As N-acyl Homoserine Lactones Research Articles

Efficient Biostimulation of Native and Introduced Quorum-Quenching Rhodococcus erythropolis Populations Is Revealed by a Combination of Analytical Chemistry, Microbiology, and Pyrosequencing

Degradation of the quorum-sensing (QS) signals known as N-acylhomoserine lactones (AHL) by soil bacteria may be useful as a beneficial trait for protecting crops, such as potato plants, against the worldwide pathogen Pectobacterium. In this work, analytical chemistry and microbial and molecular approaches were combined to explore and compare biostimulation of native and introduced AHL-degrading Rhodococcus erythropolis populations in the rhizosphere of potato plants cultivated in farm greenhouses under hydroponic conditions. We first identified gamma-heptalactone (GHL) as a novel biostimulating agent that efficiently promotes plant root colonization by AHL-degrading R. erythropolis population. We also characterized an AHL-degrading biocontrol R. erythropolis isolate, R138, which was introduced in the potato rhizosphere. Moreover, root colonization by AHL-degrading bacteria receiving different combinations of GHL and R138 treatments was compared by using a cultivation-based approach (percentage of AHL-degrading bacteria), pyrosequencing of PCR-amplified rrs loci (total bacterial community), and quantitative PCR (qPCR) of the qsdA gene, which encodes an AHL lactonase in R. erythropolis. Higher densities of the AHL-degrading R. erythropolis population in the rhizosphere were observed when GHL treatment was associated with biocontrol strain R138. Under this condition, the introduced R. erythropolis population displaced the native R. erythropolis population. Finally, chemical analyses revealed that GHL, gamma-caprolactone (GCL), and their by-products, gamma-hydroxyheptanoic acid and gamma-hydroxycaproic acid, rapidly disappeared from the rhizosphere and did not accumulate in plant tissues. This integrative study highlights biostimulation as a potential innovative approach for improving root colonization by beneficial bacteria.

The quorum-quenching N -acyl homoserine lactone acylase PvdQ is an Ntn-hydrolase with an unusual substrate-binding pocket

In many Gram-negative pathogens, their virulent behavior is regulated by quorum sensing, in which diffusible signals such as N-acyl homoserine lactones (AHLs) act as chemical messaging compounds. Enzymatic degradation of these diffusible signals by, e.g., lactonases or amidohydrolases abolishes AHL regulated virulence, a process known as quorum quenching. Here we report the first crystal structure of an AHL amidohydrolase, the AHL acylase PvdQ from Pseudomonas aeruginosa. PvdQ has a typical alpha/beta heterodimeric Ntn-hydrolase fold, similar to penicillin G acylase and cephalosporin acylase. However, it has a distinct, unusually large, hydrophobic binding pocket, ideally suited to recognize C12 fatty acid-like chains of AHLs. Binding of a C12 fatty acid or a 3-oxo-C12 fatty acid induces subtle conformational changes to accommodate the aliphatic chain. Furthermore, the structure of a covalent ester intermediate identifies Serbeta1 as the nucleophile and Asnbeta269 and Valbeta70 as the oxyanion hole residues in the AHL degradation process. Our structures show the versatility of the Ntn-hydrolase scaffold and can serve as a structural paradigm for Ntn-hydrolases with similar substrate preference. Finally, the quorum-quenching capabilities of PvdQ may be utilized to suppress the quorum-sensing machinery of pathogens.

