Quorum Sensing

During the last two decades, critical scholars in gender, migration and post-colonial studies have been engaged in attempting to dislodge the figure of the sex trafficking victim from its position of primacy in public, policy and academic debates. The body of work that stresses the agency and rights of migrant women in the sex sector has put forward a convincing critique of the passive and enslaved trafficking victim and has replaced the latter with the figures of the active migrant and the political protagonist. Despite such a shift, however, the figure of the trafficking victim continues to dominate public and policy arenas. In this chapter, I am interested in the persistence of the figure of the victim and suggest that that the figure of the victim is not a 'free-floating' but rather produced through specific codes and conventions. These issues, I argue, become visible by bringing to the fore the nexus among sexuality, gender and narrative. By building on feminist scholarship on sexuality and representation in film, visual media studies and historical studies of East/West Europe, I explore the ways in which representations are embedded within narrative tropes and discursive constructions about women's sexuality that are culturally and historically specific.

Quorum sensing (QS) inhibitory activity of 29 unifloral honeys was evaluated using the bacterial model Cromobacterium violaceum. The tested honeys were able to inhibit the production of acyl-homoserine lactones (AHLs) produced by C. violaceum at 0.1 g/ml. However, chestnut and linden honey samples showed the highest inhibitory activity, while orange and rosemary were less effective in inhibiting QS. When honey samples from the same floral origin obtained from different geographical regions were compared, they showed similar QS inhibitory activity. Thus, one of the factors which influence the inhibitory activity could be derived from the floral origin, independently of the geographic location. It was observed that unifloral honey samples showed ''non-peroxide" anti-QS activity, which was not linearly correlated with total and individual phenolic compounds. The obtained results showed that the preservative properties of honey could be due to both the antimicrobial properties and the QS inhibitory capacity.

For most marine aquaculture species, one of the main bottlenecks is the stable production of high quality juveniles. The high and unpredictable mortality in the first weeks after hatching of marine fish larvae remains a challenging problem that needs to be solved. The severity of the problem differs between species, but cannot be considered adequately solved for any species. Both scientific evidence and experience in hatcheries for a variety of fish, shrimp and shellfish species are accumulating as support for the hypothesis that detrimental fishmicrobe interactions are the cause of these problems. Host-microbe interactions in reared fish are still poorly understood, except for a few pathogens, and empirical data of the quality required to test this hypothesis, are lacking. This article provides an overview on the current knowledge of the microbial environment of fish larvae, including methodological aspects to characterize the microbial community (both using culture-dependent and culture-independent methods). Further, the current knowledge of the immunology of fish larvae is reviewed, including recent advances in the understanding of toll-like receptors, inflammatory cytokines, mast cells and piscidins, and the ontogeny of the adaptive immune system. Finally, we provide an overview of the state of the art with respect to steering of microbial communities associated with fish larvae-both steering of community composition and of its activity (e.g. by quorum sensing interference).

The emergence of multidrug-resistant bacterial strains is a severe global health issue, which is worsened by the inability of new antibiotics. Virulence inhibition is one of the novel strategies that have been proposed to combat bacterial pathogens more effectively, without the risk of exerting selection pressure on these microorganisms. Inhibition of bacterial cell-cell communication (quorum sensing; QS) is a promising approach however, rapid and cost-effective screening for compounds with QS-inhibitory activity is not yet well-established. Aims: The aim of the present study is to determine the ideal experimental conditions for the disk-diffusion based QSinhibitory assay with the most frequently used QS-signal molecule-producing and reporter strains. Methods: In our study, the effects of growth characteristics, incubation time, temperature and the used culture media were studied on the used bacterial strains and results of the disk-diffusion based QS-inhibitory assay.

Biological nanofactories, which are engineered to contain modules that can target, sense and synthesize molecules, can trigger communication between different bacterial populations. These communications influence biofilm formation 1,2 , virulence 3,4 , bioluminescence 5,6 and many other bacterial functions 7,8 in a process called quorum sensing 9 . Here, we show the assembly of a nanofactory that can trigger a bacterial quorum sensing response in the absence of native quorum molecules. The nanofactory comprises an antibody (for targeting) and a fusion protein that produces quorum molecules when bound to the targeted bacterium. Our nanofactory selectively targets the appropriate bacteria and triggers a quorum sensing response when added to two populations of bacteria. The nanofactories also trigger communication between two bacterial populations that are otherwise non-communicating. We envision the use of these nanofactories in generating new antimicrobial treatments that target the communication networks of bacteria rather than their viability.

