Semantics of Quorum Sensing and Quorum Quenching

Saima Naureen

07-arid-1191

Ph.D Zoology

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Contents

Introduction Signalling molecules Mechanism Quorum Quenching Biofilms Applications Advantages Conclusion and Future perspective References

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Introduction. Quorum Sensing

Cell to cell conversation A signaling mechanism in microorganisms Control gene expression Cell density dependent

Example:• Bacteria and some social insects

(ants and honey bees etc.)

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History

Research into AHL based quorum sensing started in the late 1960s.

The marine bioluminescent bacteria Vibrio fischeri was being grown in liquid cultures and produced light only when large numbers of bacteria were present (Greenberg, 1967).

culture media contained an inhibitor of luminescence, which was removed by the bacteria when large numbers were present (Kempner & Hanson, 1968).

 It was later shown that the luminescence was initiated by the accumulation of an activator molecule or "autoinducer" (Nealson et al, 1970, Eberhard, 1972).

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Cont….

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Vibrio fischeri, lives as a mutualistic symbiont in the photophore (or light-producing organ) of the Hawaiian bobtail squid.

free-living (or planktonic) the autoinducer is at low conc., so no luminesence.

highly concentrated in the photophore (about 1011 cells/ml) transcription of luciferase is induced, leading to bioluminescence.

Decision Making Process

Quorum sensing can function as a decision-making process in any decentralized system, as long as individual components have:

(a) a means of assessing the number of other components they interact with

(b) a standard response once a threshold number of components is detected.

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Why do bacteria talk to each other?

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Where Quorum sensing occurs?

Within and between

bacterial species

Between prokaryotes and

their eukaryotic host Animal Plant

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Cross Talk

Cross talk Interspecies communication

Cross talk has implications in many areas of microbiology Antibiotic resistance Virulence Biofilms (mixed species populations)

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Signaling Molecules

Three types of molecules:

Oligopeptides Autoincduer (AIPs)

Acyl-homoserine lactones (AHLs)

Autoinducer-2 (AI-2)

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Table: 1.1

Class Signaling Molecules Strains

Gram- Bacteria AHLs V. fisheri

V. harveyi

Gram + Bacteria AIPs B.subtilis,

S. aureuas

Gram + Bacteria and

Gram -Bacteria

Al-2 E. coli

V. harveyi

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Oligopeptide Autoinducer

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These are small peptides.

Regulates competence and sporulation gene expressions

Mechanism of Oligopeptide Autoinducers

The precursor peptide is modified, processed and an ATP-binding cassette (ABC) exporter complex secretes the mature Oligopeptides-autoinducer.

At high cell density the autoinducer is detected by a two-component signal transduction system. Specifically, the sensor kinase protein recognizes the autoinducer and subsequently autophosphorylates at a conserved histidine residue (H).

The phosphoryl group is transferred to a cognate response regulator protein, and this protein is phosphorylated on a conserved aspartate residue (D).

The phosphorylated response regulator binds to specific target promoters to modulate the expression of quorum sensing regulated genes.

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Mechanism of AIPs

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These are of several types depending on their length of acyl side chain.

Able to diffuse through membrane.

Involve virulence and biofilm formation.

Acyl-homoserine lactones (AHLs)

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Mechanism of Acylhomoserine Lactones AI-1

These are synthesized by an autoinducer synthase LuxI and recognized by a autoinducer receptor/DNA binding transcriptional activator protein LuxR.

LuxR binds to the promoter region and initiate transcription of target gene.

There is a low likelihood of a bacterium detecting its own secreted inducer

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Mechanism of AHLs

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Autoinducer-2 (AI-2)

Involve in interspecies communication among bacteria

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Mechanism of Autoinducer-2

Al-2 are synthesized by LuxS and sensed by Lsr transporter cassette (ACDB), then phosphoryrated by kinase LsrK.

The Phospho-Al-2 binds with LsrR and transcription of the lsr genes is initiated result in (lsrACBDFG) expression

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Mechanism of Al-2

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Hybrid Quorum Sensing Circuit

The hybrid quorum sensing circuit of V. harveyi.

Elements characteristic of both Gram-negative and Gram-positive bacterial quorum sensing systems are combined to make up the V. harveyi circuit.

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Mechanism of Hybrid Quorum Sensing Circuit

An acyl-HSL autoinducer (AI-1, green pentagons) is produced by the activity of LuxLM.

A second autoinducer (AI-2, red pentagons) is synthesized by the enzyme LuxS. AI-2 is proposed to be a furanone.

Both autoinducers accumulate as a function of cell density.

The sensor for AI-1 is LuxN and two proteins, LuxP and LuxQ function together to detect AI-2.

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Cont…

LuxN and LuxQ are hybrid sensor kinase/regulator protein that transduce information to a shared integrator protein called LuxU.

Lux U send the signal to the response regulator protein LuxO.

LuxO controls the transcription of a putative repressor protein (denoted X) and a transcriptional activator protein called LuxR is also required for expression of the luciferase structure operon (lux CDABE).

