Rhizosphere plant-microbe interactions under water stress.

Climate change, with its extreme temperature, weather and precipitation patterns, is a major global concern of dryland farmers, who currently meet the challenges of climate change agronomically and with growth of drought-tolerant crops. Plants themselves compensate for water stress by modifying aerial surfaces to control transpiration and altering root hydraulic conductance to increase water uptake. These responses are complemented by metabolic changes involving phytohormone network-mediated activation of stress response pathways, resulting in decreased photosynthetic activity and the accumulation of metabolites to maintain osmotic and redox homeostasis. Phylogenetically diverse microbial communities sustained by plants contribute to host drought tolerance by modulating phytohormone levels in the rhizosphere and producing water-sequestering biofilms. Drylands of the Inland Pacific Northwest, USA, illustrate the interdependence of dryland crops and their associated microbiota. Indigenous Pseudomonas spp. selected there by long-term wheat monoculture suppress root diseases via the production of antibiotics, with soil moisture a critical determinant of the bacterial distribution, dynamics and activity. Those pseudomonads producing phenazine antibiotics on wheat had more abundant rhizosphere biofilms and provided improved tolerance to drought, suggesting a role of the antibiotic in alleviation of drought stress. The transcriptome and metabolome studies suggest the importance of wheat root exudate-derived osmoprotectants for the adaptation of these pseudomonads to the rhizosphere lifestyle and support the idea that the exchange of metabolites between plant roots and microorganisms profoundly affects and shapes the belowground plant microbiome under water stress.

Root Exudates Alter the Expression of Diverse Metabolic, Transport, Regulatory, and Stress Response Genes in Rhizosphere Pseudomonas.

Plants live in association with microorganisms that positively influence plant development, vigor, and fitness in response to pathogens and abiotic stressors. The bulk of the plant microbiome is concentrated belowground at the plant root-soil interface. Plant roots secrete carbon-rich rhizodeposits containing primary and secondary low molecular weight metabolites, lysates, and mucilages. These exudates provide nutrients for soil microorganisms and modulate their affinity to host plants, but molecular details of this process are largely unresolved. We addressed this gap by focusing on the molecular dialog between eight well-characterized beneficial strains of the Pseudomonas fluorescens group and Brachypodium distachyon, a model for economically important food, feed, forage, and biomass crops of the grass family. We collected and analyzed root exudates of B. distachyon and demonstrated the presence of multiple carbohydrates, amino acids, organic acids, and phenolic compounds. The subsequent screening of bacteria by Biolog Phenotype MicroArrays revealed that many of these metabolites provide carbon and energy for the Pseudomonas strains. RNA-seq profiling of bacterial cultures amended with root exudates revealed changes in the expression of genes encoding numerous catabolic and anabolic enzymes, transporters, transcriptional regulators, stress response, and conserved hypothetical proteins. Almost half of the differentially expressed genes mapped to the variable part of the strains' pangenome, reflecting the importance of the variable gene content in the adaptation of P. fluorescens to the rhizosphere lifestyle. Our results collectively reveal the diversity of cellular pathways and physiological responses underlying the establishment of mutualistic interactions between these beneficial rhizobacteria and their plant hosts.

Functional Analysis of Phenazine Biosynthesis Genes in Burkholderia spp.

Burkholderia encompasses a group of ubiquitous Gram-negative bacteria that includes numerous saprophytes as well as species that cause infections in animals, immunocompromised patients, and plants. Some species of Burkholderia produce colored, redox-active secondary metabolites called phenazines. Phenazines contribute to competitiveness, biofilm formation, and virulence in the opportunistic pathogen Pseudomonas aeruginosa, but knowledge of their diversity, biosynthesis, and biological functions in Burkholderia is lacking. In this study, we screened publicly accessible genome sequence databases and identified phenazine biosynthesis genes in multiple strains of the Burkholderia cepacia complex, some isolates of the B. pseudomallei clade, and the plant pathogen B. glumae We then focused on B. lata ATCC 17760 to reveal the organization and function of genes involved in the production of dimethyl 4,9-dihydroxy-1,6-phenazinedicarboxylate. Using a combination of isogenic mutants and plasmids carrying different segments of the phz locus, we characterized three novel genes involved in the modification of the phenazine tricycle. Our functional studies revealed a connection between the presence and amount of phenazines and the dynamics of biofilm growth in flow cell and static experimental systems but at the same time failed to link the production of phenazines with the capacity of Burkholderia to kill fruit flies and rot onions.IMPORTANCE Although the production of phenazines in Burkholderia was first reported almost 70 years ago, the role these metabolites play in the biology of these economically important microorganisms remains poorly understood. Our results revealed that the phenazine biosynthetic pathway in Burkholderia has a complex evolutionary history, which likely involved horizontal gene transfers among several distantly related groups of organisms. The contribution of phenazines to the formation of biofilms suggests that Burkholderia, like fluorescent pseudomonads, may benefit from the unique redox-cycling properties of these versatile secondary metabolites.