Cell population density-dependent regulation of gene expression is an important determinant of bacterial pathogenesis. Staphylococci have two quorum-sensing (QS) systems. The accessory gene regulator (agr) is genus specific and uses a post-translationally modified peptide as an autoinducing signal. In the pathogens Staphylococcus aureus and Staphylococcus epidermidis, agr controls the expression of a series of toxins and virulence factors and the interaction with the innate immune system. However, the role of agr during infection is controversial. A possible second QS system of staphylococci, luxS, is found in a variety of Gram-positive and Gram-negative bacteria. Importantly, unlike many QS systems described in Gram-negative bacteria, agr and luxS of staphylococci reduce rather than induce biofilm formation and virulence during biofilm-associated infection. agr enhances biofilm detachment by up-regulation of the expression of detergent-like peptides, whereas luxS reduces cell-to-cell adhesion by down-regulating expression of biofilm exopolysaccharide. Significant QS activity in staphylococci is observed for actively growing cells at a high cell density, such as during the initial stages of an infection and under optimal environmental conditions. In contrast, the metabolically quiescent biofilm mode of growth appears to be characterized by an overall low activity of the staphylococcal QS systems. It remains to be shown whether QS control in staphylococci represents a promising target for the development of novel antibacterial agents.

Staphylococci are a major health threat because of increasing resistance to antibiotics. An alternative to antibiotic treatment is preventing virulence by inhibition of bacterial cell-to-cell communication using the quorum-sensing inhibitor RNAIII-inhibiting peptide (RIP). In this work, we identified 2',5-di-O-galloyl-d-hamamelose (hamamelitannin) as a nonpeptide analog of RIP by virtual screening of a RIP-based pharmacophore against a database of commercially available small-molecule compounds. Hamamelitannin is a natural product found in the bark of Hamamelis virginiana (witch hazel), and it has no effect on staphylococcal growth in vitro; but like RIP, it does inhibit the quorum-sensing regulator RNAIII. In a rat graft model, hamamelitannin prevented device-associated infections in vivo, including infections caused by methicillin-resistant Staphylococcus aureus and Staphylococcus epidermidis strains. These findings suggest that hamamelitannin may be used as a suppressor to st...

This study aimed at getting a deeper insight in the molecular mechanism by which the natural furanone (5Z)-4-bromo-5-(bromomethylene)-3-butyl-2(5H)furanone disrupts quorum sensing in Vibrio harveyi. Bioluminescence experiments with signal molecule receptor double mutants revealed that the furanone blocks all three channels of the V. harveyi quorum sensing system. In further experiments using mutants with mutations in the quorum sensing signal transduction pathway, the compound was found to block quorum sensing-regulated bioluminescence by interacting with a component located downstream of the Hfq protein. Furthermore, reverse transcriptase real-time polymerase chain reaction with specific primers showed that there was no effect of the furanone on luxR Vh mRNA levels in wild-type V. harveyi cells. In contrast, mobility shift assays showed that in the presence of the furanone, significantly lower levels of the LuxR Vh response regulator protein were able to bind to its target promoter sequences in wildtype V. harveyi. Finally, tests with purified LuxRVh protein also showed less shifts with furanone-treated LuxRVh, whereas the LuxRVh concentration was found not to be altered by the furanone (as determined by SDS-PAGE). Therefore, our data indicate that the furanone blocks quorum sensing in V. harveyi by rendering the quorum sensing master regulator protein LuxRVh unable to bind to the promoter sequences of quorum sensing-regulated genes.

N-acylhomoserine lactone (AHL) quorum-sensing molecules modulate the swimming behaviour of zoospores of the macroalga Ulva to facilitate the location of bacterial biofilms. Here we show that the intertidal surfaces colonized by Ulva are dominated by Alphaproteobacteria, particularly the Rhodobacteraceae family, and the Bacteroidetes family Flavobacteriaceae, and that this diverse assemblage both produces and degrades AHLs. N-acylhomoserine lactones could also be extracted from the surfaces of pebbles recovered from intertidal rock-pools. Bacteria representative of this assemblage were isolated and tested for the production and degradation of AHLs, and for their ability to modulate zoospore settlement at different biofilm densities. Of particular interest was a Shewanella sp. This strain produced three major AHLs (OC4, OC10 and OC12) in the late exponential phase, but the longer-chain AHLs were rapidly degraded in the stationary phase. Degradation occurred via both lactonase and amidase activity. A close relationship was found between AHL synthesis and Ulva zoospore settlement. The Shewanella isolate also interfered with AHL production by a Sulfitobacter isolate and its ability to enhance zoospore settlement in a polymicrobial biofilm. This influence on the attachment of Ulva zoospores suggests that AHLdegrading strains can affect bacterial community behaviour by interfering with quorum sensing between neighbouring bacteria. More importantly, these interactions may exert wider ecological effects across different kingdoms.

