In vitro evaluation of antagonistic properties of Pseudomonas corrugata

Efficacy of endophytic bacteria as promising inducers for enhancing the immune responses in tomato plants and managing Rhizoctonia root-rot disease

Around the world, a variety of crops, including tomatoes, suffer serious economic losses due to the Rhizoctonia root-rot disease. Herein, Bacillus velezensis, Bacillus megaterium, and Herpaspirillum huttiense isolated from strawberry (Fragaria chiloensis var. ananassa) plants were pragmatic as plant growth promotors for battling the Rhizoctonia root rot disease and bringing about defense mechanisms as well as growth promotional strategies in tomato plants. These endophytic bacteria demonstrated potent antifungal activity against R. solani in vitro along in vivo. Data explained that the isolated endophytic bacteria could produce Indole acetic acid, Gibberellic acid GA, and siderophore as well as solubilize phosphate in the soil. The consortium of (Bacillus velezensis, Bacillus megaterium, and Herpaspirillum huttiense) increased the protection % against Rhizoctonia infection by (79.4%), followed by B. velezensis by (73.52%), H. huttiense by (70.5%), and B. megaterium by (67.64%), respectively. There was an increase in soluble proteins and carbohydrates in infected plants treated with a consortium of endophytic bacteria by 30.7% and 100.2% over untreated infected plants, respectively. Applying endophytic bacteria either alone or in combination lowered the level of malondialdehyde MDA and hydrogen peroxide H2O2 and improved the activities of antioxidant enzymes in both infected and uninfected plants. Also, bacterial endophytes have distinctive reactions regarding the number and concentrations of isozymes in both infected and uninfected plants. It could be recommended the commercial usage of a mixture of targeted bacterial endophyte strains as therapeutic nutrients against Rhizoctonia root-rot disease as well as plant growth inducer.

Suppression of tomato wilt by cell-free supernatants of Acinetobacter baumannii isolates from wild cacao from the Colombian Amazon

Tomato vascular wilt caused by Fusarium oxysporum f. sp. lycopersici (Fol) is one of the most limiting diseases of this crop. The use of fungicides and varieties resistant to the pathogen has not provided adequate control of the disease. In this study, siderophore-producing bacteria isolated from wild cocoa trees from the Colombian Amazon were characterized to identify prominent strategies for plant protection. The isolates were taxonomically classified into five different genera. Eight of the fourteen were identified as bacteria of the Acinetobacter baumannii complex. Isolates CBIO024, CBIO086, CBIO117, CBIO123, and CBIO159 belonging to this complex showed the highest efficiency in siderophore synthesis, producing these molecules in a range of 91-129 µmol/L deferoxamine mesylate equivalents. A reduction in disease severity of up to 45% was obtained when plants were pretreated with CBIO117 siderophore-rich cell-free supernatant (SodSid). Regarding the mechanism of action that caused antagonistic activity against Fol, it was found that plants infected only with Fol and plants pretreated with SodSid CBIO117 and infected with Fol showed higher levels of PR1 and ERF1 gene expression than control plants. In contrast, MYC2 gene expression was not induced by the SodSid CBIO117 application. However, it was upregulated in plants infected with Fol and plants pretreated with SodSid CBIO117 and infected with the pathogen. In addition to the disease suppression exerted by SodSid CBIO117, the results suggest that the mechanism underlying this effect is related to an induction of systemic defense through the salicylic acid, ethylene, and priming defense via the jasmonic acid pathway.

Antimicrobial function of yeast against pathogenic and spoilage microorganisms via either antagonism or encapsulation: A review

