Abstract
BackgroundGreat excitement accompanied discoveries over the last decade in several Gram-negative and Gram-positive bacteria of the LuxS protein, which catalyzes production of the AI-2 autoinducer molecule for a second quorum sensing system (QS-2). Since the luxS gene was found to be widespread among the most diverse bacterial taxa, it was hypothesized that AI-2 may constitute the basis of a universal microbial language, a kind of bacterial Esperanto. Many of the studies published in this field have drawn a direct correlation between the occurrence of the luxS gene in a given organism and the presence and functionality of a QS-2 therein. However, rarely hathe existence of potential AI-2 receptors been examined. This is important, since it is now well recognized that LuxS also holds a central role as a metabolic enzyme in the activated methyl cycle which is responsible for the generation of S-adenosyl-L-methionine, the major methyl donor in the cell.ResultsIn order to assess whether the role of LuxS in these bacteria is indeed related to AI-2 mediated quorum sensing we analyzed genomic databases searching for established AI-2 receptors (i.e., LuxPQ-receptor of Vibrio harveyi and Lsr ABC-transporter of Salmonella typhimurium) and other presumed QS-related proteins and compared the outcome with published results about the role of QS-2 in these organisms. An unequivocal AI-2 related behavior was restricted primarily to organisms bearing known AI-2 receptor genes, while phenotypes of luxS mutant bacteria lacking these genes could often be explained simply by assuming deficiencies in sulfur metabolism.ConclusionGenomic analysis shows that while LuxPQ is restricted to Vibrionales, the Lsr-receptor complex is mainly present in pathogenic bacteria associated with endotherms. This suggests that QS-2 may play an important role in interactions with animal hosts. In most other species, however, the role of LuxS appears to be limited to metabolism, although in a few cases the presence of yet unknown receptors or the adaptation of pre-existent effectors to QS-2 must be postulated.
Highlights
Great excitement accompanied discoveries over the last decade in several Gramnegative and Gram-positive bacteria of the LuxS protein, which catalyzes production of the AI-2 autoinducer molecule for a second quorum sensing system (QS-2)
The LuxPQreceptor was absent from the genome of V. angustum, which does not respond to the AI-2 signal [39], even though the luxS gene coding for S-ribosylhomocysteinase is present
In order to be able to detect the AI-2 signal and to react in a QS-2 dependent manner bacteria must carry an appropriate receptor for the autoinducer, in the light of our analysis it is apparent that there is still a common misconception in the literature leading to the conclusion that the mere presence of the luxS gene is a sufficient condition to deduce the existence of type-2 quorum sensing
Summary
Great excitement accompanied discoveries over the last decade in several Gramnegative and Gram-positive bacteria of the LuxS protein, which catalyzes production of the AI-2 autoinducer molecule for a second quorum sensing system (QS-2). Rarely hathe existence of potential AI-2 receptors been examined This is important, since it is well recognized that LuxS holds a central role as a metabolic enzyme in the activated methyl cycle which is responsible for the generation of S-adenosyl-L-methionine, the major methyl donor in the cell. N-acyl-homoserine lactones (AHLs) are the most frequent signaling molecules found in Gramnegative bacteria. Several other type I autoinducers (AI-1), whose molecules differ only in the AHL-acyl side chain moiety, have been discovered in a number of Gram-negative bacteria [1]. A limited (but not universal) communication between different bacterial species is possible through QS-1 [4,5,6]
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