Quorum sensing, bacterial cell-to-cell communication with small signal molecules such as acyl-homoserine lactones, regulates the virulence of many pathogenic bacteria. Therefore, interfering with quorum sensing is currently being explored as a novel biocontrol strategy to fight bacterial infections. In this study, the effects of 19 micro-algal strains on acyl-homoserine lactone-regulated phenotypes of three reporter strains were investigated. Two freshwater micro-algae inhibited violacein production of quorum sensing reporter strain Chromobacterium violaceum CV026. Further tests using Escherichia coli JB523 showed that micro-algal extracts inhibited or stimulated quorum sensing, depending on the algal strain. One freshwater and five marine algae showed quorum sensing inhibitory activity, whereas two algae stimulated quorum sensing-regulated gene expression. Micro-algal strains that showed inhibitory activity in the previous assays also inhibited acylhomoserine lactone-regulated bioluminescence in the aquaculture pathogen Vibrio harveyi. The growth of all reporter strains was found to be unaffected by the micro-algal samples. The most promising micro-algal strain was found to be Chlorella saccharophila CCAP211/48, as its extracts inhibited quorum sensing-regulated gene expression in all three reporter strains.

The pectinolytic Dickeya spp. are soft-rot Gramnegative bacteria that cause severe disease in a wide range of plant species. In recent years, there has been an increase in the damage caused by Dickeya in potato crops in Europe. Soft-rot symptoms are due to the production and secretion of degradative enzymes that destroy the plant cell wall. However, an efficient colonization of the host plant requires many additional bacterial factors, including elements in the early stages allowing for the adhesion and penetration of the bacteria in the plant and different elements in the intermediate stages, involved in the adaptation to the new growth conditions encountered in the host. Dickeya pathogenicity is clearly a multifactorial process, and successful infection by these bacteria requires a temporal coordination of survival and virulence gene expression. This involves the ancestral nucleoid-associated proteins, Fis and H-NS, and modifications of DNA topology, as well as various specific regulatory systems, including a new quorumsensing pathway and regulators that sense the bacterial metabolic status or environmental stresses. This review presents new information concerning the ecology of Dickeya and the strategies used by this bacterium to coordinate its survival and virulence programmes during infection.

Bacteria can coordinate community-wide behaviors through quorum sensing, that is, the secretion and sensing of autoinducer (AI) molecules. Bacterial quorum sensing is implicated in the regulation of pathologically relevant events such as biofilm formation, bacterial virulence, and drug resistance. Inhibitors of bacterial quorum sensing could therefore be useful therapeutics. Herein we report for the first time the discovery of several pyrogallol compounds as single digit micromolar inhibitors of bacterial quorum sensing in Vibrio harveyi.

In a previous study we determined that by-products of Lactobacillus plantarum inhibited pathogenicity of Pseudomonas aeruginosa and is effective in the treatment of infected wounds. This study assesses the cytotoxic activity of acetic acid (AA), supernatants of L. plantarum and P. aeruginosa, with and without signal acyl-homoserin-lactones (AHL), and mixtures of both bacterial supernatants on human neutrophils. Cytotoxicity was determined through viability using trypan blue, apoptosis by Annexin V, necrosis by propidium iodide and intracellular pH by SNARF-1. We found that supernatants of L. plantarum caused less cytotoxicity than AA at the same extracellular pH (p<0.05). P. aeruginosa induced a remarkable drop in intracellular pH, which was independent of extracellular pH. This intracellular acidity was correlated with a significant decrease in viability and was higher than supernatants of AHL producing P. aeruginosa (p<0.05). When supernatants were mixed, the quantity of AHL diminished (p<0.001) and the cytotoxic effect induced by P aeruginosa was ameliorated by L. plantarum supernatant (p<0.001 vs p<0.01). These results are in agreement with the inflammatory in vivo assays determined by intradermal inoculations in Balb/c mice. Our findings will be useful for the formulation of effective and inexpensive products to resolve infected chronic wounds in our hospitals.

