Quorum sensing Final Term Paper
By: - Ibrahim Farouk Farag
Quorum sensing is a communication mechanism exhibited by different bacterial species to detect its population density by sensing specific signal molecules called autoinducers; when these signals present in threshold concentrations that represents certain population density it induces multiple genes expression that coordinates to perform several functions. (For example; biofilm formation, pathogenesis, production of certain compounds, virulence, symbiosis, etc...).
This mechanism is widespread among gram negative bacteria by utilizing certain class of autoinducerss called homoserine lactones, on the other hand; gram positive bacteria communicate by producing peptide- like compounds.
Recent researches discovered that quorum sensing is not only confined to be within certain bacterial species but also interspecies and inter-kingdom communications also exist.
So we can classify the quorum sensing that are present among different bacterial communities into 4 categories:-
1) The basic model system which is discovered in V.fisherii that is shared by most of the Gram negative bacteria with little variations and complexities that may be observed in some genera over the others.
2) The unique quorum sensing system that is demonstrated by gram positive bacteria which is depending on totally different signal molecules (peptide in nature), this system regulates the pathogenesis, virulence, biofilm formation and other different processes.
3) The third system discovered has a great role in interspecies communication called AI2/LUX S system.
4) The 4th one is highly unique, utilized for interkingdom communication (AI-3/ epinephrine/ norepinephrine) and is widely present in Ecoli.
The rational of this study is based on the understanding how bacteria communicate with each other's, how they develop its own consortia, how they can sense the danger and evading these factors.
After the understanding of these mechanisms, we can take this advantage to mess up these systems by searching for both natural and synthetic compounds that aid in controlling the harmful activities (such as pathogenesis, virulence, evading immune system and resistance to antibiotics).
So quorum sensing mechanisms can be the next target of the future medications to control harmful bacteria (1, 2).
History of researches conducted in Quorum sensing:-
Quorums sensing was first discovered at the late 1960s in a marine bacterium Vibrio fisherii that live in symbiotic relationship within the light organs of a marine squid, these bacteria when reach certain population density produce light.
This bioluminescence produced as a function of certain operon called luciferase operon (LUX ICDABE).
LUX I is gene responsible for the production of autoinducer (acyl homoserine lactone AHL), which diffuses in and out of the cell, when the population density increases to certain level, the environment will be rich in this autoinducer molecule.
This autoinducer binds to certain signal receptor called LUX R, which activated by the AHL forming LUXR-AHL complex and acts as transcription activators that binds to specific region on the operon system called LUX box which in turns activates the synthesis of the entire operon system including the LUX I itself (2).
Figure 1:- showing the LUXRI system (3)
After the discovery of the quorum sensing in Vibrio fisherii, it was found that this model is utilized by different types of gram negative bacteria especially alpha proteobacteria.
One of the well studied examples is quorum sensing mechanism exhibited by Pseudomonas aeruginosa which posseses 2 quorum sensing systems, one of them act downstream to the other.
The first quorum sensing system is a homolog to the model system (LUXRI) discovered in Vibrio fisherii called Las RI, this system uses different class of homoserine lactone (3-oxo-decandoyl homoserine lactone), and the other system RHLRI acts in hierarchical fashion in response to the LasRI system.
Figure 2:- Showing the interaction between the 2 systems of P.aeruginosa (4)
The figure shows us that both systems have its own genes which are responsible for autoinducer synthesis (lasI and RhlI), in addition to the presence of signal perception proteins unique for each system from the other one (LasR and RhlR), and how the first system modulate the activation of the other by giving it the first push to synthesize its own gene by the binding of the (LasR -C12) complex to the Lux box of the Rhl system.
There is other signal molecule called PQS (Pseudomonas quinolone signal) which also has a boosting effect on the activation of the Rhl system and act as additional regulatory signal(4).
Erwinia carotovora a plant pathogen possesses a quorum sensing system similar to that of P.aeruginosa, one of them act to regulate the virulence factors of the bacteria by producing the enzymes required for the maceration of the plant tissues as cellulases and pectinases, These virulence factors regulated by EXPI/EXPR system, the point behind the utilization of this mechanism is to allow the bacteria to achieve high population before secreting these exoenzymes and triggering the host defences against it. The other system is CarI/CarR which acts downstream to the first one and regulates the production of carbapenem to reduce the competition between Erwinia and its competitors.
The advantage in the Erwinia carotovora quorum sensing mechanism that both autoinducer encoding genes produce the same homoserine lactone which is N-3-oxo hexanoyl homoserine lactone, which means that both systems are triggered to adapt the bacteria for its environment after reaching a sufficient population density (2).
For salmonella and E.coli, they have a relatively different mechanism of quorums sensing as they possessa LUX R homolog called SdiA which seems to have a role in regulation of cell division genes ftsQAZ , but it was found that it lacks a homolog for LuxI which means that it can't produce its own signal molecule.
