Molecular Basis of Quorum Sensing in Virulence of Bacterial Pathogens

Tuesday, May 2, 2023 1

Shezaib Siddiqui

Research Fellow

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Introduction

What is Quorum Sensing?

Molecular basis of infectionsa. Staphylococcus aureusb. Pseudomonas aeruginosa

Biosynthesis and degradation of Quorum sensing

Therapeutic approaches

Contents:

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Introduction

KEY TERMS

Pathogenicity:

The ability of an organism to cause a disease.

Virulence:

The degree of pathogenicity or disease caused by a pathogen.OR

Total no. of pathogenic determinants an organism is collectively termed as virulence.

Phenomenon that regulates the gene expression of bacterial population in unison at particular density in response to secreted chemical molecules.

OR Synchronous gene expression of bacterial population in

an environment.

Quorum Sensing

HISTORY

o First Identified in late1960s by Nealson et al.

o Discovered in Vibrio fischeri and Vibrio harveyii

o V. fischeri is a symbiotic microbiota of Howaiian bobtail squid light organ.

o V. Harveyi is a free living marine bacterium

o Bioluminescence in Vibrio is control via Quorum sensing (QS).

o Lux operon was discovered as QS regulatory coding region.

Howaiin bobtail squid

Vibrio fischerii

Vibrio harveyi

Quorum Sensing in a Nut Shell

Production and release of signalling molecule (Auto inducer).

Diffusion of Auto inducer (AI) molecule at low cell density leaving the QS circuit in-active

Activation of QS circuit by AI at higher concentration due to Higher cell density.

Transcriptional activation of down regulating genes by AI receptor complex or other activated signalling molecule.

Activation of virulence, biofilm formation, sporulation, competence and antibiotic production.

QS IN GRAM NEGATIVE BACTERIA

The autoinducers are typically molecules called acyl-homoserine lactones (AHLs).

AHLs diffuses extracellular and intracellular via passive transport.

QS IN GRAM POSITIVE BACTERIA

The autoinducers are oligopeptides, short peptides typically 8-10 amino acids long.

Oligopeptides excretes in extracellular environment via active transport.

Schematic Representation of QS Circuits

Canonical bacterial quorum-sensing (QS) circuits. Autoinducing peptide (AIP) QS in Gram-positive bacteria by (A) two-component signalling, or (B) an AIP-binding transcription factor. Small molecule QS in Gram-negative bacteria by (C) a LuxI/LuxR-type system, or (D) two-

component signalling.

Gram positive

Gram negative

QS Circuit of Vibrio harveyi

Lux Bioluminescence System

Molecular Basis of Quorum Sensing in Bacterial Virulence

Staphylococcus aureus

(gram positive)

S. aureus is a gram positive opportunistic pathogen.

Causative agent of many cumbersome respiratory and skin infections and poisonings.

Few of high mortality infections and poisonings are Scalded skin syndrome, toxic shock syndrome, furuncles, carbuncles, sinusitis, otitis media etc.

Emerged as Multi drug resistant bug and many hospital setting.

Prevalent in > 60% of Nosocomial infections

MRSA has mortality rate of >20%

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Encode various virulence factor under the control of Quorum sensing system (Agr operon)

Scalded Skin Syndrome Impetigo

Toxic shock syndrome Otitis media

S. aureus has a peptide based quorum sensing system ,encoded by accessory gene regulator (agr) locus.

The agr locus controls transcription of two RNA transcripts RNA II and RNA III from promoter P2 and P3, respectively.

RNA II encodes Agr A, B, C and D proteins RNA III encodes δ-hemolysin and posses regulatory activity.

QS Control of S. aureus virulence

Protein FunctionAgr A Transcription factor (Activated

by Agr C)Agr B Triming and transport of pre-

AIP (Agr D)Agr C Histidine Kinase recetor of

AIPAgr D Pre- Auto inducer peptide

Table 1: components of agr system

The Agr System

QS Circuit of Staphylococcus aureus

Role of RNAIII in virulence:

5’ end upregulates transcription of α- hemolysin.

3’ end is responsible for the repression of rot (repressor of toxin) protein and Protein A.

Rot inhibits the transcription of various toxins.

Inhibition of rot upregulates the translation of toxins and increases virulence.

Repression of Protein A (cell surface protein) decease adhesion of biofilm.

Role of Agr in Biofilm control:

Agr A up regulates the translation of PSMs (Phenol soluble modulins).

PSMs are peptide toxin that help in channel formation in tissues.

PSMs also acts as surfactants that allow biofilm dispersal.

Biofilm dispersal helps S. aureus to colonize and infect new sites.