The present status of research on the molecular basis of microbe-plant interactions is discussed. Principles and mechanisms which play a role in the interactions of microbial pathogens, biofertilizers, phytostimulators, rhizoremediators and biocontrol agents with the plants are treated. Special emphasis is given to colonization, phase variation, two-component systems, quorum sensing, complex regulation of the syntheses of extracellular enzymes and secondary metabolites, Type 4 pili and Type III and Type IV secretion systems.

Quorum sensing is the regulation of gene expression in response to flu sensing bacteria produce and release chemical signal molecules called autoinducers that increase in concentration as a function of cell density. The detection of a minimal threshold stimulatory concentration of alteration in gene expression. behavior on a population-wide scale in response to changes in the number and/or species present in a quorum sensing controlled processes are unproductive when undertaken by an individual bacterium acting alone but become beneficial when carried out simultaneously by a large number of cells. Thus, quorum sensing confuses the distinction between prokaryotes and eukaryotes because it enables bacteria to act as multicellular organisms. These processes include symbiosis, virulence, competence, conjugation, antibiotic production, motility, sporulation, and biofilm formation. Recent advances in th between bacterial species. Furthermore, bacterial autoinducers elicit specific responses from host organisms. Although the nature of the chemical signals, the signal rela sensing systems differ, in every case the ability to communicate with one another allows bacteria to coordinate the gene expression, and therefore the behavior, of the entire community. Pres the qualities of higher organisms. Quorum sensing and its inhibition (quorum quenching) have significant clinical implications in terms of antimicrobial therapy.

Inter-cell communication aided by released chemical signals when cell density reaches a critical concentration has been investigated for over 30 years as quorum sensing. Originally discovered in Gramnegative bacteria, quorum-sensing systems have also been studied extensively in Gram-positive bacteria and dimorphic fungi. Microbial communities communicating via quorum sensing employ various chemical signals to supervise their surrounding environment, alter genetic expression and gain advantage over their competitors. These signals vary from acylhomoserine lactones to small modified or unmodified peptides to complex γ -butyrolactone molecules. The scope of this review is to give an insight into some of the quorum-sensing systems now known and to explore their role in microbial physiology and development of pathogenesis. Particular attention will be dedicated to the signalling molecules involved in quorum-sensingmediated processes and the potential shown by some of their natural and synthetic analogues in the treatment of infections triggered by quorum sensing.

El quorum sensing es un sistema de comunicación intercelular entre bacterias con el fin de coordinarse y realizar acciones de forma conjunta. Dicho sistema se basa en la secreción al medio extracelular de unas moléculas autoinductoras que son percibidas por la población bacteriana. Asimismo, se distinguen varios tipos de quorum sensing según la cantidad de autoinductores utilizados en el proceso, su composición química o las relaciones entre ellos. Los procesos regulados por el quorum sensing son muy variados, y pueden ser desde la producción de bioluminiscencia hasta la expresión de factores de virulencia.
Además, se tratan algunos ejemplos en los que está presente el quorum sensing, haciendo hincapié en el quorum quenching o inhibición del quorum sensing realizada por algunos organismos eucariotas, y en los biofilms formados por las poblaciones del género Streptococcus.
Por último, se realiza una parte experimental en la que se pone en manifiesto la existencia de este sistema de comunicación. Para ello se realizan tres cultivos en los que la bacteria Chromobacterium violaceum
determina si hay presencia o no de Acil-homoserina lactonas de cadena corta, un tipo de autoinductor, presentando tinción violácea. Además, se da el caso de una interacción por medio del quorum sensing
entre bacterias de distintas especies.

Aquaculture is emerging as one of the most viable and promising enterprises for keeping pace with the surging need for animal protein, providing nutritional and food security to humans, particularly those residing in regions where livestock is relatively scarce. With every step toward intensification of aquaculture practices, there is an increase in the stress level in the animal as well as the environment. Hence, disease outbreak is being increasingly recognized as one of the most important constraints to aquaculture production in many countries, including India. Conventionally, the disease control in aquaculture has relied on the use of chemical compounds and antibiotics. The development of non-antibiotic and environmentally friendly agents is one of the key factors for health management in aquaculture.

