Quorum Sensing

Gram Negative Cell-Cell Communication:

Bacteria are not single entities that act in groups.

Communication occurs through acylhomoserine lactones (HSLs)

Was first discovered in Vibrio fischeri

-cells constitutively make HSLs which diffuse into the surrounding environment.

-As the population increases, so does the concentration of HSLs

-Once the population ( and HSLs) reach a certain level the HSLs move back down its concentration gradient into the cell, causing an increase in the amount of HSLs in the cell.

-The HSL binds its transcriptional regulator, which in turn activates gene expression (autoinduction).

-Different species make different HSLs which differ by their acyl side chain and substitutions at the C3 position of the N-linked acyl side chain.

-The side chain can range from 4-18 carbons.

Roles of HSLs:

-Virulence

-Biofilm formation

-conjugation

-antibiotic synthesis

Gram Positive Cell-Cell Communication:

Oligopeptides used as signaling molecules not HSLs.

Secreted through Type I Secretion System (ABC transporter complex)

Detectors- 2 component systems

Example S. aureus: agr system

AgrD- produces autoinducer peptide (AIP)

AgrB-secretes AIP

AgrC-sensor protein that binds AIP, phosphorylates on its histidine

AgrA-response regulator, phosphorylated on its aspartate activating it.

AgrA and SarA- binds to P2 promoter initiating transcription causing the production of other agr proteins and virulence factors.

Vibrio Harveyi: Gram Negative that uses HSLs and 2 component proteins

-Like other gram negatives it uses HSLs, only they are detected through apparatus’ that consist of 2 component proteins.

-Also responds to AI-2, which is designed for interspecies communication

-AI-2 is unique in that it contains boron