2. What is quorum sensing? Bacteria can behave either as individual single-celled organisms or as multicellular populations. Bacteria exhibit these behaviors by chemically "talking" to one another through a process called quorum sensing. Quorum sensing involves the production, release, and community-wide sensing of molecules called autoinducers ( AI ) that modulate gene expression, and ultimately bacterial behavior, in response to the density of a bacterial population.
3. A brief overview of the process Bacterial genes code for the production of signaling molecules called autoinducers that are released into the bacterium's surrounding environment. These signaling molecules then bind to signaling receptors either on the bacterial surface or in the cytoplasm. When these autoinducers reach a critical threshold level, they activate bacterial quorum sensing genes that enable the bacteria to behave as a multicellular population rather than as individual single-celled organisms. The autoinducer/receptor complex is able to bind to DNA promoters and activate the transcription of quorum sensing-controlled genes in the bacterium. In this way, individual bacteria within a group are able to benefit from the activity of the entire group.
4. MECHANISM IN GRAM NEGATIVE BACTERIA. InGram-negative bacteria, the autoinducers are typically molecules called acyl- homoserine lactones or AHL. AHLs diffuse readily out of and into bacterial cells where they bind to AHL receptors in the cytoplasm of the bacteria. When a critical level of AHL is reached, the cytoplasmic autoinducer/receptor complex functions as a DNA- binding transcriptional activator. MECHANISM IN GRAM POSITIVE BACTERIA. InGram-positive bacteria, the autoinducers are oligopeptides, short peptides typically 8-10 amino acids long. Oligopeptides cannot diffuse in and out of bacteria like AHLs, but rather leave bacteria via specific exporters. They then bind to autoinducer receotors on the surface of the bacterium. When a critical level of oligopeptide is reached, the binding of the oligopeptide to its receptor starts a phosphorylation cascade that activates DNA- binding transcriptional regulatory proteins called response regulators.
5. Canonical bacterial quorum-sensing (QS) circuits. Canonical bacterial quorum-sensing (QS) circuits. Autoinducing peptide (AIP) QS in Gram-positive bacteria by (A) two-component signaling, or (B) an AIP- binding transcription factor. Small molecule QS in Gram-negative bacteria by (C) a LuxI/LuxR-type system, or (D) two-component signaling. Gram positive Gram negative
6. ADVANTAGES OF MULTICELLULAR BEHAVIOUR As the entire population of bacteria simultaneously turn on their virulence genes, the body's immune systems are much less likely to have enough time to counter those virulence factors before harm is done. Virulence factors such as exoenzymes and toxins can damage host cells enabling the bacteria in the biofilm to obtain nutrients. Quorum sensing enables some of the bacteria to escape the biofilm and return to individual single-celled organism behavior in order to find a new site to colonize. Bioluminiscence is also produced by some bacteria by quorum sensing.
7. Role of quorum sensing in bacterial virulence
8. Staphylococcus aureus (gram positive)
9. The accessory gene regulator (agr) quorum sensing system of S. aureus
10. Important features Despite its widespread prevalence in healthy subjects, S. aureus is also a very dangerous opportunistic pathogen which has been increasingly associated with antibiotic resistance. S . aureus has multiple virulence factors and can display very rapid transmission, aiding its importance as a human pathogen. S. aureus forms biofilms on many surfaces, including indwelling devices such asurethral stents . These indwelling devices, and subsequent biofilms formed on them, pose a serious risk for Staphylococcus infection
11. Virulence factors of Quorum sensing S. aureus has a peptide based quorum sensing system ,encoded by accessory gene regulator (agr) locus. The autoinducer (AI) is an oligopeptide termed as Auto inducer peptide (AIP). AIP is encoded by agrD. Genes involved in agr system are agr A,agrB,agrC,agrD. agrD, encodes pre-AIP. AgrB protein (membrane bound) trims and releases AIP from its precursor form to the extracellular environment. AgrC (a membrane bound sensor kinase) to which extracellular AIP binds,leading to autophosphorylation of AgrC- activates AgrA. AgrA (phosphorylated -> active) , induces transcription at the promoters P2 (higher affinity) and P3.
12. Regulation of virulence genes in agr systeem is predominantly involved around promoters P2 & P3 ( oppositely directed). P2 promotes transcription of agr operon from the RNAII transcript ,which includes agrA,B,C,D. P3 transcription leads to production of RNAIII (regulatory RNA). RNAIII is the effector molecule of the agr system. RNAIII reduces the expression of surface adhesins and increases the production of capsule toxins and proteases. **** RNAIII also functions as the mRNA for -toxin ( 5 end upregulates - haemolysin while 3 is required for repression of protein A synthesis). 4 distinct group of agr polymorphism found agrI,II,III,IV . AIP s of each group cross-inhibits other groups. (exception-group II & IV cross activates each other).
13. The accessory gene regulator (agr) quorum sensing system of S. aureus
14. Impact of agr on virulence RNAIII has the dual-function of activating production of a-toxin and repressing expression of rot, fibronectin binding proteins A and B, protein A, coagulase, and other surface proteins. Repression of rot, which encodes a repressor of toxins, leads to de-repression of additional toxins, proteases, lipases, enterotoxins, superantigens, and urease . The net result of this QS regulatory cascade is downregulation of surface virulence factors (such as protein A), and up-regulation of secreted virulence Factors (such as a-toxin). Most of the effects of QS on regulation of virulence in S. aureus Are mediated through direct and indirect regulation by RNAIII, however, phosphorylated AgrA also directly activates at least two additional virulence genes encoding phenol- soluble modulines. Biofilm formation : When agr nonfunctional(non active),bacteria S.aureus ha enhanced adhesion abilities (i.e. first stage of Biofilm formation). Agr also plays a role in detachment of cells from biofilm ,important for dissemination of during an infectious process. Active form triggers virulence . contribution of agr to disease has been studied in models like septic arthiritis, osteomyelitis, endopthalmitis, pulmonary infections, toxic shock syndromes.
15. (GRAM NEGATIVE) Pseudomonas aeruginosa
16. INTRODUCTION P.aeruginosa is a Gram-negetive bacterium. This organism is an opportunistic pathogen & commonly associated with nosocomial infection & infection of severely burned individuals, and is leading to cause of death in severe respiratory infections, such as chronic lung infections in CF patients. Infections with P. aeruginosa are difficult to eradicate due to their high levels of antibiotic resistance and growth in biofilms. Pseudomonas aeruginosa is a very versatile organism that can adapt too many different environments and can cause diseases in plants, animals and humans. This organism produces a broad range of exoproducts, which are regulated in a population density-dependent manner via cell-to-cell communication or quorum sensing".
17. Quorum sensing in P. aeruginosa: a complex regulatory network resulting in fine signal tuning
18. In P.aeruginosa at least three interwined quorum sensing (QS) system and one orphan autoinducer receptor affect the ability of this organism to cause disease. Two intertwined QS systems (the las and the rhl systems),rely on the production of acyl homoserine lactones(AHLs) as the signaling molecules(auto inducers-AIs) have been shown to be involved in virulence, biofilm development, and many other processes in P. aeruginosa. In the las system lasI gene encodes an enzyme which produce an AIs molecule called N-3-oxododecanoyl-homoserine lactone (3-O-C12-HSL). In the other hand, the rhlI gene of rhl system encodes an enzyme that produce another AIs molecule called N-butyryl-homoserine lactone (C4-HSL). Then it regulates its gene exp