Food spoilage is a complex process and excessive amounts of foods are lost due to microbial spoilage even with modern day preservation techniques. Despite the heterogeneity in raw materials and processing conditions, the microflora that develops during storage and in spoiling foods can be predicted based on knowledge of the origin of the food, the substrate base and a few central preservation parameters such as temperature, atmosphere, a w and pH. Based on such knowledge, more detailed sensory, chemical and microbiological analysis can be carried out on the individual products to determine the actual specific spoilage organism. Whilst the chemical and physical parameters are the main determining factors for selection of spoilage microorganisms, a level of refinement may be found in some products in which the interactive behavior of microorganisms may contribute to their growth and/or spoilage activity. This review gives three such examples. We describe the competitive advantage of Pseudomonas spp. due to the production of iron-chelating siderophores, the generation of substrates for spoilage reactions by one organism from another microorganism (so-called metabiosis) and the up-regulation of phenotypes potentially involved in spoilage through cell-to-cell communication. In particular, we report for the first time the widespread occurrence of N-acyl homoserine lactones (AHL) in stored and spoiling fresh foods and we discuss the potential implications for spoilage and food preservation. D

The rhizosphere is the soil-plant root interphase and in practice consists of the soil adhering to the root besides the loose soil surrounding it. Plant growth-promoting rhizobacteria (PGPR) are potential agents for the biological control of plant pathogens. A biocontrol strain should be able to protect the host plant from pathogens and fulfill the requirement for strong colonization. Numerous compounds that are toxic to pathogens, such as HCN, phenazines, pyrrolnitrin, and pyoluteorin as well as, other enzymes, antibiotics, metabolites and phytohormones are the means by which PGPR act, just as quorum sensing and chemotaxis which are vital for rhizosphere competence and colonization. The presence of root exudates has a pronounced effect on the rhizosphere where they serve as an energy source, promoting growth and influencing the root system for the rhizobacteria. In certain instances they have products that inhibit the growth of soil-borne pathogens to the advantage of the plant root. A major source of concern is reproducibility in the field due to the complex interaction between the plant (plant species), microbe and the environment (soil fertility and moisture, day length, light intensity, length of growing season, and temperature). This review listed most of the documented PGPR genera and discussed their exploitation.

The increasing resistance of microorganisms to conventional chemicals and drugs is a serious and evident worldwide problem that has prompted research into the identification of new biocides with broad activity. Plants and their derivatives, such as essential oils, are often used in folk medicine. In nature, essential oils play an important role in the protection of plants. Essential oils contain a wide variety of secondary metabolites that are capable of inhibiting or slowing the growth of bacteria, yeasts and moulds. Essential oils and their components have activity against a variety of targets, particularly the membrane and cytoplasm, and in some cases, they completely change the morphology of the cells. This brief review describes the activity of essential oils against pathogenic bacteria.

Biofilms are colonies of bacteria or fungi that adhere to a surface, protected by an extracellular polymer matrix composed of polysaccharides and extracellular DNA. They are highly complex and dynamic multicellular structures that resist traditional means of killing planktonic bacteria. Recent developments in nanotechnology provide novel approaches to preventing and dispersing biofilm infections, which are a leading cause of morbidity and mortality. Medical device infections are responsible for approximately 60% of hospital acquired infections. In the United States, the estimated cost of caring for healthcare-associated infections is approximately between $28 billion and $45 billion per year. In this review, we will discuss our current understanding of biofilm formation and degradation, its relevance to challenges in clinical practice, and new technological developments in nanotechnology that are designed to address these challenges.

Bacteria use the language of low-molecular-weight ligands to assess their population densities in a process called quorum sensing (QS). Different types of quorum sensing pathways are present in Gram-negative and Gram-positive bacteria. Signal molecules most commonly used in Gram-negative bacteria are acyl homoserine lactones. In recent years, a substantial amount of literature and data have been available on bacterial QS. Recently, interest in modulation of quorum sensing with different approaches has increased among scientific communities. In this chapter, we provide an updated overview on bacterial QS, assays and methods for detecting signal molecules, and various approaches to inhibit AHL-based quorum sensing. Significance of QS interference by prokaryotic and eukaryotic organisms in relation to plant health and the environment is discussed here.

Bacteria can switch between planktonic forms (single cells) and biofilms, i.e., bacterial communities growing on solid surfaces and embedded in a matrix of extracellular polymeric substance. Biofilm formation by pathogenic bacteria often results in lower susceptibility to antibiotic treatments and in the development of chronic infections; thus, biofilm formation can be considered an important virulence factor. In recent years, much attention has been directed towards understanding the biology of biofilms and towards searching for inhibitors of biofilm development and of biofilm-related cellular processes. In this report, we review selected examples of target-based screening for anti-biofilm agents: We focus on inhibitors of quorum sensing, possibly the most characterized target for molecules with anti-biofilm activity, and on compounds interfering with the metabolism of the signal molecule cyclic di-GMP metabolism and on inhibitors of DNA and nucleotide biosynthesis, which represent a novel and promising class of biofilm inhibitors. Finally, we discuss the activation of biofilm dispersal as a novel mode of action for anti-biofilm compounds.

