**4.1 Anti-quorum sensing activity**

It has now become apparent that different types of microorganisms have evolved the ability to recognize and act in response to the presence of other microorganisms in their neighborhood. Most Gram-negative bacteria produce and respond to *N*-acyl homoserine lactone (AHLs) signal molecules to regulate production of secondary metabolites in order to monitor their own population density. These molecules, at a threshold population density, act together with cellular receptors and elicit the expression of target genes such as those involved in virulence, antimicrobial production, motility and swarming, sporulation, bioluminescence and biofilm formation. The concept of quorum sensing (QS) was initially described in *Vibrio fischeri*, a luminescent marine bacterium. It was observed that the organisms express genes controlling light emission (the luciferase enzyme) when in symbiotic association with its hosts, the squid [83]. At low population densities (i.e. free-living in seawater) *Vibrio fischeri* does not express luciferase and so is non-luminescent. However, when cultured in the laboratory to high cell densities, they express bioluminescence with a blue-green light. They do not emit light unless they detect a concentration high enough of their own AHL. These organisms usually form symbiotic relationships with some fish and squid species such as *Euprymna scolopes*. *Euprymna scolopes* appears bioluminescent in dark surroundings because of high-population of the cells (*Vibrio fischeri*) in a specialized light organ. *Euprymna scolopes,* in return, offers nutrients to the *Vibrio fischeri* population. The QS system originally identified in *Vibrios* involved two genes, luxl and luxR. The Luxl codes for an enzyme, which synthesizes 3-oxo-C6-homoserine lactone (an auto-inducer as they are produced by the same cells whose metabolism they regulate) [82].

The unpleasant side effects of antibiotics (such as ototoxicity and nephrotoxicity associated with the aminoglycosides) have led to preference for preventive rather than curative approach towards fighting infectious diseases. Inhibition of quorum sensing activity has been hypothesized as one approach that can be useful in preventing bacterial infection. It could provide an additional approach to antibiotic mediated bactericidal or bacteriostatic activity thereby reducing the risk of successful establishment of infections or resistance development in the bacteria. This is supported by the protective effect of QS inhibition demonstrated

**123**

*Combating Biofilm and Quorum Sensing: A New Strategy to Fight Infections*

**5. Medicinal plants with biofilm inhibition activity**

thereby blocking QS network and biofilm development [85].

Lamiaceae Aerial

Commelinaceae Whole

**Plant name Family Part used Solvent Biofilm** 

parts

plant

*Curcuma longa* L. Zingiberaceae Rhizome Aqueous Removed 30 to

in animal infection models. A simple animal infection model on QS was launched in *Caenorhabditis elegans*, a nematode that feeds on bacteria. When fed on opportunistic pathogens such as *P. aeruginosa*, the worm was mostly destroyed within a short time after taking in the bacteria; presumably annihilated by the actions of cyanide and phenazines produced by the bacteria [84]. However, in instances where the worms ingested *P. aeruginosa* with mutations in the QS-controlling systems, they were not killed but were rather sustained on the bacteria. This model highlights the involvement of QS-regulated virulence factors in pathogenicity of *Pseudomonas aeruginosa*. It is obvious from such models that interruption of the QS apparatus of bacteria by plant extracts or other chemical compounds may offer a novel and an exciting approach to fight the existing problems associated with antimicrobial

Many bacteria produce AHL molecules in response to QS and so could be used as biomonitor organisms in screening of compounds for anti-QS activity. Such bacteria include *Chromobacterium violaceum, Erwinia carotovora* and *Pseudomonas* 

Natural products have been identified to inhibit biofilm formation in microorganisms. The exact mechanism for most of the agents is yet to be elucidated. Medicinal plants have been identified as rich source of bioactive compounds that have the capability of interfering with biofilm formation but most of these studies are still in the early stages of drug development. The anti-biofilm effects of medicinal plants have been proposed to be due to the inhibition of formation of polymer matrix, suppression of cell adhesion and attachment, interruption of extracellular matrix formation and reduction in virulence factors production and activation,

Medicinal plants belonging to various plant families reported to have biofilm inhibitory activity are listed in **Table 1**; the part of the plant (leaves, fruits, stem

Lythraceae Fruit Methanol Inhibit biofilm

Ericaceae Fruit Decoction Reducing 47%

Distilled water

**inhibition activity**

formation by 60.9% at 0.78 mg/mL

MRSA biofilm viable counts. 12.5 mg/mL

40% of biofilm at 5–0.63 μg/mL

Inhibited the biofilm formation at 250 μg/mL

Ethanol Inhibit biofilm

formation in *E. coli* by 70% at 150 μg/mL

**Reference**

[86]

[87]

[88]

[89]

*DOI: http://dx.doi.org/10.5772/intechopen.89227*

chemotherapy.

