Lecture-10M.Sc 2nd Semester (Environmental Microbiology)

Paper EM-202: Microbial physiology and adaptation

Unit IV: Signal Transduction (Quorum Sensing)

Quorum sensing (QS)

Quorum sensing is the regulation of gene expression in response to fluctuations incell-population density. Quorum sensing bacteria produce and releasechemical signal molecules called autoinducers that increase in concentration asa function of cell density. The detection of a minimal threshold stimulatoryconcentration of an autoinducer leads to an alteration in gene expression.Gram-positive and Gram-negative bacteria use quorum sensingcommunication circuits to regulate a diverse array of physiological activities.These processes include symbiosis, virulence, competence, conjugation,antibiotic production, motility, sporulation, and biofilm formation.

Quorum sensing is a system of stimulus and response correlated to populationdensity. Many species of bacteria use quorum sensing to coordinate geneexpression according to the density of their local population, using manysignaling molecules.

There are many different kinds of signal molecules in the bacterial community.Among those signal molecules, N-acyl homoserine lactones (HSLs also referredto as AHLs, acy-HSLs etc.) are often employed as QS signal molecules formany Gram-negative bacteria.

Due to the specific structure and tiny amount of those HSL signal molecules, thecharacterization of HSLs has been the subject of extensive investigations in thelast decades and has become a paradigm for bacteria intercellular signaling.

Bioluminescence

Quorum sensing

Virulence

Biofilm

Secondary metabolites

or antibiotic production

Sporulation

Fruiting Body Development

Motility & clumping

Exopolysaccharide production

Pigmentation

Bacterial cross talk

Siderophore formationBiosurfactant

N2 Fixation

Pollution degradation

Exoenzyme production

Plasmid transfer

Production of viable but nonculturable (VBNC) cells

Different functions related govern by quorum sensing in microorganisms

AHL

No expression

cLux I C D A B ELux

R

AHL

receptor

AHL

cLux I C D A B ELux

R

Fig. Bacterial quorum sensing circuits. (a) QS in Gram negative bacteria at low cell density

and acyl homoserine lactone (AHL) results in no expression (b) at high AHL concentration

lead to expression of gene

Mechanisms of Signal Transduction1. Signal transduction is the means by which cells respond to

extracellular information.

2. Signal transduction at the cellular level refers to the movement ofsignals from outside the cell to inside.

3. The movement of signals can be simple, like that associated withreceptor molecules of the acetylcholine class: receptors that constitutechannels which, upon ligand interaction, allow signals to be passed inthe form of small ion movement, either into or out of the cell.

4. These ion movements result in changes in the electrical potential ofthe cells that, in turn, propagates the signal along the cell.

5. More complex signal transduction involves the coupling of ligand-receptor interactions to many intracellular events.

These events include

• Phosphorylations by tyrosine kinases and/or serine/threonine kinases.

• Protein phosphorylations change enzyme activities and proteinconformations.

• The eventual outcome is an alteration in cellular activity and changesin the program of genes expressed within the responding cells.

Signal Transduction machinery in bacteria

• The centerpiece of this control mechanism is composed of“two-component”.

A. Receptor protein: Senses environmental parameters.B. Response regulator protein. Metabolic activities• In a typical two component system, a receptor protein

transduces environmental signals into metabolic changesthrough phosyphorylation of response regulator protein.

• The signal cascade requires the transfer of a phosphategroup from an autophosphorylating protein kinase to aregulator protein.

• This is the most common type of signal transduction systemin bacteria and controls diverse processes such as geneexpression, sporulation, and chemotaxis.

Histidine

Kinase

ATP

ADPP

Response regulator

P

Gene regulation

Pro-AIP

AIP

Pro

cess

ed Auto inducer receptor

complex

Gene regulation

Pro-AIP

Pro-AIP

Process

AIP

Fig. Bacterial quorum sensing circuits. Autoinducing peptide (AIP) in gram positive bacteria

by (a) two component signaling (b) an AIP-binding transcription factor

Lux

M

Lux

S

Cqs

S

Cqs

A

LuxU

LuxO

Hfqs54sRNA

LuxR

mRNA

Biolumenescence

LuxN

HAI-

1 AI-

2CAI-

1

a

AH

L-1

La

s R

Rhl

R

Target

gene

Target

gene

AH

L-2

b

Lux

Q

Fig. Parallel (a) and series (b) quorum sensing circuits in V. harveyi

LuxL

M

Lux

O

Lux

S

P

H

D

H

D

H

P

D

Lux

RLux

CDABE

X

P

LuxN LuxQ

LuxU

Fig. The hybrid quorum sensing circuit of Vibrio harveyi

Two component system(TCSs)

• Typical TCSs comprise a receptor protein (a histidine protein kinase, or HK), and an effector or transmitter protein (response regulator or RR).

• Both the HK and the RR are multi-domain proteins.• The N-termini of HKs are diverse, and usually contain sensory or ‘input’

domains, which respond to changes in environmental stimuli. HKs are definedby the possession of a transmitter domain, which is a combination of two sub-domains (HisKA/Hpt and HATPase), typically at the C-terminus of the HKprotein.

• RRs typically comprise an N-terminal receiver domain, with diverse C-terminaltransmitter or ‘output’ domains.

• Upon stimulus perception by an input domain, the transmitter domain of an HKis activated, leading to autophosphorylation of the HK at a conserved Hisresidue within the transmitter domain.

• Phosphorylated transmitter domains are able to interact with the receiver domains of their partner RRs.

• Formation of a phospho transmitter–receiver domain complex allows transferof the phosphoryl group from the transmitter His residue onto a conserved Aspresidue within the RR receiver domain. Phosphorylation of the RR receiverdomain then alters its interaction with the RR output domain, up- or down-regulating effector activity.

Receiver and Transmitter proteins and signal cascade

• The signal cascade is directed by phosphorylation anddephosphorylation that originates when specificcompounds bind to transmembrane receptors in theperiplasmic space.

• There are four homologous transmembrane receptorsknown as methyl-accepting chemotaxis proteins (MCPs),Tsr, Tar, Trg, and Tap.

• They span the inner membrane as dimers and transmitthe chemotactic signal to the cytoplasm through anunknown mechanism.

• The MCPs are reversibly methylated through binding ofattractants and repellents so that the cell can detectchanges in their concentration and adjust its swimmingstyle accordingly.

• The main cellular proteins responsible for thetransduction of the chemical signal to the flagellar switchare CheA, CheW, CheY, CheZ, CheR, and CheB.

Organic Pollutants In-organic Pollutants

Other organic

compounds

Co-

metaboli

zationSugars

Fatty acids Pyruvate

Acetyl Co-A

TCA

CO2 + H2O

respiration

Cell growth

β-oxidation

Exopolysaccharides

Biosurfactant

Emulsify

EPS SequestrationCd2+Cd2+

Outer membrane binding

i.e. biosorption

Pb2+ Pb2+

Pb2+Pb2+

Pb2+

Efflux pump

Cd2+

Cd2+

Bioassimilation through

siderophore formation

Precipitation as metal salts

& metal reduction i.e.

Biotransformation

Intacellular Sequestration i.e

bioaccumulation

Cd2+

Mechanisms of organic and inorganic pollutants removal by biofilm forming bacteria