Seafood forms a part of a healthy diet. However, seafood can be contaminated with foodborne pathogens,
resulting in disease outbreaks. Because people consume large amounts of seafood, such disease outbreaks
are increasing worldwide. Seafood contamination is largely due to the naturally occurring phenomenon
of biofilm formation. The common seafood bacterial pathogens that form biofilms are Vibrio
spp., Aeromonas hydrophila, Salmonella spp., and Listeria monocytogenes. As these organisms pose a
global health threat, recent research has focused on elucidating methods to eliminate these biofilmforming
bacteria from seafood, thereby improving food hygiene. Therefore, we highlight recent advances
in our understanding of the underlying molecular mechanisms of biofilm formation, the factors
that regulate biofilm development and the role of quorum sensing and biofilm formation in the virulence
of foodborne pathogens. Currently, several novel methods have been successfully developed for controlling
biofilms present in seafood. In this review, we also discuss the epidemiology of seafood-related
diseases and the novel methods that could be used for future control of biofilm formation in seafood.

Phytochemical research has gained a lot of momentum in the medical field for the discovery of new, safe, and effective remedies. In the context of antimicrobial research, various plant sources have been discovered with the potential to disrupt bacterial quorum sensing (QS), which plays a key role in the regulation of virulence in many Gram positive and Gram negative bacteria. Pseudomonas aeruginosa, a Gram negative bacterium, is known to produce multiple QS systems that control the expression of virulence determinants and biofilm development in this pathogen. Hence, the inhibition of QS has been pursued as a promising therapy for treatment of drug resistant Pseudomonas infections. A comprehensive review of the research data available for plant products as QS inhibitors of the organism has been presented here, while further suggesting the future prospects for these inhibitors.

Many bacteria grow on surfaces forming biofilms. In this structure, they are well protected and often high dosages of antibiotics cannot clear infectious biofilms. The formation and stabilization of biofilms are mediated by diffusible autoinducers (e.g. N-acyl homoserine lactones, small peptides, furanosyl borate diester). Metabolites interfering with this process have been identified in plants, animals and microbes, and synthetic analogues are known. Additionally, this seems to be not the only way to control biofilms. Enzymes capable of cleaving essential components of the biofilm matrix, e.g. polysaccharides or extracellular DNA, and thus weakening the biofilm architecture have been identified. Bacteria also have mechanisms to dissolve their biofilms and return to planktonic lifestyle. Only a few compounds responsible for the signalling of these processes are known, but they may open a completely novel line of biofilm control. All these approaches lead to the destruction of the biofilm but not the killing of the pathogens. Therefore, a combination of biofilm-destroying compounds and antibiotics to handle biofilm infections is proposed. In this article, different approaches to combine biofilm-controlling compounds and antibiotics to fight biofilm infections are discussed, as well as the balance between biofilm formation and virulence.

Inoculation of plants with beneficial microbes can be traced back for centuries. Although bacteria were not proven to exist until in 1683 von Leeuwenhoek discovered microscopic ‘animals’ under the lens of his microscope, their utilisation to stimulate plant growth in agriculture has been exploited since ancient times. Theophrastus (372-287 BC) suggested the mixing of different soils as a means of ‘remedying defects and adding heart to the soil’ (Vessey 2003). Colonisation of plant root system is the very first step in nearly all interactions between plants and soil borne microbes. The importance of this compartment for plant growth and soil microbiology had already been realised in the very pioneer times of microbiology in the late 19th century. In 1888, Hellriegel and Wilfarth proved the special case of nitrogen nutrition of legumes through their root nodule bacteria, which Beijerinck finally isolated in 1889. In 1887–1888 Hiltner was certainly fascinated by the findings of Hellriegel and Wilfarth about the special case of nitrogen nutrition in legumes. Together with Professor Nobbe, intensive studies were conducted about the nature of the symbiotic interaction of nodule bacteria and legume roots (Nobbe and Hiltner 1896). Apparently during these studies, Hiltner became aware of the importance of the ecological interactions in the root zone. Together with his teacher he developed the first inoculant based on root nodule bacteria for agricultural practice which they called ‘Nitragin’ in 1890. As agricultural practice was in close contact to the new achievements of basic sciences, the challenge quickly arose whether at all, and how, these discoveries of the — at that time — very young science ‘soil bacteriology’ could be applied in the field. Region of contact between root and soil where soil is affected by roots was designated as “rhizosphere” by Hiltner (1904).

