Quorum sensing: Listen to the bacterial talk!

Do bacteria talk?

Yes, they do!

Which language do they speak?

Unlike the language of humans, bacteria use chemical signaling molecules to communicate. These diffusible chemical signals are termed as auto-inducers. Many microbes use this type of cell-to-cell communication called as quorum sensing (QS). The QS molecules are released into the environment and are sensed by other bacteria of the same or different species. They are said to co-ordinate population activities. Studies are ongoing and scientists are interested to know more about these chemical signals and are trying to decode.

What is the nature of talk?

The bacterial cell-to-cell communication is possible chemically by means of peptides (in case of most Gram positive bacteria) and N-acyl homoserine lactones, AHLs (in case of Gram negative bacteria). At higher cell concentrations bacteria use these chemical signals to switch from a nomadic existence to that of a multicellular community normally termed as biofilms. The accumulation of signaling molecules enables the cells to sense the number of bacteria and in turn controls the cell density. In the natural surroundings, there are many different bacteria living together which use various classes of signaling molecules. As they employ different languages, they cannot necessarily talk to all other bacteria.

Why do they talk?

In natural environments bacteria need to adapt to changes in nutrient availability, competition, limited living space and other toxicants, or to fight the immune response of the host where they colonize. Also, there is an obligation for these tiny creatures to communicate and co-ordinate in order to overcome these problems. The important processes related to production of virulence factors and antimicrobial resistance in case of pathogenic bacteria has been credited to quorum sensing.

Quorum sensing is an important part in the formation of biofilms.

What are biofilms?

In environments the microbial cells are always in the form of complex multicellular communities. These surface attached (sessile) microbial communities are termed as “biofilms”. They thrive on environmental settings such as cooling towers, heat exchanger surfaces and most of the corrosion related problems are entitled to their credit. They are also seen present on various medical settings such as catheter surfaces, prosthetic devices etc. They are known to be the major culprits for disease spread through catheter surfaces and prosthetic devices. In medical and environment related problems they are sanctified with the powers of higher resistance towards antimicrobials, metals and other disinfecting agents. They usually form an unavoidable part of the system with which they associate and this property makes them a nasty personality in the microbial world. Biofilm growth and bio-fouling are global problems causing a tremendous economic problem in medicine and industry.

In biofilms too interestingly there is a talk in between the communities. The talk in case of pathogenic communities is focused more towards initial adherence, formation of multicellular structures and to establish pathogenesis for disease condition spread.

Whom do they talk to?

The bacteria talk to each other (same species) and to bacteria from different species. The talk between different bacterial species is normally termed as “quorum sensing cross talk”. It is also observed that different bacterial species use different quorum sensing molecules to communicate. A single species can have more than one quorum sensing system and the bacteria might respond to each molecule in a different fashion. 

Are there any standard quorum sensing bacterial systems?

The production of these chemical signals or quorum sensing molecules has typically been checked by bacteriological monitor systems. The characteristics of bacterial systems most commonly used are:

Pigment production: Chromobacterium violaceum is the most commonly studied bacteria for quorum sensing. It is a soil-borne Gram negative bacterium. The indication shown by this bacterium is the production of a pigment violacein when reacted with a quorum sensing auto-inducer molecule, N-hexanoyl homoserine lactone (HHL). The HHL is released into the environment and diffuses back into the bacterium when quorum sensing has been reached. This bacterium is studied with respect to many industrial and pharmaceutical perspectives.

Bioluminescence: Vibrio fischeri is another bacterium used for the quorum sensing studies. It is a marine bioluminescent bacterium, produces light only when large numbers of bacteria are present. The luminescence was initiated by the accumulation of auto-inducer in the medium. In this case the mechanism of cell density sensing was observed for producing luminescence.

Antibiotic production: Erwinia carotovora is a bacterium known to produce antibiotic as a result of quorum sensing. A class of mutant could not make antibiotic on their own but could do so when cross-fed by a second group of mutants. It was observed that second type was supplying a signaling molecule, which triggered antibiotic synthesis in the first group.

Enzyme production: Pseudomonas aeruginosa is a Gram negative bacterium known to be a human pathogen. It shows similar quorum sensing system as V. fischeri. It is shown to regulate the production of an important virulence factor such as the production of an enzyme elastase.

Why is this study necessary?

In accordance with the interfering mechanisms and properties posed by controlling the talk or cross talk, most of the diseases can be controlled. By use of mediators that might help disrupt the talk, most of the communications in between these bacteria can be curbed. The initial work in this field was more focused on the symbiotic and pathogenesis as well as the AHL regulation of traits required for the symbiosis or virulence. The recent research has been more focused on the broad range of bacteria looking at their phenotypic expressions like production of anti-fungal compounds, swarming behavior, biofilm formation and production of extra-cellular polysaccharides. Researchers have shown the use of quorum sensing molecules to increase fermentation yields. The inhibition of quorum sensing using antagonist results in cell growth and hence is beneficial for the production of desired product.

There is a continuous search for potential quorum sensing inhibiting agents in case of pathogenic and detrimental bacteria and hence a need to know more about their communication signaling molecules.

Quorum sensing inhibitors

The quorum sensing inhibitors are similar to AHL molecules which are generally used to inhibit the bacterial talk and disrupt biofilms. Quorum sensing inhibition may represent a natural, widespread, antimicrobial strategy utilized by plants and other organisms with significant impact on biofilm formation. Quorum signaling is now recognized as a global regulatory mechanism in bacteria which in some human and plant pathogens such as Pseudomonas aeruginosa and Erwinia carotovora has been shown to regulate virulence. Some of the representative agents that show inhibition of bacterial talk are:

Antibiotics: The agents belonging to macrolide group of antibiotics such as Azithromycin have shown to disrupt quorum sensing in Pseudomonas aeruginosa, a major causative agent of cystic fibrosis in humans. The antibiotic disrupts communication in Pseudomonas as Azithromycin acts as antagonist inhibiting the QS and in-turn reducing the biofilm formation.

Medicinal Plants: Researchers suggested the use of vanilla seed extracts for the disruption of quorum sensing in P. aeruginosa biofilms.

Marine systems: One of the marine seaweed, Delisea pulchra produces a number of halogenated furanones and enones that interfere with the formation of biofilms by inhibiting the talk between bacterial systems. Since the halogens produced cannot be suited to human systems, there are efforts for search of other natural quorum sensing inhibitors. Researchers say that chemical signaling controls the behavior of biofilms. They have begun to manipulate both biofilm formation and detachment of the biofilms using these signals and their analogues.

This is a new avenue in the era of managing environmental and medical related problems. Let us understand the language of these tiny creatures and decode it. Let’s listen to the bacterial talk and recognize whether they are our friends or foe.