A new compound has been shown toprevent communication amongPseudomonas aeruginosa. This researchhas the potential to lead to a new class of antibiotics.

P. aeruginosa infections rarelytrouble healthy individuals. However,the bacteria thrive in the lungs of cysticfibrosis patients, and they also targetimmunocompromized patients andburn victims – they are also extremelydifficult to treat. Not only are thesebacteria resistant to most of theantibiotics currently available but theycan also produce a slimy biofilm thatcoats surfaces and shields the bacterialcommunity from the few antibioticsthat would otherwise still work.

At present, these bugs can not bekilled; therefore, scientists are lookingfor other ways to keep them at bay.A tempting approach is to block theproduction of virulence factors andbiofilm formation by interfering withquorum sensing.

Strength in numbersQuorum sensing systems enablepathogens to measure their owndensity and launch an attack on hosttissues as a large population, ratherthan as individual cells. The bacteriaemit a signal molecule (autoinducer)that, at low cell density, diffuses into the surrounding media. But with increasing cell density, theconcentration of the autoinducer risesas well. When a threshold concentrationis reached, the autoinducer binds to anintracellular receptor molecule that,when complexed with the autoinducer,acts as a transcriptional activator.Among other things, this mechanismhelps to set off biofilm formation andthe production of virulence factors.

P. aeruginosa has two quorum-sensingcircuits, the las system and the rhlsystem. The las system consists ofautoinducer synthase LasI, bindingprotein LasR and autoinducer AI1; it controls the production of LasBelastase and other virulence factors and is required for the activation of thesecond quorum sensing system (rhl ).

A new class of antibiotics?Although some researchers remainsceptical as to whether a new class ofantibiotics can be found by targetingquorum-sensing systems, others haveembraced the idea with enthusiasm.‘The exciting thing is that there hasbeen talk for about a decade of tryingto develop antivirulence drugs, ratherthan antibacterial drugs that killbacteria,’ says Everett Greenberg at the University of Iowa (http://www.uiowa.edu), one of the pioneers ofquorum sensing research inP. aeruginosa. ‘I think these quorumsensing signals represent a realopportunity to test the idea, becausewe know they are important incontrolling virulence.’

Several academic groups and a few companies are pursuing variousstrategies to interfere with quorumsensing. In the commercial arena,Vertex (http://www.vpharm.com) are screening huge libraries ofsmall-molecule compounds thatmight do the trick, and Biosignal(http://www.biosignal.com.au) areworking with furanones, which arenatural quorum-sensing inhibitorsemitted by a seaweed to preventbacterial growth on its surface.

Other groups have tried to rationallydesign autoinducer analogues thatblock quorum sensing. The signal

molecules of Gram-negative bacteriaare homoserine lactone (HSL)-basedmolecules that differ in their acyl sidechains. Previous efforts have centredon altering this acyl structure, but thisapproach did not deliver potentantagonists. Therefore, Hiroaki Sugaand colleagues at the University ofBuffalo (http://www.buffalo.edu)decided to tackle the HSL group. Suga believes that it is this structurethat is actually important for R proteinactivation.

Breaking the codeTo explore this hypothesis, Suga andcolleagues synthesized a library of96 AI1 analogues in which the HSLgroup is replaced by amines oralcohols. To screen for biologicalactivity, they used a P. aeruginosa strainin which lasI and rhlI were knocked outso that the bacteria could not produceany autoinducers. In addition, thebacteria carried a GFP reporter geneunder the control of the lasI promoter.Using this approach, they found anagonist that was as potent as AI1.

Based on the structure of thisagonist, they designed three othercompounds that were similar instructure and again screened for agonistactivity. They were hoping that thisexperiment would tell them detailsabout the structural elements requiredfor R protein activation.

But they were in for a surprise: thereporter protein remained silent whenthe strains were exposed to compound 3[1]. ‘We could not believe that thismolecule failed to bind to the targetprotein, because the structure wechanged is really tiny,’ recalls Suga.

He suspects that compound 3 acts as a competitive inhibitor, and results

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Stop talking at the backMartina Habeck, freelance writer

of an antagonist assay support thishypothesis. Other assays revealed that compound 3 blocks theproduction of various virulence factors and prevents biofilm formation (Fig. 1).

Greenberg believes the result ispromising. However, he is concernedthat this compound itself might notbe useful as a drug: ‘It is too weak aninhibitor’. Indeed, compound 3inhibited lasI induction by only 35%.‘But it may be possible to furtherdevelop it into a drug,’ reckonsGreenberg.

According to Suga, this won’t be toodifficult. ‘The approach we used is veryflexible, and it is possible to expand itby working with other focused libraries.Therefore, we can look for more potentantagonists in the future.’

Reference1 Smith, K.M. et al. (2003) Induction and

inhibition of Pseudomonas aeruginosa quorumsensing by synthetic autoinducer analogs.Chem. Biol. 10, 81–89

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Figure 1. Effect of compounds on biofilm formation of Pseudomonas aeruginosa; the bacteria,which express the green fluorescent protein, are grown on cover slips in a static culture.When grown in the presence of autoinducers, the bacteria form a biofilm. The addition ofcompound 3 blocks biofilm formation. (a) P. aeruginosa PAO-JP2 (laboratory strain). (b) P. aeruginosa PAO-1 (wildtype strain). Figure reproduced, with permission, from Ref. [1].

Drug Discovery Today

1 µM AI1, 10 µM AI2 1 µM AI1, 10 µM AI2,50 µM compound 3

Negative control 50 µM compund 3

Negative control(a)

(b)

Centenarians provide genetic clue toage-related diseaseSabine Louët, freelance writer

Studies on peoplewho have passedtheir 100th birthdayare revealing the roleof genetics in age-related disease.

Many age-related diseases result frominflammatory processes, and centenariansenjoy unusually low inflammatoryprofiles, says Italian immunologist,Claudio Franceschi, Professor of

Immunology at the University ofBologna, Italy (http://www.unibo.it).

Link between inflammation and ageingFranceschi, says that his work oncentenarians has revealed a genetic link between a mechanism for chronicinflammation and ageing. ‘People areprone to develop inflammation on agenetic basis.’

Franceschi discovered thatcentenarians present low levels of thepro-inflammatory cytokine interleukin 6(IL-6) and high levels of the anti-inflammatory cytokine IL-10. High levelsof IL-6, which is produced in musclesand bones, is related to loss of musclemass and power with age. Such statustranslates into frailty and disability andthe occurrence of diseases, such asosteoporosis.