Contaminations of pathogenic and spoilage microbes on foods are threatening food safety and quality, highlighting the importance of developing antimicrobial agents. According to different working mechanisms, the antimicrobial activities of yeast-based agents were summarized from two aspects: antagonism and encapsulation. Antagonistic yeasts are usually applied as biocontrol agents for the preservation of fruits and vegetables via inactivating spoilage microbes, usually phytopathogens. This review systematically summarized various species of antagonistic yeasts, potential combinations to improve the antimicrobial efficiency, and the antagonistic mechanisms. The wide applications of the antagonistic yeasts are significantly limited by undesirable antimicrobial efficiency, poor environmental resistance, and a narrow antimicrobial spectrum. Another strategy for achieving effective antimicrobial activity is to encapsulate various chemical antimicrobial agents into a yeast-based carrier that has been previously inactivated. This is accomplished by immersing the dead yeast cells with porous structure in an antimicrobial suspension and applying high vacuum pressure to allow the agents to diffuse inside the yeast cells. Typical antimicrobial agents encapsulated in the yeast carriers have been reviewed, including chlorine-based biocides, antimicrobial essential oils, and photosensitizers. Benefiting from the existence of the inactive yeast carrier, the antimicrobial efficiencies and functional durability of the encapsulated antimicrobial agents, such as chlorine-based agents, essential oils, and photosensitizers, are significantly improved compared with the unencapsulated ones.

Potential for Biological Control of Pythium schmitthenneri Root Rot Disease of Olive Trees (Olea europaea L.) by Antagonistic Bacteria

Several diseases affect the productivity of olive trees, including root rot disease caused by Pythium genera. Chemical fungicides, which are often used to manage this disease, have harmful side effects on humans as well as environmental components. Biological management is a promising control approach that has shown its great potential as an efficient eco-friendly alternative to treating root rot diseases. In the present study, the antagonistic activity of ten bacterial isolates was tested both in vitro and in planta against Pythium schmitthenneri, the causal agent of olive root rot disease. These bacterial isolates belonging to the genera Alcaligenes, Pantoea, Bacillus, Sphingobacterium, and Stenotrophomonas were chosen for their potential antimicrobial effects against many pathogens. Results of the in vitro confrontation bioassay revealed a high reduction of mycelial growth exceeding 80%. The antifungal effect of the volatile organic compounds (VOCs) was observed for all the isolates, with mycelial inhibition rates ranging from 28.37 to 70.32%. Likewise, the bacterial cell-free filtrates showed important inhibition of the mycelial growth of the pathogen. Overall, their efficacy was substantially affected by the nature of the bacterial strains and their modes of action. A greenhouse test was then carried out to validate the in vitro results. Interestingly, two bacterial isolates, Alcaligenes faecalis ACBC1 and Bacillus amyloliquefaciens SF14, were the most successful in managing the disease. Our findings suggested that these two antagonistic bacterial isolates have promising potential as biocontrol agents of olive root rot disease.

Extracts from Environmental Strains of Pseudomonas spp. Effectively Control Fungal Plant Diseases

The use of synthetic chemical products in agriculture is causing severe damage to the environment and human health, but agrochemicals are still widely used to protect our crops. To counteract this trend, we have been looking for alternative strategies to control plant diseases without causing harm to the environment or damage to our health. However, these alternatives are still far from completely replacing chemical products. Microorganisms have been widely known as a biological tool to control plant diseases, but their use is still limited due to the high variability in their efficacy, together with issues in product registration. However, the metabolites produced by these microorganisms can represent a novel tool for the environment-friendly management of plant diseases, while reducing the issues mentioned above. In this study, we explore the soil microbial diversity in natural systems to look for microorganisms with the potential to be used in pre- and post-harvest protection against fungal plant pathogens. Using a simple workflow, we isolated 22 bacterial strains that were tested both in vitro and in vivo for their ability to counteract the growth of common plant pathogens. The three best isolates, identified as members of the bacterial genus Pseudomonas, were used to produce a series of alcoholic extracts, which were then tested for their action against plant pathogens in simulated real-world applications. Results show that extracts from these isolates have an exceptional biocontrol activity and can be successfully used to control plant pathogens in operational setups. Thus, this study shows that the environmental microbiome is an important source of microorganisms producing metabolites that might provide an alternative strategy to synthetic chemical products.

