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Quorum sensing can be defined as the process of cell-to-cell signaling which causes coordinated behavior in bacteria. The field of quorum sensing is said to have begun with Vibrio which is a genus in gamma-proteobacteria. The genus is believed to translate at least two systems. The main system is known as LuxR/Luxl of Vibro fischeri where Luxl is the signal producer. LuxR, on the other hand, receives the synthesized signal. The signal that Luxl generates is known as AHL. The second system is LuxPQ/LuxS where LuxS synthesizes the signal which is Al-2. With the prevalence of cell-to-cell signaling behaviors among many bacterial species, scientists realized that the traits had not been discovered in Salmonella or Escherichia Coli. However, there were two complications regarding quorum sensing in the two bacterial species. The first complication is the presence of components of quorum sensing which are said to be incomplete. The second one is that some of the systems are metabolic reactions and not quorum sensing processes. In both species, sdiA is used for gene regulation.
To determine which genes are synchronized by SdiA in the Salmonella system, a genetic screen was performed. MudJ insertions that produce lacZY fusions used for transcription were then separated. Among the fusions synthesized, seven of them are located in the resistance to compliment killing region that is found in Salmonella typhimurium. The exhibition of the MudJ fusions was analyzed with reference to the mutant strains of S. enterica to determine whether they would be activated by SdiA. In case the genes became activated by SdiA, then they would form part of the quorum sensing regulon. In the experiment, none of the fusions displayed significant expressions that were dependent on SdiA. The results were therefore not consistent with genome sensing. In another experiment carried out to determine the response of rck and srGE operon to AHLs, it was confirmed that srGE and rck operon respond to AHLs in a manner that is dependent on SdiA. SdiA regulon varies with both temperature and AHL 3-oxo-octanoyl-homoserine lacton concentration. For instance, rck can only be expressed at 37°C and not at 22°C or 30°C. SrGE on the other hand, is expressed at 37°C and 30°C. At 37 degrees Celsius, the expression of rck and srGE are both dependent on AHL and sdiA. Resistance to Compliment killing operon is a protein with eight strands which is located on the outer membrane.
The sdiA gene expression in E. coli has been divided into three selection schemes: a set of genes which have the ability to evade a cell-division block that developed as a result of expressing minCD minus minE; a set of genes which when expressed from a plasmid, are capable of conferring resistance against mitomycin C and a set of genes which had the ability to resist quinolones when expressed from a plasmid.
Two hypothesis exist that describe the distinctions between Salmonella E. coli species. The first assumption states that when sdiA is overexpressed in species E. coli, it results in irrelevant reactions in the cell. The second proposition explains that the genes pointed out when sdiA is overexpressed in the same species, are part of the regulon in sdiA. On the other hand, both the two species have been found to synthesize Al-2. Recent studies have focused on identifying novel quorum-sensing systems. The first system identified involves the use of indole which is used to regulate genes tnaAB, gabT, and astD. The other system only exists in species P. stuarti. However, other homologous systems are said to exist in the two species.
In summary, there exist many signaling systems which scientists have recognized in Salmonella and in E. coli. However, most of these systems are not the normal quorum sensing processes but are metabolic in nature. An example of such processes is one that uses protease to produce peptide like elements. Salmonella and E. coli have unique techniques of detecting AHL synthesis in other species in the form of sidA. Today, scientists are working on developing better understanding of host-microbe interactions, cell-to-cell signaling and microbial community structures.
Ahmer, B. M. (2004). Cell‐to‐cell signalling in Escherichia coli and Salmonella enterica. Molecular microbiology, 52(4), 933-945.
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