Quorum sensing, virulence and secondary metabolite production in plant soft-rotting bacteria

Quorum sensing describes the ability of bacteria to sense their population density and respond by modulating gene expression. In the plant soft-rotting bacteria, such as Erwinia, an arsenal of plant cell wall-degrading enzymes is produced in a cell density-dependent manner, which causes maceration of plant tissue. However, quorum sensing is central not only to controlling the production of such destructive enzymes, but also to the control of a number of other virulence determinants and secondary metabolites. Erwinia synthesizes both N-acylhomoserine lactone (AHL) and autoinducer-2 types of quorum sensing signal, which both play a role in regulating gene expression in the phytopathogen. We review the models for AHL-based regulation of carbapenem antibiotic production in Erwinia. We also discuss the importance of quorum sensing in the production and secretion of virulence determinants by Erwinia, and its interplay with other regulatory systems.     

Quorum Sensing,or How Bacteria “Talk” to Each Other

Reviewed are recent advances in studying the quorum-sensing systems, which regulate gene expression depending on population density. Low-molecular-weight acyl derivatives of L-homoserine lactone (N-AHL) freely diffuse through cell membranes and determine cell-to-cell communication in bacteria. The quorum-sensing systems have first been found to regulate bioluminescence in marine bacteria Photobacterium(Vibrio) fischeriand Vibrio harveyi. Such systems are widespread and control expression of genes for virulence factors, proteases, antibiotics, etc., in various Gram-negative bacteria, including plant, animal, and human pathogens. Quorum sensing is a prominent example of social behavior in bacteria, as signal exchange among individual cells allows the entire population to choose an optimal way of interaction with the environment and with higher organisms.     

Small molecule-mediated density-dependent control of gene expression in prokaryotes: bioluminescence and the biosynthesis of carbapenem antibiotics.

Sophisticated signal transduction systems enable prokaryotes to sense their growth environment and mount an appropriate adaptive response. Signal transduction and gene regulation through the phosphorylation of two regulatory components is now recognised as one of the major global regulatory networks in bacteria. However, not all types of sensor-regulator circuits relay information via phosphoryl transfer. The Vibrio fischeri LuxR protein which has previously been characterised as a member of the response-regulator superfamily responds to a small diffusible signal molecule N-(3-oxohexanoyl)homoserine lactone (HSL). Biosynthesis of HSL in V. fischeri is dependent on the expression of the luxI gene. Until recently, the role of HSL as an 'autoinducer' was thought to be restricted to V. fischeri and a few related marine bacteria in which it controls the onset of bioluminescence. However, we have discovered that a diverse group of terrestrial bacteria: (1) produce HSL; (2) possess genes analogous to luxI; and (3) exhibit cell density-dependent induction of bioluminesence when transformed with a recombinant plasmid carrying V. fischeri lux genes but lacking luxI. In one of these, Erwinia carotovora, HSL is shown to mediate the cell density-dependent biosynthesis of a carbapenem antibiotic.     

Integration of environmental and host-derived signals with quorum sensing during plant-microbe interactions

Many plant-associated microbes use secreted autoinducer molecules, including N-acylhomoserine lactones (AHLs), to regulate diverse behaviours in association with their population density (quorum sensing). Often, these responses are affected by environmental conditions, including the presence of other AHL-producing bacterial species. In addition, plant-derived metabolites, including products that arise as a direct result of the bacterial infection, may profoundly influence AHL-regulated behaviours. These plant products can interact directly and indirectly with the quorum-sensing network and can profoundly affect the quorum-sensing behaviour. Local conditions on a microscopic scale may affect signal molecule longevity, stability and accumulation, and this could be used to give information in addition to cell density. Furthermore, in many Gram-negative bacteria, AHL signalling is subservient to an additional two-component signalling system dependent upon homologues of GacS and GacA. The signal(s) to which GacS responds are not known, but recent research suggests that a self-produced ligand may be being detected. This review will focus on two well-studied examples of AHL-regulated plant-associated behaviour, Erwinia carotovora and Agrobacterium tumefaciens, to illustrate the complexity of such signalling networks.     

The Rap and Hor proteins of Erwinia, Serratia and Yersinia: a novel subgroup in a growing superfamily of proteins regulating diverse physiological processes in bacterial pathogens

The enteric bacterium Serratia marcescens is an opportunistic human pathogen. The strain ATCC39006 makes the red pigment, prodigiosin (Pig), and the β-lactam antibiotic carbapenem (Car). Mutants were isolated that were concomitantly defective for Pig and Car production. These mutants were found to have a mutation in the rap gene (regulation of antibiotic and pigment). Sequence analysis of the rap gene revealed a predicted protein product showing strong homology to SlyA, originally thought to be a haemolytic virulence determinant in Salmonella typhimurium. Homologues of rap were detected in several bacterial genera, including Salmonella, Yersinia, Enterobacter , and species of the plant pathogen, Erwinia. The Erwinia hor_Er (homologue of rap ) and the Yersinia hor_Ye genes were also found to be very similar to rap and slyA. Marker exchange mutagenesis of hor_Er revealed that it encoded a regulatory protein controlling the production of antibiotic and exoenzyme virulence determinants in the phytopathogen, Erwinia carotovora subspecies carotovora. We have shown that these new homologues of SlyA form a highly conserved subgroup of a growing superfamily of bacterial regulatory proteins controlling diverse physiological processes in human, animal and plant pathogens.     