Draft Genome Sequences of Six Strains Isolated from the Rhizosphere of Wheat Grown in Cadmium-Contaminated Soil.

This study presents high-quality draft genome assemblies of six bacterial strains isolated from the roots of wheat grown in soil contaminated with cadmium. The results of this study will help to elucidate at the molecular level how heavy metals affect interactions between beneficial rhizobacteria and crop plants.

Draft Genome Sequences of Xylella fastidiosa subsp. fastidiosa Strains OK3, VB11, and NOB1, Isolated from Bunch and Muscadine Grapes Grown in Southern Mississippi.

We report here high-quality draft whole-genome assemblies of Xylella fastidiosa subsp. fastidiosa strains OK3, VB11, and NOB1, which were isolated from symptomatic bunch and muscadine grape plants grown in southern Mississippi.

Comparative Analysis of Rhizosphere Microbiomes of Southern Highbush Blueberry (Vaccinium corymbosum L.), Darrow's Blueberry (V. darrowii Camp), and Rabbiteye Blueberry (V. virgatum Aiton).

Plants are inhabited by millions of parasitic, commensal, and mutualistic microorganisms that coexist in complex ecological communities, and profoundly affect the plant's productivity, health, and capacity to cope with environmental stress. Therefore, a better understanding of the rhizosphere microbiome may open a yet untapped avenue for the rational exploitation of beneficial plant-microbe interactions in modern agriculture. Blueberries encompass several wild and cultivated species of shrubs of the genus Vaccinium that are native to North America. They are grown commercially for the production of fruits, which are considered a health food due to the rich content of minerals, trace elements, and phenolic compounds with antioxidant, antitumor, and anti-inflammatory properties. Despite a long history of breeding and extensive commercial use, remarkably little is known about the composition and function of the blueberry root microbiome. To address this gap, we employed molecular approaches to characterize and compare microbial communities inhabiting the roots of rabbiteye blueberry (Vaccinium virgatum), Darrow's blueberry (Vaccinium darrowii), and southern highbush blueberry (SHB; an interspecific hybrid of Vaccinium corymbosum and V. darrowii). Our results revealed that these plant species share a common core rhizobiome, but at the same time differ significantly in the diversity, relative abundance, richness, and evenness of multiple groups of prokaryotic and eukaryotic microorganisms. Although the host signature effects were especially pronounced at the plant species level, we also observed genotype-level variations in the distribution of specific microbial taxa, which suggests that the assembly of the blueberry microbiome is shaped by the plant genotype and modifications associated with the domestication and breeding of members of the Vaccinium genus. We also demonstrated that the studied Vaccinium species differ in the abundance of beneficial rhizobacteria and ericoid mycorrhizal fungi, which play a vital role in their adaptation to soils with low pH and slow turnover of organic matter.

Pseudomonas synxantha 2-79 Transformed with Pyrrolnitrin Biosynthesis Genes Has Improved Biocontrol Activity Against Soilborne Pathogens of Wheat and Canola.

