Max Teplitski

Plants Secrete Substances That Mimic Bacterial N-Acyl Homoserine Lactone Signal Activities and Affect Population Density-Dependent Behaviors in Associated Bacteria

In gram-negative bacteria, many important changes in gene expression and behavior are regulated in a population density-dependent fashion by N-acyl homoserine lactone (AHL) signal molecules. Exudates from pea (Pisum sativum) seedlings were found to contain several separable activities that mimicked AHL signals in well-characterized bacterial reporter strains, stimulating AHL-regulated behaviors in some strains while inhibiting such behaviors in others. The chemical nature of the active mimic compounds is currently unknown, but all extracted differently into organic solvents than common bacterial AHLs. Various species of higher plants in addition to pea were found to secrete AHL mimic activities. The AHL signal-mimic compounds could prove to be important in determining the outcome of interactions between higher plants and a diversity of pathogenic, symbiotic, and saprophytic bacteria.

Production of Substances by Medicago truncatula that Affect Bacterial Quorum Sensing

Earlier work showed that higher plants produce unidentified compounds that specifically stimulate or inhibit quorum sensing (QS) regulated responses in bacteria. The ability of plants to produce substances that affect QS regulation may provide plants with important tools to manipulate gene expression and behavior in the bacteria they encounter. In order to examine the kinds of QS active substances produced by the model legume M. truncatula, young seedlings and seedling exudates were systematically extracted with various organic solvents, and the extracts were fractionated by reverse phase C18 high-performance liquid chromatography. M. truncatula appears to produce at least 15 to 20 separable substances capable of specifically stimulating or inhibiting responses in QS reporter bacteria, primarily substances that affect QS regulation dependent on N-acyl homoserine lactone (AHL) signals. The secretion of AHL QS mimic activities by germinating seeds and seedlings was found to change substantially with developmental age. The secretion of some mimic activities may be dependent upon prior exposure of the plants to bacteria.

Mini-review: quorum sensing in the marine environment and its relationship to biofouling

Bacterial quorum sensing (QS) is a cell–cell communication and gene regulatory mechanism that allows bacteria to coordinate swarming, biofilm formation, stress resistance, and production of toxins and secondary metabolites in response to threshold concentrations of QS signals that accumulate within a diffusion-limited environment. This review focuses on the role of QS signaling and QS inhibition in marine bacteria by compounds derived from marine organisms. Since the formation of a biofilm is considered to be an initial step in the development of fouling, direct and indirect effects of QS signals and inhibitors on the process of marine biofouling are discussed. Directions for future investigations and QS-related biotechnological applications are highlighted.

Chlamydomonas reinhardtii secretes compounds that mimic bacterial signals and interfere with quorum sensing regulation in bacteria.

The unicellular soil-freshwater alga Chlamydomonas reinhardtii was found to secrete substances that mimic the activity of the N-acyl-l-homoserine lactone (AHL) signal molecules used by many bacteria for quorum sensing regulation of gene expression. More than a dozen chemically separable but unidentified substances capable of specifically stimulating the LasR or CepR but not the LuxR, AhyR, or CviR AHL bacterial quorum sensing reporter strains were detected in ethyl acetate extracts of C. reinhardtii culture filtrates. Colonies of C. reinhardtii and Chlorella spp. stimulated quorum sensing-dependent luminescence in Vibrio harveyi, indicating that these algae may produce compounds that affect the AI-2 furanosyl borate diester-mediated quorum sensing system of Vibrio spp. Treatment of the soil bacterium Sinorhizobium meliloti with a partially purified LasR mimic from C. reinhardtii affected the accumulation of 16 of the 25 proteins that were altered in response to the bacteriums own AHL signals, providing evidence that the algal mimic affected quorum sensing-regulated functions in this wild-type bacterium. Peptide mass fingerprinting identified 32 proteins affected by the bacteriums AHLs or the purified algal mimic, including GroEL chaperonins, the nitrogen regulatory protein PII, and a GTP-binding protein. The algal mimic was able to cancel the stimulatory effects of bacterial AHLs on the accumulation of seven of these proteins, providing evidence that the secretion of AHL mimics by the alga could be effective in disruption of quorum sensing in naturally encountered bacteria.