The major cause of mortality and morbidity in human beings is bacterial infection. Bacteria have developed resistance to most of the antibiotics primarily due to largescale and "indiscriminate" usage. The need is to develop novel mechanisms to treat bacterial infections. The expression of pathogenicity during bacterial infections is mediated by a cell density-dependent phenomenon known as quorum sensing (QS). A wide array of QS systems (QSS) is operative in expressing the virulent behavior of bacterial pathogens. Each QSS may be mediated largely by a few major signals along with others produced in minuscule quantities. Efforts to target signal molecules and their receptors have proved effective in alleviating the virulent behavior of such pathogenic bacteria. These QS inhibitors (QSIs) have been reported to be effective in influencing the pathogenicity without affecting bacterial growth. However, evidence is accumulating that bacteria may develop resistance to QSIs. The big question is whether QSIs will meet the same fate as antibiotics.

Bacterial cells can produce and sense signal molecules, allowing the whole population to initiate a concerted action once a critical concentration (corresponding to a particular population density) of the signal has been reached, a phenomenon known as quorum sensing. One of the possible quorum sensing-regulated phenotypes is swarming, a flagella-driven movement of differentiated swarmer cells (hyperflagellated, elongated, multinucleated) by which bacteria can spread as a biofilm over a surface. The glycolipid or lipopeptide biosurfactants thereby produced function as wetting agent by reducing the surface tension. Quorum sensing systems are almost always integrated into other regulatory circuits. This effectively expands the range of environmental signals that influence target gene expression beyond population density. In this review, we first discuss the regulation of AHL-mediated surface migration and the involvement of other low-molecular-mass signal molecules (such as the furanosyl borate diester AI-2) in biosurfactant production of different bacteria. In addition, population density-dependent regulation of swarmer cell differentiation is reviewed. Also, several examples of interspecies signalling are reported. Different signal molecules either produced by bacteria (such as other AHLs and diketopiperazines) or excreted by plants (such as furanones, plant signal mimics) might influence the quorum sensingregulated swarming behaviour in bacteria different from the producer. On the other hand, specific bacteria can reduce the local available concentration of signal molecules produced by others. In the last part, the role and regulation of a surface-associated movement in biofilm formation is discussed. Here we also describe how quorum sensing may disperse existing biofilms and control the interaction between bacteria and higher organisms (such as the Rhizobium-bean symbiosis).

The discovery of novel classes of antibiotics has slowed dramatically. This has occurred during a time when the appearance of resistant strains of bacteria has shown a substantial increase. Concern is therefore mounting over our ability to continue to treat infections in an effective manner using the antibiotics that are currently available. While ongoing efforts to discover new antibiotics are important, these must be coupled with strategies that aim to maintain as far as possible the spectrum of activity of existing antibiotics. In many instances, the resistance to antibiotics exhibited by bacteria in chronic infections is mediated not by direct resistance mechanisms, but by the adoption of modes of growth that confer reduced susceptibility. These include the formation of biofilms and the occurrence of subpopulations of 'persister' cells. As our understanding of these processes has increased, a number of new potential drug targets have been revealed. Here, advances in our ability to disrupt these systems that confer reduced susceptibility, and in turn increase the efficacy of antibiotic therapy, are discussed.

Through a mechanism called quorum sensing, bacteria are able to express specific genes in response to population density. Cell-to-cell communication in bacteria is mediated by signal molecules such as acylated homoserine lactones (AHLs). This work aimed to detect AHL production in Gram-negative psychrotrophic bacteria isolated from raw milk. A total of 84.9% of the bacteria were identified as AHL producers eliciting a diversity of responses in the AHL-monitor systems. These results demonstrate that AHL-production is common among psychrotrophic bacteria isolated from milk, and indicate that quorum sensing may play an important role in the spoilage of this product.