A four-gene operon (prnABCD) from Pseudomonas protegens Pf-5 encoding the biosynthesis of the antibiotic pyrronitrin was introduced into P. synxantha (formerly P. fluorescens) 2-79, an aggressive root colonizer of both dryland and irrigated wheat roots that naturally produces the antibiotic phenazine-1-carboxylic acid and suppresses both take-all and Rhizoctonia root rot of wheat. Recombinant strains ZHW15 and ZHW25 produced both antibiotics and maintained population sizes in the rhizosphere of wheat that were comparable to those of strain 2-79. The recombinant strains inhibited in vitro the wheat pathogens Rhizoctonia solani anastomosis group 8 (AG-8) and AG-2-1, Gaeumannomyces graminis var. tritici, Sclerotinia sclerotiorum, Fusarium culmorum, and F. pseudograminearum significantly more than did strain 2-79. Both the wild-type and recombinant strains were equally inhibitory of Pythium ultimum. When applied as a seed treatment, the recombinant strains suppressed take-all, Rhizoctonia root rot of wheat, and Rhizoctonia root and stem rot of canola significantly better than did wild-type strain 2-79.

Exploring the Pathogenicity of Pseudomonas brassicacearum Q8r1-96 and Other Strains of the Pseudomonas fluorescens Complex on Tomato.

Pseudomonas brassicacearum and related species of the P. fluorescens complex have long been studied as biocontrol and growth-promoting rhizobacteria involved in suppression of soilborne pathogens. We report here that P. brassicacearum Q8r1-96 and other 2,4-diacetylphloroglucinol (DAPG)-producing fluorescent pseudomonads involved in take-all decline of wheat in the Pacific Northwest of the United States can also be pathogenic to other plant hosts. Strain Q8r1-96 caused necrosis when injected into tomato stems and immature tomato fruits, either attached or removed from the plant, but lesion development was dose dependent, with a minimum of 106 CFU ml-1 required to cause visible tissue damage. We explored the relative contribution of several known plant-microbe interaction traits to the pathogenicity of strain Q8r1-96. Type III secretion system (T3SS) mutants of Q8r1-96, injected at a concentration of 108 CFU ml-1, were significantly less virulent, but not consistently, as compared with the wild-type strain. However, a DAPG-deficient phlD mutant of Q8r1-96 was significantly and consistently less virulent as compared with the wild type. Strain Q8r1-96acc, engineered to over express ACC deaminase, caused a similar amount of necrosis as the wild type. Cell-free culture filtrates of strain Q8r1-96 and pure DAPG also cause necrosis in tomato fruits. Our results suggest that DAPG plays a significant role in the ability of Q8r1-96 to cause necrosis of tomato tissue, but other factors also contribute to the pathogenic properties of this organism.

Antimicrobial Activity of, and Cellular Pathways Targeted by, p-Anisaldehyde and Epigallocatechin Gallate in the Opportunistic Human Pathogen Pseudomonas aeruginosa.

Plant-derived aldehydes are constituents of essential oils that possess broad-spectrum antimicrobial activity and kill microorganisms without promoting resistance. In our previous study, we incorporated p-anisaldehyde from star anise into a polymer network called proantimicrobial networks via degradable acetals (PANDAs) and used it as a novel drug delivery platform. PANDAs released p-anisaldehyde upon a change in pH and humidity and controlled the growth of the multidrug-resistant pathogen Pseudomonas aeruginosa PAO1. In this study, we identified the cellular pathways targeted by p-anisaldehyde by generating 10,000 transposon mutants of PAO1 and screened them for hypersensitivity to p-anisaldehyde. To improve the antimicrobial efficacy of p-anisaldehyde, we combined it with epigallocatechin gallate (EGCG), a polyphenol from green tea, and demonstrated that it acts synergistically with p-anisaldehyde in killing P. aeruginosa We then used transcriptome sequencing to profile the responses of P. aeruginosa to p-anisaldehyde, EGCG, and their combination. The exposure to p-anisaldehyde altered the expression of genes involved in modification of the cell envelope, membrane transport, drug efflux, energy metabolism, molybdenum cofactor biosynthesis, and the stress response. We also demonstrate that the addition of EGCG reversed many p-anisaldehyde-coping effects and induced oxidative stress. Our results provide insight into the antimicrobial activity of p-anisaldehyde and its interactions with EGCG and may aid in the rational identification of new synergistically acting combinations of plant metabolites. Our study also confirms the utility of the thiol-ene polymer platform for the sustained and effective delivery of hydrophobic and volatile antimicrobial compounds.IMPORTANCE Essential oils (EOs) are plant-derived products that have long been exploited for their antimicrobial activities in medicine, agriculture, and food preservation. EOs represent a promising alternative to conventional antibiotics due to their broad-range antimicrobial activity, low toxicity to human commensal bacteria, and capacity to kill microorganisms without promoting resistance. Despite the progress in the understanding of the biological activity of EOs, our understanding of many aspects of their mode of action remains inconclusive. The overarching aim of this work was to address these gaps by studying the molecular interactions between an antimicrobial plant aldehyde and the opportunistic human pathogen Pseudomonas aeruginosa The results of this study identify the microbial genes and associated pathways involved in the response to antimicrobial phytoaldehydes and provide insights into the molecular mechanisms governing the synergistic effects of individual constituents within essential oils.