*aeruginosa*.

*Punica granatum*

*Salvia fruticosa* Mill.

*Vaccinium corymbosum* L

*Commelina benghalensis* L.

L

*Combating Biofilm and Quorum Sensing: A New Strategy to Fight Infections DOI: http://dx.doi.org/10.5772/intechopen.89227*

*Bacterial Biofilms*

their survival in complex environments [82].

and other chemical compounds),

• Biodegradation of signal molecule.

**4.1 Anti-quorum sensing activity**

ment. Such methods of interruption of the QS include:

• Disruption of biosynthesis of signal molecules,

• Chemical inactivation of quorum sensing signals,

the same cells whose metabolism they regulate) [82].

and interruption of signals. The ability of bacteria to dispatch, pull together, and process information allow them to act as "multicellular" organisms and enhance

Any mechanism capable of disrupting QS signals can be used to reduce survival of the microorganism thereby preventing or reducing virulence in the host environ-

• Application of QS antagonists (e.g. use of extracts from higher plants and algae

Agents capable of inhibiting the growth of microorganisms or disrupting the quorum sensing mechanisms of the microorganisms or interrupting the biofilm

It has now become apparent that different types of microorganisms have evolved the ability to recognize and act in response to the presence of other microorganisms in their neighborhood. Most Gram-negative bacteria produce and respond to *N*-acyl homoserine lactone (AHLs) signal molecules to regulate production of secondary metabolites in order to monitor their own population density. These molecules, at a threshold population density, act together with cellular receptors and elicit the expression of target genes such as those involved in virulence, antimicrobial production, motility and swarming, sporulation, bioluminescence and biofilm formation. The concept of quorum sensing (QS) was initially described in *Vibrio fischeri*, a luminescent marine bacterium. It was observed that the organisms express genes controlling light emission (the luciferase enzyme) when in symbiotic association with its hosts, the squid [83]. At low population densities (i.e. free-living in seawater) *Vibrio fischeri* does not express luciferase and so is non-luminescent. However, when cultured in the laboratory to high cell densities, they express bioluminescence with a blue-green light. They do not emit light unless they detect a concentration high enough of their own AHL. These organisms usually form symbiotic relationships with some fish and squid species such as *Euprymna scolopes*. *Euprymna scolopes* appears bioluminescent in dark surroundings because of high-population of the cells (*Vibrio fischeri*) in a specialized light organ. *Euprymna scolopes,* in return, offers nutrients to the *Vibrio fischeri* population. The QS system originally identified in *Vibrios* involved two genes, luxl and luxR. The Luxl codes for an enzyme, which synthesizes 3-oxo-C6-homoserine lactone (an auto-inducer as they are produced by

The unpleasant side effects of antibiotics (such as ototoxicity and nephrotoxicity associated with the aminoglycosides) have led to preference for preventive rather than curative approach towards fighting infectious diseases. Inhibition of quorum sensing activity has been hypothesized as one approach that can be useful in preventing bacterial infection. It could provide an additional approach to antibiotic mediated bactericidal or bacteriostatic activity thereby reducing the risk of successful establishment of infections or resistance development in the bacteria. This is supported by the protective effect of QS inhibition demonstrated

formation may be useful in the fight against microbial pathogenicity.

**122**

in animal infection models. A simple animal infection model on QS was launched in *Caenorhabditis elegans*, a nematode that feeds on bacteria. When fed on opportunistic pathogens such as *P. aeruginosa*, the worm was mostly destroyed within a short time after taking in the bacteria; presumably annihilated by the actions of cyanide and phenazines produced by the bacteria [84]. However, in instances where the worms ingested *P. aeruginosa* with mutations in the QS-controlling systems, they were not killed but were rather sustained on the bacteria. This model highlights the involvement of QS-regulated virulence factors in pathogenicity of *Pseudomonas aeruginosa*. It is obvious from such models that interruption of the QS apparatus of bacteria by plant extracts or other chemical compounds may offer a novel and an exciting approach to fight the existing problems associated with antimicrobial chemotherapy.

Many bacteria produce AHL molecules in response to QS and so could be used as biomonitor organisms in screening of compounds for anti-QS activity. Such bacteria include *Chromobacterium violaceum, Erwinia carotovora* and *Pseudomonas aeruginosa*.