Biosurfactant (BS)/bioemulsifier (BE) produced by varied microorganisms exemplify immense structural/functional diversity and consequently signify the involvement of particular molecular machinery in their biosynthesis. The present chapter aims to compile information on molecular genetics of BS/BE production in microorganisms. Polymer synthesis in Acinetobacter species is controlled by an intricate operon system and its further excretion being controlled by enzymes. Quorum sensing system (QSS) plays a fundamental role in rhamnolipid and surfactin synthesis. Depending upon the cell density, signal molecules (autoinducers) of regulatory pathways accomplish the biosynthesis of BS. The regulation of serrawettin production by Serratia is believed to be through non ribosomal peptide synthetases (NRPSs) and N-acylhomoserine lactones (AHLs) encoded by QSS located on mobile transposon. This regulation is under positive as well as negative control of QSS operon products. In case of yeast and fungi, glycolipid precursor production is catalyzed by genes that encode enzyme cytochrome P450 monooxygenase. BS/BE production is dictated by genes present on the chromosomes. This chapter also gives a glimpse of recent biotechnological developments which helped to realize molecular genetics of BS/BE production in microorganisms. Hyper-producing recombinants as well as mutant strains have been constructed successfully to improve the yield and quality of BS/BE. Thus promising biotechnological advances have expanded the applicability of BS/BE in therapeutics, cosmetics, agriculture, food, beverages and bioremediation etc. In brief, our knowledge on genetics of BS/BE production in prokaryotes is extensive as compared to yeast and fungi. Meticulous and concerted study will lead to an understanding of the molecular phenomena in unexplored microbes. In addition to this, recent promising advances will facilitate in broadening applications of BS/BE to diverse fields. Over the decades, valuable information on molecular genetics of BS/BE has been generated and this strong foundation would facilitate application oriented output of the surfactant industry and broaden its use in diverse fields. To accomplish our objectives, interaction among experts from diverse fields likes microbiology, physiology, biochemistry, molecular biology and genetics is indispensable.

Quorum sensing (QS) is a mechanism by which diverse microorganisms can control specific processes in response to population density. A relatively well-known form of QS among Proteobacteria involves production and subsequent response to acylated homoserine lactones (AHLs). Quorum sensing inhibition (QSI), targeting AHL-dependent signaling, has been reported as a strategy for the control of biofilm formation used by several marine organisms. We developed a simple soft agar overlay protocol, based on pigmentation inhibition, to rapidly screen for the presence of potential QSI by bacteria and plants. For bacterial screens, test organisms are first streaked onto their appropriate media and incubated overnight. For plant screens, the plant material (leaf, stem, flower, etc.) is placed onto LB agar. The bacterial growth or plant samples are then covered with an overlay of LB soft agar containing an inoculum of either Pseudomonas aureofaciens 30 -84 or Chromobacterium violaceum ATCC 12472 (indicator cultures) and then incubated overnight. These indicator bacteria regulate pigment production by N-hexanoyl-HSL (C6-HSL) QS and are readily inhibited by AHL analogues and other antagonists. QSI is indicated by the lack of pigment production of the indicator culture in the vicinity of the test sample. Growth inhibition of the indicator culture indicates possible antibiotic production. Two different biosensor organisms based on derivatives of Agrobacterium tumefaciens and C. violaceum, capable of detecting a range of AHLs were used to determine whether QSI is due to the production of interfering AHLs competing with the C6-HSL regulation of C. violaceum and P. aureofaciens pigment production. This simple protocol will facilitate the screening of multiple organisms for the production of potential antifouling compounds. D

The process of biofouling of marine structures and substrates, such as platforms or ship hulls, proceeds in multiple steps. Soon after the formation of an initial conditioning film, formed via the adsorption of organic particles to natural or manmade substrates, a population of different bacterial taxa associates under the formation of a biofilm. These microorganisms communicate through a complex quorum sensing network. Macro-foulers, e.g., barnacles, then settle and form a fouling layer on the marine surfaces, a process that globally has severe impacts both on the economy and on the environment. Since the ban of tributyltin, an efficient replacement of this antifouling compound by next-generation antifouling coatings that are environmentally more acceptable and also showing longer half-lives has not yet been developed. The sponges, as sessile filter-feeder animals, have evolved antifouling strategies to protect themselves against micro-and subsequent macrobiofouling processes. Experimental data are summarized and suggest that coating of the sponge surface with bio-silica contributes to the inhibition of the formation of a conditioning film. A direct adsorption of the surfaces by microorganisms can be impaired through poisoning the organisms with direct-acting secondary metabolites or toxic peptides. In addition, first, compounds from sponges have been identified that interfere with the anti-quorum sensing network. Sponge secondary metabolites acting selectively on diatom colonization have not yet been identified. Finally, it is outlined that direct-acting secondary metabolites inhibiting the growth of macro-fouling animals and those that poison the multidrug resistance pump are available. It is concluded that rational screening programs for inhibitors of the complex and dynamic problem of biofilm production, based on multidisciplinary studies and using sponges as a model, are required in the future.