Phenazine-1-carboxylic acid and soil moisture influence biofilm development and turnover of rhizobacterial biomass on wheat root surfaces.

Phenazine-1-carboxylic acid (PCA) is produced by rhizobacteria in dryland but not in irrigated wheat fields of the Pacific Northwest, USA. PCA promotes biofilm development in bacterial cultures and bacterial colonization of wheat rhizospheres. However, its impact upon biofilm development has not been demonstrated in the rhizosphere, where biofilms influence terrestrial carbon and nitrogen cycles with ramifications for crop and soil health. Furthermore, the relationships between soil moisture and the rates of PCA biosynthesis and degradation have not been established. In this study, expression of PCA biosynthesis genes was upregulated relative to background transcription, and persistence of PCA was slightly decreased in dryland relative to irrigated wheat rhizospheres. Biofilms in dryland rhizospheres inoculated with the PCA-producing (PCA+ ) strain Pseudomonas synxantha 2-79RN10 were more robust than those in rhizospheres inoculated with an isogenic PCA-deficient (PCA- ) mutant strain. This trend was reversed in irrigated rhizospheres. In dryland PCA+ rhizospheres, the turnover of 15 N-labelled rhizobacterial biomass was slower than in the PCA- and irrigated PCA+ treatments, and incorporation of bacterial 15 N into root cell walls was observed in multiple treatments. These results indicate that PCA promotes biofilm development in dryland rhizospheres, and likely influences crop nutrition and soil health in dryland wheat fields.

Long-Term Irrigation Affects the Dynamics and Activity of the Wheat Rhizosphere Microbiome.

The Inland Pacific Northwest (IPNW) encompasses 1. 6 million cropland hectares and is a major wheat-producing area in the western United States. The climate throughout the region is semi-arid, making the availability of water a significant challenge for IPNW agriculture. Much attention has been given to uncovering the effects of water stress on the physiology of wheat and the dynamics of its soilborne diseases. In contrast, the impact of soil moisture on the establishment and activity of microbial communities in the rhizosphere of dryland wheat remains poorly understood. We addressed this gap by conducting a three-year field study involving wheat grown in adjacent irrigated and dryland (rainfed) plots established in Lind, Washington State. We used deep amplicon sequencing of the V4 region of the 16S rRNA to characterize the responses of the wheat rhizosphere microbiome to overhead irrigation. We also characterized the population dynamics and activity of indigenous Phz+ rhizobacteria that produce the antibiotic phenazine-1-carboxylic acid (PCA) and contribute to the natural suppression of soilborne pathogens of wheat. Results of the study revealed that irrigation affected the Phz+ rhizobacteria adversely, which was evident from the significantly reduced plant colonization frequency, population size and levels of PCA in the field. The observed differences between irrigated and dryland plots were reproducible and amplified over the course of the study, thus identifying soil moisture as a critical abiotic factor that influences the dynamics, and activity of indigenous Phz+ communities. The three seasons of irrigation had a slight effect on the overall diversity within the rhizosphere microbiome but led to significant differences in the relative abundances of specific OTUs. In particular, irrigation differentially affected multiple groups of Bacteroidetes and Proteobacteria, including taxa with known plant growth-promoting activity. Analysis of environmental variables revealed that the separation between irrigated and dryland treatments was due to changes in the water potential (Ψm) and pH. In contrast, the temporal changes in the composition of the rhizosphere microbiome correlated with temperature and precipitation. In summary, our long-term study provides insights into how the availability of water in a semi-arid agroecosystem shapes the belowground wheat microbiome.