Polymorphism in Agr Locus

S. aureus can be classified in to 4 distinct groups based on Agr locus.

Agr locus

Agr I Agr II Agr III Agr IV

(Gram negative)

Pseudomonas aeruginosa

P.aeruginosa is a Gram-negative opportunistic pathogen.  Responsible for severe nosocomial infections cellulitis, UTIs,

endocarditis and epidermidis bullosa.

Accounts for more than 10% of nosocomial infections.  Patients with Cystic Fibrosis, injuries (especially burns/boils) are

highly susceptible to P. aeruginosa infections.

Second leading cause of ventilator associated pneumonia.    Difficult to treat due to emerging drug resistance.

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Cellulitis Epidermidis bullosa

Endocarditis

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Quorum Sensing In P. aeruginosa

• Three QS circuits are identified

• LasI/R (3-0C12HL synthase/ Receptor)

• RhlI/R (C4HL synthase/ Receptor)

• Pseudomonas Quinolone Signal (PQS)

Las Inducer/Repressor (I/R) system:

• Auto-induced by LasI

• Controls RhlR operon

• Activates transcription of Las B (Elastase), LasA (Protease), tox A (Exotoxin A) and aprA (alkaline phosphatase).

• LasI inhibits Rhl R.

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RhlI/R system:

Upregulates rhl I◦

Positive regulator of Las B (elastase), RhlA and B (Rhamnosyl transferase), RpoS (Stationary phase σ factor) and Pyocanin (niche molecule)

Quorum sensing in P. aeruginosa

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Negative regulation NFκB.

Induces apoptosis in macrophages.

Down regulates TNF-α and IL-12.

Induces IL-8 from human bronchial epithelial cells.

Additional role of 3-0-C12HSL (Role in anaphylactic shock)

Biosynthesis and degradation of Acyl Homo serine Lactones Fatty acid

pathway

AHL synthase

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Types of Auto inducers in gram negative bacteria

PAME= propionyl amphotericin B methyl ester, DSF= Diffusible signal factor, THMF= tetra hydroxyl methyl furan

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Therapeutic Potential of Quorum Sensing

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Novel therapeutic tartest against multi drug resistant bacteria.

Reported to decrease virulence only, thus empower host immune system to combat attenuated pathogen.

Low chances of drug resistance against quorum quenchers as they do not kill bacteria.

Quorum sensing as Therapeutic Target

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Strategies for Quroum Sensing Inhibition

1. Elimination of signal

By stimulating enzymatic degradation of auto inducer By inhibition of auto inducer biosynthesis and receptor

molecule biosynthesis

2. Inhibition of signal receptor complex

By using auto inducer analogues By treating with antibodies against auto inducer

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Current Approaches for QS inhibition

AHL acylases, paraxonases and AHL lactonases are secreted by many bacteria to degrade AHL , therefore negatively control QS.

AHL and AIP analogues have been synthesized to inhibits QS in S. aureus, P. aeruginosa and other pathogens

Synthesis of inhibitor molecules against enzymes involved in AHL biosynthesis, eg: tricolsan inhibits enoyl-ACP reductase.

Inhibition of auto inducer receptors by using RNAi technology.

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Auto Inducer and Inhibitors of P. aeruginosa

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Auto inducers and analogue Inhibitors of S. aureus

s

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QS circuits serve as new and potential anti virulence target to control infections with reduces risks of resistance evolution.

CONCLUSION

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Miller, Melissa B., and Bonnie L. Bassler. "Quorum sensing in bacteria."Annual Reviews in Microbiology 55.1 (2001): 165-199.

Waters, Christopher M., and Bonnie L. Bassler. "Quorum sensing: cell-to-cell communication in bacteria." Annu. Rev. Cell Dev. Biol. 21 (2005): 319-346.

Whiteley, Marvin, Kimberly M. Lee, and E. P. Greenberg. "Identification of genes controlled by quorum sensing in Pseudomonas aeruginosa."Proceedings of the National Academy of Sciences 96.24 (1999): 13904-13909.

Bassler, Bonnie L. "How bacteria talk to each other: regulation of gene expression by quorum sensing." Current opinion in microbiology 2.6 (1999): 582-587.

Yarwood, Jeremy M., et al. "Quorum sensing in Staphylococcus aureus biofilms." Journal of bacteriology 186.6 (2004): 1838-1850.

Rutherford, Steven T., and Bonnie L. Bassler. "Bacterial quorum sensing: its role in virulence and possibilities for its control." Cold Spring Harbor Perspectives in Medicine 2.11 (2012): a012427.

References:

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