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Mechanism of hybrid quorum sensing

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Quorum Quenching

1.Bacterial Virulence:

QSI is a potential method for inhibition of bacterial

virulence both In-vitro In-vivo

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Cont…

• Recent research into quorum sensing systems has produced compounds that can disrupt the bacteria's ability to communicate, thereby disabling or diminishing the bacteria's ability to become pathogenic.

• Therefore, the body is not compromised by cell damage, inflammation, toxicity, or other detrimental effects of the bacteria.

• This gives the body time to eradicate the bacteria naturally through normal immune system functions.

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2. Anti-biotic resistanceNow a days most of bacteria are antibiotic resistant Penicillin resistant bacteria developed in 1942, just after 2 years of its introduction

Antibiotic sensitive bacteria

Antibiotic resistant bacteria

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3 strategies can be applied

Targeting signal

Generation

Targeting AHL signal

disseminationTargeting the signal

ReceptorSignal precursor

Signal Signal Signal

Signal receptor Signal receptorSignal receptor

Strategies for quorum sensing inhibition

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Conversation between bacteria

To Talk? Or Not To Talk?

Quorum sensing Quorum Quenching

Autoinducers

Regulate QS-controlled phenotypes

Natural Synthetic AHL QSI QSI QSI degradation system

QS Inhibitors

Block Quorum Sensing Pathway

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Applications

QS research has many potential applications:

Models to understand biology of sociality Understanding bacteria To develop new medicines to treat devastating

bacterial infection like Cystic fibrosis Otitis Media Bacteria Endocarditis etc.

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Biofilms

Bacteria use quorum sensing in biofilm

formation. A structured community of bacterial

cells enclosed in self-produced

polymeric matrix. They are protective mode of growth

that allows survival in hostile

environment. Bacteria in the biofilms are inherently

resistant to killing.

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Why research on Biofilms?

Due to morphology of

biofilms, bacteria capable

of forming them are

highly resistant to

Antibiotics, making

treatment difficult.

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Advantages of Anti-Quorum Sensing

The advantage of the anti-quorum sensing approach to controlling infection is that there are few evolutionary forces that select for resistance—there is little in the process that would create resistant strains.

Since the compounds kill none of the bacteria, any resistant mutations must compete with living, non-resistant individuals.

In other words, there is no survival advantage to the resistant mutations, and natural selection does not come into play. Resistant strains will be unlikely to occur.

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Cont…

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Next to the potential antimicrobial functionality, quorum-sensing molecules, especially the peptides, are investigated for their use in other therapeutic domains as well, including immunology and oncology.

This hypothesis is based on

(1) the recent evidence of prokaryote–eukaryote signaling by the use of quorum-sensing signaling molecules

(2) the apoptotic phenomenon seen in bacteria

(3) the clear similarities between the bacterial quorum-sensing mechanisms and the metastatic process tumor cells initiate

(4) the multiple receptor targeting and

(5) the possibility of pharmacologic manipulation of peptides, resulting in increased receptor targeting.

Conclusions and future perspectives

QS inhibitors have provided evidence of alternative method for fighting bacterial infections.

QS inhibitors can be isolated from the huge natural pool of chemicals.

unsuitable compounds lead to lack in selection of human compatible QS inhibitors.

Further studies are needed on QS gene expression QS inhibitors, may replace the antibiotics.

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Abbreviations

• DPD: (4,5-dihydroxy-2,3-pentanedione)• ABC: (ATP binding cassette)• AI: Autoinducing protein• AHLs: N-acylhomoserine lactones• H: Histidine• D: Aspartate

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References

• Abdelnour, A., Arvidson, S., Bremell, T., Ryden, C. & Tarkowski, A. (1993). The accessory gene regulator (agr) controls Staphylococcus aureus virulence in a murine arthritis model. Infect Immun 61, 3879–3885.

• Thomas, G. Platt. & Clay Fuqua. (2010). Whats in a name? The semantics of quorum Sensing. 18(9): Microbiol. 383-387.

• Antunes, L. C. & Ferreira, R. B. (2009). Intercellular communication in bacteria. Crit Rev Microbiol 35, 69–80.

• Bendiak, G. N. & Ratjen, F. (2009). The approach to Pseudomonas aeruginosa in cystic fibrosis. Semin Respir Crit Care Med 30, 587–595.

• Otto, M., Sussmuth, R., Jung, G. & Gotz, F. (1998). Structure of the pheromone peptide of the Staphylococcus epidermidis agr system. FEBS Lett 424, 89–94.

• Smith, D., Wang, J. H., Swatton, J. E., Davenport, P., Price, B., Mikkelsen, H., Stickland, H., Nishikawa, K., Gardiol, N. & other authors (2006). Variations on a theme: diverse N-acyl homoserine lactone-mediated quorum sensing mechanisms in Gram-negative bacteria. Sci Prog 89, 167–211.

• http://www.sciencedaily.com

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