Rhizosphere Microbial Communities of Spartina alterniflora and Juncus roemerianus From Restored and Natural Tidal Marshes on Deer Island, Mississippi.

The U. S. Gulf of Mexico is experiencing a dramatic increase in tidal marsh restoration actions, which involves planting coastal areas with smooth cordgrass (Spartina alterniflora) and black needlerush (Juncus roemerianus) for erosion control and to provide habitat for fish and wildlife. It can take decades for sedimentary cycles in restored marshes to approach reference conditions, and the contribution of the sediment microbial communities to these processes is poorly elucidated. In this study, we addressed this gap by comparing rhizosphere microbiomes of S. alterniflora and J. roemerianus from two restored marshes and a natural reference marsh located at Deer Island, MS. Our results revealed that plants from the restored and reference areas supported similar microbial diversity indicating the rapid colonization of planted grasses with indigenous soil microbiota. Although close in composition, the microbial communities from the three studied sites differed significantly in the relative abundance of specific taxa. The observed differences are likely driven by the host plant identity and properties of sediment material used for the creation of restored marshes. Some of the differentially distributed groups of bacteria include taxa involved in the cycling of carbon, nitrogen, and sulfur, and may influence the succession of vegetation at the restored sites to climax condition. We also demonstrated that plants from the restored and reference sites vary in the frequency of culturable rhizobacteria that exhibit traits commonly associated with the promotion of plant growth and suppression of phytopathogenic fungi. Our findings will contribute to the establishment of benchmarks for the assessment of the outcome of coastal restoration projects in the Gulf of Mexico and better define factors that affect the long-term resiliency of tidal marshes and their vulnerability to climate change.

Using Aldehyde Synergism To Direct the Design of Degradable Pro-Antimicrobial Networks.

We describe the design and synthesis of degradable, dual-release, pro-antimicrobial poly(thioether acetal) networks derived from synergistic pairs of aromatic terpene aldehydes. Initially, we identified pairs of aromatic terpene aldehyde derivatives exhibiting a synergistic antimicrobial activity against Pseudomonas aeruginosa by determining fractional inhibitory concentrations. Synergistic aldehydes were converted into dialkene acetal monomers and copolymerized at various ratios with a multifunctional thiol via thiol-ene photopolymerization. The step-growth nature of the thiol-ene polymerization ensures every cross-link junction contains a degradable acetal linkage enabling a fully cross-linked polymer network to revert into its small molecule constituents upon hydrolysis, releasing the synergistic aldehydes as active antimicrobial compounds. A three-pronged approach was used to characterize the poly(thioether acetal) materials: (i) determination of the degradation/aldehyde release behavior, (ii) evaluation of the antimicrobial activity, and (iii) identification of the cellular pathways impacted by the aldehydes on a library of mutated bacteria. From this approach, a polymer network derived from a 40:60 p-bromobenzaldehyde/p-anisaldehyde monomer ratio exhibited potent antimicrobial action against Pseudomonas aeruginosa, a common opportunistic human pathogen. From a transposon mutagenesis assay, we showed that these aldehydes target porins and multidrug efflux pumps. The aldehydes released from the poly(thioether acetal) networks exhibited negligible toxicity to mammalian tissue culture cells, supporting the potential development of these materials as dual-release antimicrobial biomaterial platforms.

A bio-based pro-antimicrobial polymer network via degradable acetal linkages.

