QUORUM SENSING

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Quorum sensing (QS) is defined as the cell-to-cell interaction (communication) between one bacterium and another, and which allows them to share genetic information through the release of signaling molecules (e.g., acylated homoserine lactones, AHLs) that enables them to evade inconveniences in their surrounding environment. It is a cell-to-cell interaction that occurs amongst bacterial community, and in which they release signal molecules that enable them to form formidable structures (e.g., biofilms) that gives them protection from untoward effects in their surrounding environment. Quorum sensing is a mechanism by which bacteria access their population density in a particular environment; and it allows bacterial cells to form a quorum before initiating a particular response especially one that is detrimental to their host and protective to the microbial community in the biofilm. Through quorum sensing, bacterial cells in a population can initiate a coordinated attack such as the production of harmful toxins during the initiation of a particular disease process in a host. Biofilm development has been documented in a number of Gram-positive and Gram-negative pathogenic bacteria including Klebsiella pneumoniae, Staphylococcus aureus, E. coli and Pseudomonas aeruginosa. Microorganisms including bacteria communicate amongst themselves by the release of signaling molecules which allows them to form clusters, and evade any untoward effect in their environment.

QS is mediated through diffusible signal molecules released by microorganisms (both pathogens and non-pathogens), and this phenomenon allows bacteria to form biofilms, virulence factors and a population of microbes resistant to antimicrobial agents. These inconveniences or untoward effect that bacteria (including pathogenic microbes) dodge in their surrounding environs may include antibiotic action, disinfectants, antiseptics, and the action of other antimicrobial agents whose activity is geared towards abating the deleterious effect of microorganisms (especially those that are of clinical importance) to humanity. Quorum sensing involves the communications that go on amongst bacterial population, and which trigger them to change their behaviour in a particular environment (especially to the advantage of the microbes). This change in behaviour is usually stimulated by the release of chemical signaling molecules which are generally known as auto-inducers (AIs). Acylated homoserine lactone (AHLs) is a typical auto-inducer produced by bacteria as they try to communicate with each other in their environment. AIs are cellular signaling molecules produced by microbes (particularly bacteria), and which give them the exceptional ability to direct and monitor gene expression in their immediate environment. These signaling molecules binds to specific activating protein molecules within the bacterial cells; and this binding triggers the transcription of specific gene molecules that are translated to produce particular proteins such as toxins. Acylated homoserine lactones (AHLs) produced by bacteria, aromatic alcohols (produced by yeast cells), gamma-butyrolactones (produced by Streptomyces)and 4-hydroxy alkyl quinolone (produced by Pseudomonads) are typical examples of other auto-inducers produced by microbial cells during quorum sensing activities. With quorum sensing, pathogenic bacteria including those that form biofilms in the environment can detect or sense when the bacterial population in their environment is sufficient enough to initiate a targeted microbial attack.

Quorum sensing (which was first discovered in bioluminescent bacteria) occurs in prokaryotic and eukaryotic cells and in Archaea; and they generally help microbial cells to monitor their immediate environment for signaling molecules of their own kind which allows them to work cooperatively as a community instead of working as single cells. Through quorum sensing, microorganisms are able to form biofilms, control spore formation, produce virulent factors, produce antibiotics and emit light in their surrounding environment. The production of AIs through quorum sensing has been recorded in both Gram-positive and Gram-negative pathogenic microorganisms such as Staphylococcus aureus, Pseudomonas aeruginosa, Vibrio cholerae and Bacillus cereus. QS have currently been studied and considered as a potent tool for the containment of antimicrobial resistance in microbes due to the cell-to-cell interaction amongst microorganisms which make them attractive targets for novel therapeutic attack. It is also believed that therapies or drugs that are directed against the signaling systems of microbes will enhance the prognosis of patients through reduced biofilm formation and demobilization of microorganisms recruited for biofilm formation. Since biofilm formation have been implicated in a handful of chronic bacterial infections including cystic fibrosis, dental caries, S. aureus mediated endocarditis, infections in catheterized patients and other gum diseases; novel antimicrobial agents or therapy incorporating quorum sensing inhibitory compounds directed against microbes will go a long way in weakening the virulence and pathogenicity factors of bacteria by inhibiting them from communicating amongst each other and forming biofilms.

Further reading

Brooks G.F., Butel J.S and Morse S.A (2004). Medical Microbiology, 23rd edition. McGraw Hill Publishers. USA.

Gilligan P.H, Shapiro D.S and Miller M.B (2014). Cases in Medical Microbiology and Infectious Diseases. Third edition. American Society of Microbiology Press, USA.

Madigan M.T., Martinko J.M., Dunlap P.V and Clark D.P (2009). Brock Biology of Microorganisms, 12th edition. Pearson Benjamin Cummings Inc, USA.

Mahon C. R, Lehman D.C and Manuselis G (2011). Textbook of Diagnostic Microbiology. Fourth edition. Saunders Publishers, USA.

Patrick R. Murray, Ellen Jo Baron, James H. Jorgensen, Marie Louise Landry, Michael A. Pfaller (2007). Manual of Clinical Microbiology, 9th ed.: American Society for Microbiology.

Wilson B. A, Salyers A.A, Whitt D.D and Winkler M.E (2011). Bacterial Pathogenesis: A molecular Approach. Third edition. American Society of Microbiology Press, USA.

Woods GL and Washington JA (1995). The Clinician and the Microbiology Laboratory. Mandell GL, Bennett JE, Dolin R (eds): Principles and Practice of Infectious Diseases. 4th ed. Churchill Livingstone, New York.

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