Root Mucilage from Pea and Its Utilization by Rhizosphere Bacteria as a Sole Carbon Source

Plant roots secrete a complex polysaccharide mucilage that may provide a significant source of carbon for microbes that colonize the rhizosphere. High molecular weight mucilage was separated by high-pressure liquid chromatography gel filtration from low molecular weight components of pea root exudate. Purified pea root mucilage generally was similar in sugar and glycosidic linkage composition to mucilage from cowpea, wheat, rice, and maize, but appeared to contain an unusually high amount of material that was similar to arabinogalactan protein. Purified pea mucilage was used as the sole carbon source for growth of several pea rhizosphere bacteria, including Rhizobium leguminosarum 8401 and 4292, Burkholderia cepacia AMMD, and Pseudomonas fluorescens PRA25. These species grew on mucilage to cell densities of three- to 25-fold higher than controls with no added carbon source, with cell densities of 1 to 15% of those obtained on an equal weight of glucose. Micromolar concentrations of nod gene-inducing flavonoids specifically stimulated mucilage-dependent growth of R. leguminosarum 8401 to levels almost equaling the glucose controls. R. leguminosarum 8401 was able to hydrolyze p-nitrophenyl glycosides of various sugars and partially utilize a number of purified plant polysaccharides as sole carbon sources, indicating that R. leguminosarum 8401 can make an unexpected variety of carbohydrases, in accordance with its ability to extensively utilize pea root mucilage.

A LuxR Homolog Controls Production of Symbiotically Active Extracellular Polysaccharide II by Sinorhizobium meliloti

Production of complex extracellular polysaccharides (EPSs) by the nitrogen-fixing soil bacterium Sinorhizobium meliloti is required for efficient invasion of root nodules on the host plant alfalfa. Any one of three S. meliloti polysaccharides, succinoglycan, EPS II, or K antigen, can mediate infection thread initiation and extension (root nodule invasion) on alfalfa. Of these three polysaccharides, the only symbiotically active polysaccharide produced by S. meliloti wild-type strain Rm1021 is succinoglycan. The expR101 mutation is required to turn on production of symbiotically active forms of EPS II in strain Rm1021. In this study, we have determined the nature of the expR101 mutation in S. meliloti. The expR101 mutation, a spontaneous dominant mutation, results from precise, reading frame-restoring excision of an insertion sequence from the coding region of expR, a gene whose predicted protein product is highly homologous to the Rhizobium leguminosarum bv. viciae RhiR protein and a number of other homologs of Vibrio fischeri LuxR that function as receptors for N-acylhomoserine lactones (AHLs) in quorum-sensing regulation of gene expression. S. meliloti ExpR activates transcription of genes involved in EPS II production in a density-dependent fashion, and it does so at much lower cell densities than many quorum-sensing systems. High-pressure liquid chromatographic fractionation of S. meliloti culture filtrate extracts revealed at least three peaks with AHL activity, one of which activated ExpR-dependent expression of the expE operon.

Pathways Leading from BarA/SirA to Motility and Virulence Gene Expression in Salmonella

The barA and sirA genes of Salmonella enterica serovar Typhimurium encode a two-component sensor kinase and a response regulator, respectively. This system increases the expression of virulence genes and decreases the expression of motility genes. In this study, we examined the pathways by which SirA affects these genes. We found that the master regulator of flagellar genes, flhDC, had a positive regulatory effect on the primary regulator of intestinal virulence determinants, hilA, but that hilA had no effect on flhDC. SirA was able to repress flhDC in a hilA mutant and activate hilA in an flhDC mutant. Therefore, although the flhDC and hilA regulatory cascades interact, sirA affects each of them independently. A form of BarA lacking the two N-terminal membrane-spanning domains, BarA198, autophosphorylates in the presence of ATP and transfers the phosphate to purified SirA. Phosphorylated SirA was found to directly bind the hilA and hilC promoters in gel mobility shift assays but not the flhD, fliA, hilD, and invF promoters. Given that the CsrA/csrB system is known to directly affect flagellar gene expression, we tested the hypothesis that SirA affects flagellar gene expression indirectly by regulating csrA or csrB. The sirA gene did not regulate csrA but did activate csrB expression. Consistent with these results, phosphorylated SirA was found to directly bind the csrB promoter but not the csrA promoter. We propose a model in which SirA directly activates virulence expression via hilA and hilC while repressing the flagellar regulon indirectly via csrB.