Rapid Hydrolysis of Quorum‐Sensing Molecules in the Gut of Lepidopteran Larvae

Microorganisms compete for nutrients and living space in the gut of plant-feeding insect larvae, such as Spodoptera spp. Their physiological activities and their organization are generally controlled or synchronised by "autoinducers", such as N-acylhomoserinelactones (AHLs). Due to the strongly alkaline milieu in the insect gut, the lactone ring of AHLs is rapidly and spontaneously opened. Further degradation to the inactive components homoserine and the acyl moiety is then achieved by a microbial N-acylamino acid hydrolase (AAH) and related enzymatic activities in the insect gut. Initialised by the alkaline milieu, such activities might account for the complete absence of AHLs in the intestinal fluid of the studied Spodoptera spp. The AHL-recognition system of E. coli RV308pSB40, but not that of Agrobacterium tumefaciens NT1/pZLR4 and Chromobacterium violaceum CV026, was found to be inhibited by the structurally related N-acylglutamines, which are abundantly present in the gut of many lepidopteran larvae. Our observations suggest an active role of the insect in interfering with the quorum sensing of their gut microbiota by several independent strategies.

New trends in marine chemical ecology

This essay is the outcome of a colloquium convened in November 2005 at the Benthos Laboratory of the Stazione Zoologica Anton Dohrn in Ischia, Italy, on chemical ecology and the role of secondary metabolites in the structuring and functioning of marine biodiversity. The participants of the workshop are part of the European Network of Excellence MarBEF (Marine Biodiversity and Ecosystem Function), a consortium of 56 European marine institutes to integrate and disseminate knowledge and expertise on marine biodiversity. Here we review some of the new trends and emerging topics in marine chemical ecology. The first section deals with microbial chemical interactions. Microbes communicate with each other using diffusible molecules such as N-acylhomoserine lactones (AHL). These are regulators in cell-density-dependent gene regulation (quorum sensing) controlling microbial processes. In chemical interactions with higher organisms, microbes can act either as harmful pathogens that are repelled by the host’s chemical defense or as beneficial symbionts. These symbionts are sometimes the true producers of the host’s secondary metabolites that have defensive and protective functions for their hosts. We also describe how allelochemicals can shape phytoplankton communities by regulating competition for available resources, and also interactions among individuals of the same species. Compounds such as the diatom-derived unsaturated aldehydes have been demonstrated to act as info chemicals, and they possibly function as a diffusible bloom-termination signal that triggers an active cell death and bloom termination at sea. The same molecules have also been shown to interfere with the reproductive capacity of grazing animals deterring future generations of potential predators. Such compounds differ from those that act as feeding deterrents since they do not target the predator but its offspring. Many of the neurotoxins produced by dinoflagellates act as feeding deterrents, and laboratory experiments have shown that ingestion of these algae by some microzooplankton and macrozooplankton can cause acute, responses such as death, incapacitation, altered swimming behavior, and reduced fecundity and egg-hatching success. These effects may rarely occur in nature because of low individual grazing rates on dinoflagellate cells and grazing on other food sources such as microflagellates and diatoms. We also consider the nutritional component of marine plant-herbivore interactions, especially in the plankton, and the information available on the effects of growing conditions of algae on the production of toxic metabolites. Species producing saxitoxins seem to consistently produce the highest amounts of toxins (on a per cell basis) in the exponential phase of growth, and there is a decrease in their production under nitrogen, but not under phosphorus stress, where the production actually increases. We try to explain the circumstances under which organisms defend themselves chemically and argue that the most likely explanatory model for the production of secondary metabolites used for defense in planktonic organisms is the carbon nutrient balance hypothesis, which predicts that most algae produce their toxins mainly under conditions where carbon is in excess and nitrogen (or other nutrients) is limiting. We also discuss chemically mediated macroalgal-herbivore interactions in the benthos and the large variation in concentration of seaweed defense metabolites at different spatial and temporal scales. Seaweeds have been shown to produce a large variety of secondary metabolites with highly variable chemical structures such as terpenoids, acetogenins, amino acid derivates, and polyphenols. Many of these compounds probably have multiple simultaneous functions for the seaweeds and can act as allelopathic, antimicrobial, and antifouling or ultraviolet-screening agents, as well as herbivore deterrents. We also provide examples of interactions between marine benthic invertebrates, especially sponges, molluscs, and cnidarians, that are mediated by specific secondary metabolites and discuss the role of these in shaping benthic communities.