The precise role of SdiA in quorum sensing is not exactly identified till reported by Michael et.al. (2001), who stated that SdiA responsible for sensing the signal molecules which not produced by salmonella and Ecoli itself, and on other hand; it is responsible for regulation of some virulent genes like rck which involved in developing resistance against a human complement (5).
Ecoli also demonstrates an ability to utilize indole as a signal molecule in poor media as amino acid utilization becomes an important factor for bacterial survival (5).
On the other hand, quorum sensing mechanisms followed by Gram positive bacteria to regulate population density based genes expression show slight variations from the gram negative one on the basis of the nature of the autoinducer itself, its exporting mechanism, signal molecules post-translation processing and the emergence of membrane bound sensors that activated by the autoinducer rather than the presence of receptor protein distributed in the cell cytoplasm (7).
Table 1:- Examples on the most common quorum sensing systems among gram negative bacteria (10).
Quorum sensing of gram positive bacteria: - focusing on staphylococci as a model system:-
Staphylococcus aureus is a very good example for the quorum sensing in gram positive bacteria as it exists in two forms within its host:-
The first phase: - it lives as commensals when it is attached externally to the host tissues (low density population), so that it needs to express genes responsible for colonization and attachment.
The second phase: - when it gained access within the host tissues (high density population), it needs to express genes responsible for its pathogenicity as production of toxins and exoenzymes.
The shifting between those two phases is regulated by its quorum sensing system (Agr system).
The signal molecule is an oligopeptide in nature, binds to histidine kinase receptors that activate the downstream cascade by phosphorylation.
Among the activated proteins, there is a transcriptional regulatory protein called response regulator.
These peptides are not diffusible from in and out of the cell as homoserine lactones utilized by gram negative bacteria, but the transport mechanism of these peptides is mediated by certain exporters encoded by the Agr system itself. At the same time of signal exportation, the signal peptide is further processed and modified to bind to its cognate receptor.
This processing can be summarized as 1) cleavage of the signal peptide from larger peptide complex.
2) Addition of lactone rings, sulphur containing lactones and isoprenyl groups.
Figure 3:- summarizes Agr system of staphylococcus aureus.
1- The autoinducing peptide (AIP) is encoded by AgrD.
2- The AIP production is followed by its exportation outside the cell by the action of a membrane associated protein called Agr B.
3- Agr B has a dual functional as it mediates the AIP exportation and carries out the processing step to AIP by adding lactones, thiolactones or isoprenyl group.
4- The increase in the cell density will allow the binding between AIP and other membrane associated protein (AgrC) which is a histidine kinase that phosphorylates a downstream protein known as AgrA
5- The phosphorylated version of AgrA is response regulator that activates the transcription of 2 different mRNAs from 2 different promoters.
6- One of the RNAs trigger further production of AIP, while other RNA product can be either colonization or virulence factors (1,7).
Universal Interspecies language:- (LUX S / Autoinducer 2) system.
It is one of the most common quorum sensing among different type of bacteria (shared by both gram positive and gram negative), the point behind that is LuxS has other function beside quorum sensing roles.
Lux S is one of the main components involved in the Active methyl cycle (AMC), this could reveal the mystery behind the widespread of the LuxS among species and its role in the interspecies communication.
AI-2 is a secondary product of the active methyl cycle, as the S-adenosyl L-methionine (SAM) (the major methyl donor) is converted into S-Adenosyl L-homocysteine (SAH).
SAH is detoxified by the action of pfs enzyme, the detoxification product (s- ribosyl homocysteine(SRH) is the sole precursor of the LuxS.
Lux S plays an important role in the formation of autoinducer 2 precursor (4,5 dihydroxy 2,3 pentanedione (DPD)) during the conversion of s-Ribosyl homocysteine (SRH) to Homocysteine (HCY).
AI-2 is a cyclic derivative of DPD that can be produced spontaneously.
Figure 4:- chemical synthesis of DPD the precursor of AI-2 by the Lux S (8).
The first discovery of the Lux S and AI-2 come from studies carried out on Vibrio spp., it performs 3 different quorum sensing systems one of them is LuxS/AI-2.
AI-2 penetrates the periplasm of V.harvei and binds to a membrane bound protein called Lux P, the complex binds to a protein belongs to histidine kinases called Lux Q.
Lux Q in the absence of AI-2 phosphorylates one of its downstream proteins called LuxU, which in turns phosphorylate LuxO, phosphorylated version of LuxO binds to and activates s54 to synthesis short regulatory RNA (sRNA).
Both sRNA and Hfq chaperone inactivates LUXvh mRNA which is important for the successfulness of light generation as it regulates the transcription of luciferase operon.
As the concentration of AI2 increases, it binds to the membrane bound LuxP, which activates LuxQ phosphatase activity to remove the phosphate group from LuxH, consequently LuxH removes the phosphate group from Lux O leading to sRNA synthesis inhibition, and the light produced.
The other model that will describe AI-2 uptake can be demonstrated in E.coli and S.typhimurium:-
In this example, the LuxS /AI2 system is utilized to regulate the transcription of genes responsible for ABC transporters of these bacteria.