The synthesis of a fully degradable, bio-based, sustained release, pro-antimicrobial polymer network comprised of degradable acetals (PANDA) is reported. The active antimicrobial agent - p-anisaldehyde (pA) (an extract from star anise) - was converted into a UV curable acetal containing pro-antimicrobial monomer and subsequently photopolymerized into a homogenous thiol-ene network. Under neutral to acidic conditions (pH < 8), the PANDAs undergo surface erosion and exhibit sustained release of pA over 38 days. The release of pA from PANDAs was shown to be effective against both bacterial and fungal pathogens. From a combination of confocal microscopy and transmission electron microscopy, we observed that the released pA disrupts the cell membrane. Additionally, we demonstrated that PANDAs have minimal cytotoxicity towards both epithelial cells and macrophages. Although a model platform, these results point to promising pathways for the design of fully degradable sustained-release antimicrobial systems with potential applications in agriculture, pharmaceuticals, cosmetics, household/personal care, and food industries. STATEMENT OF SIGNIFICANCE: With the increasing number of patients prescribed immunosuppressants coupled with the rise in antibiotic resistance - life-threatening microbial infections are a looming global threat. With limited success within the antibiotic pipeline, nature-based essential oils (EOs) are being investigated for their multimodal effectiveness against microbes. Despite the promising potential of EOs, difficulties in their encapsulation, limited water solubility, and high volatility limit their use. Various studies have shown that covalent attachment of these EO derivatives to polymers can mitigate these limitations. The current study presents the synthesis of a fully-degradable, sustained release, cytocompatible, pro-antimicrobial acetal network derived from p-anisaldehyde. This polymer network design provides a pathway toward application-specific EO releasing materials with quantitative encapsulation efficiencies, sustained release, and broad-spectrum antimicrobial activity.

Draft genome sequences of strains Salinicola socius SMB35T, Salinicola sp. MH3R3-1 and Chromohalobacter sp. SMB17 from the Verkhnekamsk potash mining region of Russia.

Halomonads are moderately halophilic bacteria that are studied as models of prokaryotic osmoadaptation and sources of enzymes and chemicals for biotechnological applications. Despite the progress in understanding the diversity of these organisms, our ability to explain ecological, metabolic, and biochemical traits of halomonads at the genomic sequence level remains limited. This study addresses this gap by presenting draft genomes of Salinicola socius SMB35T, Salinicola sp. MH3R3-1 and Chromohalobacter sp. SMB17, which were isolated from potash mine tailings in the Verkhnekamsk salt deposit area of Russia. The analysis of these genomes confirmed the importance of ectoines and quaternary amines to the capacity of halomonads to tolerate osmotic stress and adapt to hypersaline environments. The study also revealed that Chromohalobacter and Salinicola share 75-90% of the predicted proteome, but also harbor a set of genus-specific genes, which in Salinicola amounted to approximately 0.5 Mbp. These genus-specific genome segments may contribute to the phenotypic diversity of the Halomonadaceae and the ability of these organisms to adapt to changing environmental conditions and colonize new ecological niches.

Relationships between Root Pathogen Resistance, Abundance and Expression of Pseudomonas Antimicrobial Genes, and Soil Properties in Representative Swiss Agricultural Soils.

Strains of Pseudomonas that produce antimicrobial metabolites and control soilborne plant diseases have often been isolated from soils defined as disease-suppressive, i.e., soils, in which specific plant pathogens are present, but plants show no or reduced disease symptoms. Moreover, it is assumed that pseudomonads producing antimicrobial compounds such as 2,4-diacetylphloroglucinol (DAPG) or phenazines (PHZ) contribute to the specific disease resistance of suppressive soils. However, pseudomonads producing antimicrobial metabolites are also present in soils that are conducive to disease. Currently, it is still unknown whether and to which extent the abundance of antimicrobials-producing pseudomonads is related to the general disease resistance of common agricultural soils. Moreover, virtually nothing is known about the conditions under which pseudomonads express antimicrobial genes in agricultural field soils. We present here results of the first side-by-side comparison of 10 representative Swiss agricultural soils with a cereal-oriented cropping history for (i) the resistance against two soilborne pathogens, (ii) the abundance of Pseudomonas bacteria harboring genes involved in the biosynthesis of the antimicrobials DAPG, PHZ, and pyrrolnitrin on roots of wheat, and (iii) the ability to support the expression of these genes on the roots. Our study revealed that the level of soil disease resistance strongly depends on the type of pathogen, e.g., soils that are highly resistant to Gaeumannomyces tritici often are highly susceptible to Pythium ultimum and vice versa. There was no significant correlation between the disease resistance of the soils, the abundance of Pseudomonas bacteria carrying DAPG, PHZ, and pyrrolnitrin biosynthetic genes, and the ability of the soils to support the expression of the antimicrobial genes. Correlation analyses indicated that certain soil factors such as silt, clay, and some macro- and micronutrients influence both the abundance and the expression of the antimicrobial genes. Taken together, the results of this study suggests that pseudomonads producing DAPG, PHZ, or pyrrolnitrin are present and abundant in Swiss agricultural soils and that the soils support the expression of the respective biosynthetic genes in these bacteria to various degrees. The precise role that these pseudomonads play in the general disease resistance of the investigated agricultural soils remains elusive.

