors deleted
2. Disabled quorum sensing
3. Pyocyanin production is suppressed

Question 20

How can Biofilm formation be supressed

Answer 20

1. Clogging of microfluidic channels used to assess biofilm formation
2. Mutants take longer to clog
3. Shows quorum sensing inhibited

Question 21

How is G+ve QS different to G-ve

Answer 21

1. Similar principles but circuits slightly more complex due to nature of signal molecule
2. Addition steps related to transport of AIPs into or out of cell- compared to G-ve
3. AIPs synthesised as inactive precursors

Question 22

Describe G-ve QS system

Answer 22

1. Production of precursor peptides in cell- AIP
2. Transported to outside surface of cell
3. During transport or uptake precursors are processed into mature cyclic form- ensure receptors are receiving signals only from incoming molecules
4. Recognised by two component system
5. Regulate target genes

Question 23

Quorum sensing and infection in S. aureus

Answer 23

1. At low cell densities the concentration of the AI is low: the cells express their adhesins
2. At high cell densities the concentration of the AI peptide is high: the cells express their toxins
3. This is believed to allow the bacteria to “stick” during the initial phases of infection
4. And once they have established the infection, they express toxins which allow them to release more nutrients and move onto a new site

Question 24

Describe the QS system in Staphylococcus aureus

Answer 24

1. QS control of virulence in Staphylococcus aureus
2. Agr system
3. Agr D- responsible for generation of inactive precursory peptide
4. Agr B- transporter
5. Agr C and A – make up two component system
6. RNA 111- under control of Agr circuit
7. Supresses rot which is a repressor of toxins
8. Inhibits virulence factor production
9. Direct upregulation of virulence factors by Agr A