The AI-2 binds to a periplasmic bound protein (LsrB), which is a part of ABC transporter.
The repressor in this system is LsrR which is encoded by lsrR gene found upstream to the ABC operon (lsrACDBFGE).
Immediately downstream to lsrR gene, there is a gene encoding for xylokinase protein called lsrK, which phosphorylates AI-2 upon its entrance in the cytoplasm and promotes it to be sequestered within the cell.
The phosphorylated AI-2 inhibits the repression exerted by the LsrR on the lsrACDBFGE operon, and can be catabolised by LsrG, E and F to a product that is no longer be able to repress LsrR.
The most interesting point here in the both mechanisms exhibited by V.harvei and S.typhymurium is that AI-2 has the same chemical structure but they are different in the presence of boron in V.harvei (s-THMF-borate), and the absence of boron in S.typhymurium(s-THMF), this is may be due to the availability of the boron in the environment of the former and its absence from the environment of the later (8).
Figure 5:- showing a sketch summarizing the different responses of both bacterial species to the same autoinducer (AI-2) (8).
AI-3/ epinephrine/ norepinephrine as a model system for interkingdom quorum sensing:-
This is a very good example to describe the interrelations that can be constructed between bacteria and higher eukaryotic organisms, in order to modulate their lives within their hosts and respond quickly to the changes that occur in the surrounding environment.
Both epinephrine and norepinephrine are hormones secreted in the intestine to regulate the smooth muscle contractions, blood flow in the submucosal region and controlling sodium and potassium levels by regulating ion channels.
Those hormones in combination with the autoinducer-3 play in the same ground to modulate the activation of 2 component systems responsible for the virulence genes (LEE genes) of enterohemorrhagic E.coli (EHEC) which is responsible for haemolytic uremic syndromes and the genes responsible for flagellar synthesis.
Both hormones and AI-3 bind to periplasm bounded receptor which activates two sensor kinases QseE and QseC, the former transduces the signals to activate the LEE genes (locus of enterocyte effacement genes), while the later transduces the signals to activate the flagellar regulon by phosphorylating QseB which binds to the regulon promoter flhDC that promotes genes transcription (9).
Figure 6:- showing the 2 component system activated by the epinephrine/ norepinephrine and AI-3 (9).
Recent and future discoveries in quorum sensing researches:-
Figure 7:- summarizes the future and the current trends in the quorum sensing researches (10).
1- Interspecies communication by AHL:-
AHLs are a signal molecule that are widely utilized by gram negative bacteria, but the question now is, can a bacterium utilize AHL of other one??
This question is obviously answered in the interaction between Burkholderia cepacia and P. aeruginosa in the mixed environment occupies by both species.
As Burkholderia cepacia can utilize the AHL produced by P.aeruginosa and activates its own genes transcription.
The other example of interspecies communication is demonstrated in E.coli and Salmonella typhimurium as they are lacking the genes responsible for generation of AHL but it has a LuxR homolog SdiA, these evidences told us that these bacteria can utilize different AHLs produced by other bacterial species.
There is a large number of uncoupled LuxR homologues discovered among gram negative bacteria as (P.aeruginosa QscR, Rhizobium leguminosarum BisR and Sinorhizobium meliloti ExpR)
These two examples open up a broad area of research to elucidate the significance of this interspecies communication and the distance between the bacterial species that must be held to ensure significance interaction between them.
The interspecies interactions can be demonstrated in other bacterial species by using different classes of molecules as described previously in AI-2/Lux S system(10)..
Interactions between plant kingdom and Bacterial AHLs:-
Plants produce compounds that mimics the acyl homoserine lactone produced by bacterial pathogens to antagonize its effect in activating its quorum sensing systems or to promote its functions prematurely.
There are no clear in vivo evidences that prove that these mimicking compounds interact with the bacterial LuxR, as the only in vivo experiment done to prove this hypothesis between Tomato and A.tumifaciens shows no inhibition for the quorum sensing of the later.
Other examples of quorum sensing mimicking compounds produced by plants can be demonstrated in two plant pathogens Xanthomonas compestris and Xanthomonas oryzae, both bacteria posses an orphan LUX R homolog (XccR, OryR respectively), show a good responsiveness to the plant products which mimic the AHL structure.
All these examples can open up a window for further researches to explore these complicated interactions between the bacteria and other higher organisms.
Quorum sensing and bacterial social life:-
Better understanding of the quorum sensing mechanisms will allow us to expand our knowledge about the complex life between bacterial communities.
Also it will allow us to know which is necessary for certain consortia to be established in certain environment, to know who the co-operators are and who the invaders are.
Quorum sensing as a future target for drugs:-
The discovery of quorum sensing and the complete understanding of the relationships between bacteria and their hosts can open up new targets for drugs to target the quorum sensing signal molecules and its cognate sensors.
These new approaches of utilizing quorum sensing as a target for therapeutics seem to be promising for treating hard persistent infections (10).
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