Pro-Antimicrobial Networks via Degradable Acetals (PANDAs) Using Thiol-Ene Photopolymerization.

We describe the synthesis of pro-antimicrobial networks via degradable acetals (PANDAs) as a new paradigm for sequestration and triggered release of volatile, bioactive aldehydes. PANDAs derived from diallyl p-chlorobenzaldehyde acetal degrade and release p-chlorobenzaldehyde as an antibacterial and antifungal agent under mild conditions (pH 7.4/high humidity). We show that PANDAs enable facile access to materials with tunable release profiles, potent antimicrobial activity without triggering antimicrobial resistance, and minimal cytotoxicity.

Destruction of Opportunistic Pathogens via Polymer Nanoparticle-Mediated Release of Plant-Based Antimicrobial Payloads.

The synthesis of antimicrobial thymol/carvacrol-loaded polythioether nanoparticles (NPs) via a one-pot, solvent-free miniemulsion thiol-ene photopolymerization process is reported. The active antimicrobial agents, thymol and carvacrol, are employed as "solvents" for the thiol-ene monomer phase in the miniemulsion to enable facile high capacity loading (≈50% w/w), excellent encapsulation efficiencies (>95%), and elimination of all postpolymerization purification processes. The NPs serve as high capacity reservoirs for slow-release and delivery of thymol/carvacrol-combination payloads that exhibit inhibitory and bactericidal activity (>99.9% kill efficiency at 24 h) against gram-positive and gram-negative bacteria, including both saprophytic (Bacillus subtilis ATCC 6633 and Escherichia coli ATCC 25922) and pathogenic species (E. coli ATCC 43895, Staphylococcus aureus RN6390, and Burkholderia cenocepacia K56-2). This report is among the first to demonstrate antimicrobial efficacy of essential oil-loaded nanoparticles against B. cenocepacia - an innately resistant opportunistic pathogen commonly associated with debilitating respiratory infections in cystic fibrosis. Although a model platform, these results point to promising pathways to particle-based delivery of plant-derived extracts for a range of antimicrobial applications, including active packaging materials, topical antiseptics, and innovative therapeutics.

Biocontrol and plant growth-promoting activity of rhizobacteria from Chinese fields with contaminated soils.

The aim of this study was to inventory the types of plant growth-promoting rhizobacteria (PGPR) present in the rhizosphere of plants grown in soils contaminated with heavy metals, recalcitrant organics, petroleum sewage or salinity in China. We screened 1223 isolates for antifungal activity and about 24% inhibited Rhizoctonia solani or Sclerotinia sclerotiorum. Twenty-four strains inhibitory to R. solani, Gaeumannomyces graminis var. tritici and/or S. sclerotiorum and representing the dominant morphotypes were assayed for PGPR activity. Seven strains contained phlD, prnD, pltC or phzF genes and produced the antibiotics 2,4-diacetylphloroglucinol, pyrrolnitrin, pyoluteorin and phenazines respectively. Six strains contained acdS, which encodes 1-aminocyclopropane-1-carboxylic acid deaminase. Phylogenetic analysis of 16S rDNA and phlD, phzF and acdS genes demonstrated that some strains identified as Pseudomonas were similar to model PGPR strains Pseudomonas protegens Pf-5, Pseudomonas chlororaphis subsp. aureofaciens 30-84 and P. brassicacearum Q8r1-96. Pseudomonas protegens- and P. chlororaphis-like strains had the greatest biocontrol activity against Rhizoctonia root rot and take-all of wheat. Pseudomonas protegens and P. brassicacearum-like strains showed the greatest promotion of canola growth. Our results indicate that strains from contaminated soils are similar to well-described PGPR found in agricultural soils worldwide.