Detection of quorum-sensing-related molecules in <Emphasis Type="Italic">Vibrio scophthalmi</Emphasis>

Background  

Cell-to-cell communication (also referred to as quorum sensing) based on N-acyl-homoserine lactones (AHLs) is a widespread response to environmental change in Gram-negative bacteria. AHLs seem to be highly variable, both in terms of the acyl chain length and in the chemical structure of the radicals. Another quorum sensing pathway, the autoinducer-2-based system, is present both in Gram-positive and Gram-negative bacteria. In this study the presence of signal molecules belonging to both quorum sensing signalling pathways was analysed in the marine symbiotic species Vibrio scophthalmi.     

Potato plants genetically modified to produce N-acylhomoserine lactones increase susceptibility to soft rot erwiniae

Many gram-negative bacteria employ N-acylhomoserine lactones (AHL) to regulate diverse physiological processes in concert with cell population density (quorum sensing [QS]). In the plant pathogen Erwinia carotovora, the AHL synthesized via the carI/expI genes are responsible for regulating the production of secreted plant cell wall-degrading exoenzymes and the antibiotic carbapen-3-em carboxylic acid. We have previously shown that targeting the product of an AHL synthase gene (yenI) from Yersinia enterocolitica to the chloroplasts of transgenic tobacco plants caused the synthesis in planta of the cognate AHL signaling molecules N-(3-oxohexanoyl)-L-homoserine lactone (3-oxo-C6-HSL) and N-hexanoylhomoserine lactone (C6-HSL), which in turn, were able to complement a carI-QS mutant. In the present study, we demonstrate that transgenic potato plants containing the yenI gene are also able to express AHL and that the presence and level of these AHL in the plant increases susceptibility to infection by E. carotovora. Susceptibility is further affected by both the bacterial level and the plant tissue under investigation.     

Inhibition of Quorum Sensing–Regulated Behaviors by <Emphasis Type="Italic">Scorzonera sandrasica</Emphasis>

Many Gram-negative bacteria use N-acyl homoserine lactone signal molecules to monitor their own population density and coordinate gene regulation in a process called quorum sensing (QS). Increasing evidence implies that certain eukaryotes produce QS-inhibitory compounds. In this work, we tested 46 terrestrial plants materials for their ability to inhibit QS-regulated behaviors in different bacterial species. Plant materials were dried and extracted using different solvents. The chloroform-soluble compounds extracted from Scorzonera sandrasica were found to inhibit violacein production, a QS-regulated behavior in Chromobacterium violaceum. In addition, the chloroform extract was also able to inhibit QS-regulated carbapenem antibiotic production in Erwinia carotovora. Because the regulation of many bacterial processes is controlled by QS systems, the finding of natural compounds acting as QS inhibitors suggests an attractive tool to control and handle detrimental infections caused by human, animal, and plant pathogens.     

Cooperation of two distinct ExpR regulators controls quorum sensing specificity and virulence in the plant pathogen Erwinia carotovora

Quorum sensing, the population density-dependent regulation mediated by N-acylhomoserine lactones (AHSL), is essential for the control of virulence in the plant pathogen Erwinia carotovora ssp. carotovora (Ecc). In Erwinia carotovora ssp. the AHSL signal with an acyl chain of either 6 or 8 carbons is generated by an AHSL synthase, the expI gene product. This work demonstrates that the AHSL receptor, ExpR1, of Ecc strain SCC3193 has strict specificity for the cognate AHSL 3-oxo-C8-HSL. We have also identified a second AHSL receptor (ExpR2) and demonstrate a novel quorum sensing mechanism, where ExpR2 acts synergistically with the previously described ExpR1 to repress virulence gene expression in Ecc. We show that this repression is released by addition of AHSLs and appears to be largely mediated via the negative regulator RsmA. Additionally we show that ExpR2 has the novel property to sense AHSLs with different acyl chain lengths. The expI expR1 double mutant is able to act in response to a number of different AHSLs, while the expI expR2 double mutant can only respond to the cognate signal of Ecc strain SCC3193. These results suggest that Ecc is able to react both to the cognate AHSL signal and the signals produced by other bacterial species.     

"Quorum sensing", or how bacteria "talk" to each other

Recent advances in studying the quorum-sensing systems, which regulate gene expression depending on population density, are reviewed. Low-molecular-weight acyl derivatives of L-homoserine lactone (N-AHL) freely diffuse through cell membranes and determine cell-to-cell communications in bacteria. The quorum-sensing systems have first been found to regulate bioluminescence in marine bacteria Photobacterium (Vibrio) fischeri and Vibrio harveyi. Such systems are widespread and control expression of genes for virulence factors, proteases, antibiotics, etc., in various Gram-negative bacteria, including plant, animal, and human pathogens. Quorum sensing is a prominent example of social behavior in bacteria, as signal exchange among individual cells allows the entire population to choose an optimal way of interaction with the environment and with higher organisms.