Coral-associated micro-organisms and their roles in promoting coral health and thwarting diseases

Over the last decade, significant advances have been made in characterization of the coral microbiota. Shifts in its composition often correlate with the appearance of signs of diseases and/or bleaching, thus suggesting a link between microbes, coral health and stability of reef ecosystems. The understanding of interactions in coral-associated microbiota is informed by the on-going characterization of other microbiomes, which suggest that metabolic pathways and functional capabilities define the ‘core’ microbiota more accurately than the taxonomic diversity of its members. Consistent with this hypothesis, there does not appear to be a consensus on the specificity in the interactions of corals with microbial commensals, even though recent studies report potentially beneficial functions of the coral-associated bacteria. They cycle sulphur, fix nitrogen, produce antimicrobial compounds, inhibit cell-to-cell signalling and disrupt virulence in opportunistic pathogens. While their beneficial functions have been documented, it is not certain whether or how these microbes are selected by the hosts. Therefore, understanding the role of innate immunity, signal and nutrient exchange in the establishment of coral microbiota and in controlling its functions will probably reveal ancient, evolutionarily conserved mechanisms that dictate the outcomes of host–microbial interactions, and impact the resilience of the host.

Perception and Degradation of N-Acyl Homoserine Lactone Quorum Sensing Signals by Mammalian and Plant Cells

Cell-to-cell signaling via small chemical signals is an ancient process shared by most living organisms. Whether these signals are produced by mammals, insects, plants, or bacteria and are called hormones, pheromones, or autoinducers, they convey vital information about the status of the cell and its extracellular environment. The discovery of bacterial cell-to-cell signaling, or quorum sensing (QS), revealed that single-celled prokaryotic organisms possess sophisticated methods for coordinating their behavior by secreting chemical or peptide signals and ushered in a new field of Microbiology. The study of quorum sensing has mainly focused on identifying the components making up quorum sensing circuits in bacteria, and significant progress has been made in elucidating the structures of the chemical signals, identifying the receptors that detect these signals and affect QSdependent gene expression, and mapping the signal transduction pathways involved. However, we are also beginning to understand that these bacterial quorum sensing signals (QSS) are perceived, responded to, and degraded by both eukaryotes and members of the host-associated microbial communities. Thus, the question arises, “How do host cells perceive QSS and what are the consequences of this crosskingdom communication?” QSS are structurally and functionally similar to some hormones produced by mammals and phytohoromones made by plants, and there is strong evidence to suggest that they * To whom correspondence should be addressed. Mailing address: Texas Tech University Health Sciences Center, Department of Surgery, MS 8312, 3601 4th Street, Lubbock, Texas 79430. Phone: 806-743-2460, ext. 264. Fax: 806-743-2370. E-mail: kendra.rumbaugh@ttuhsc.edu. † University of Florida. ‡ Australian National University. § Texas Tech University Health Sciences Center. Max Teplitski is an associate professor in the Soil and Water Science Department at the Genetics Institute of the University of Florida. The Teplitski group focuses on defining the roles of chemical signals and nutritional cues in structuring and functions of host-associated bacterial communities. Mechanisms by which plant and animal hosts manipulate cell-to-cell signaling and population density-dependent gene regulation in the associated symbiotic and pathogenic bacteria are of special interest to his group. To address these questions, members of the Teplitski team investigate the role of quorum sensing and post-transcriptional regulation in the Sinorhizobium-Medicago symbiosis, functional genomics of the interactions between Salmonella and plant hosts, and the interspecies interactions within microbiota of the coral mucopolysaccharide layer. Chem. Rev. 2011, 111, 100–116 100

Mini-review: Inhibition of biofouling by marine microorganisms

Any natural or artificial substratum exposed to seawater is quickly fouled by marine microorganisms and later by macrofouling species. Microfouling organisms on the surface of a substratum form heterogenic biofilms, which are composed of multiple species of heterotrophic bacteria, cyanobacteria, diatoms, protozoa and fungi. Biofilms on artificial structures create serious problems for industries worldwide, with effects including an increase in drag force and metal corrosion as well as a reduction in heat transfer efficiency. Additionally, microorganisms produce chemical compounds that may induce or inhibit settlement and growth of other fouling organisms. Since the last review by the first author on inhibition of biofouling by marine microbes in 2006, significant progress has been made in the field. Several antimicrobial, antialgal and antilarval compounds have been isolated from heterotrophic marine bacteria, cyanobacteria and fungi. Some of these compounds have multiple bioactivities. Microorganisms are able to disrupt biofilms by inhibition of bacterial signalling and production of enzymes that degrade bacterial signals and polymers. Epibiotic microorganisms associated with marine algae and invertebrates have a high antifouling (AF) potential, which can be used to solve biofouling problems in industry. However, more information about the production of AF compounds by marine microorganisms in situ and their mechanisms of action needs to be obtained. This review focuses on the AF activity of marine heterotrophic bacteria, cyanobacteria and fungi and covers publications from 2006 up to the end of 2012.