Volatile Organic Compounds Produced by Cacao Endophytic Bacteria and Their Inhibitory Activity on Moniliophthora roreri

Several studies have reported that bacteria produce anti-fungal volatiles. We identified the organic volatile compounds produced by six cacao endophytic bacteria (CEB) strains (Bacillus amyloliquefaciens CFFSUR-B35, Bacillus megaterium CFFSUR-B32, Bacillus muralis CFFSUR-B39, Bacillus pumilus CFFSUR-B34, Bacillus subtilis CFFSUR-B31, and Novosphingobium lindaniclasticum CFFSUR-B36). We evaluated their inhibitory effect on mycelium growth and spore germination of the phytopathogenic fungus Moniliophtora roreri. The volatiles produced by these six CEB, were collected and identified by SPME and GC-MS. Moreover, the inhibitory effect of five synthetic volatile organic compounds, individually and in mixtures (dimethyl disulfide, 2-5 dimethyl pyrazine, α-pinene, 2-heptanone and 2-ethyl hexanol) on M. roreri mycelium growth and spore germination was evaluated. All strains examined produced volatiles in different amounts; 13 to 10 compounds were identified, including sulfide, alcohol benzene derivate, pyrazine, ketone, nitrogen and terpene compounds. The B. subtilis CFFSUR-B31 strain produced the largest number of volatiles, while B. pumilus CFFSUR-B34 produced the fewest and the lowest amounts. The volatile organic compounds produced by B. pumilus CFFSUR-B34, B. muralis CFFSUR-B39 and N. lindaniclasticum CFFSUR-B36 inhibited M. roreri mycelium growth by more than 35%, sporulation by more than 81% and spore germination by more than 74%. However, when synthetic compounds were evaluated individually and in mixtures, 2-ethyl hexanol at 100,000 ppm (20 mg/filter paper disc) inhibited M. roreri mycelium growth by 100%, followed by organic volatile compound mixtures C (dimethyl disulfide, 2,5-dimethyl pyrazine, α-pinene, 2-ethyl-hexanol, 2-Heptanone) and D (only the top four) at 100,000 ppm (4 and 5 mg/filter paper disc) which inhibited spore germination by 97 and 89%, respectively.

Frenemies: Interactions between Rhizospheric Bacteria and Fungi from Metalliferous Soils

Is it possible to improve the efficiency of bioremediation technologies? The use of mixed cultures of bacteria and fungi inoculated at the rhizosphere level could promote the growth of the associated hyperaccumulating plant species and increase the absorption of metals in polluted soils, broadening new horizons on bioremediation purposes. This work investigates interactions between Ni-tolerant plant growth-promoting bacteria and fungi (BF) isolated from the rhizosphere of a hyperaccumulating plant. The aim is to select microbial consortia with synergistic activity to be used in integrated bioremediation protocols. Pseudomonas fluorescens (Pf), Streptomyces vinaceus (Sv) Penicillium ochrochloron (Po), and Trichoderma harzianum group (Th) were tested in mixes (Po-Sv, Po-Pf, Th-Pf, and Th-Sv). These strains were submitted to tests (agar overlay, agar plug, and distance growth co-growth tests), tailored for this aim, on Czapek yeast agar (CYA) and tryptic soy agar (TSA) media and incubated at 26 ± 1 °C for 10 days. BF growth, shape of colonies, area covered on plate, and inhibition capacity were evaluated. Most BF strains still exhibit their typical characters and the colonies separately persisted without inhibition (as Po-Sv) or with reciprocal confinement (as Th-Sv and Th-Pf). Even if apparently inhibited, the Po-Pf mix really merged, thus obtaining morphological traits representing a synergic co-growth, where both strains reached together the maturation phase and developed a sort of mixed biofilm. Indeed, bacterial colonies surround the mature fungal structures adhering to them without any growth inhibition. First data from in vivo experimentation with Po and Pf inocula in pot with metalliferous soils and hyperaccumulator plants showed their beneficial effect on plant growth. However, there is a lack of information regarding the effective co-growth between bacteria and fungi. Indeed, several studies, which directly apply the co-inoculum, do not consider suitable microorganisms consortia. Synergic rhizosphere BFs open new scenarios for plant growth promotion and soil bioremediation.

Application of Bio-Friendly Formulations of Chitinase-Producing Streptomyces cellulosae Actino 48 for Controlling Peanut Soil-Borne Diseases Caused by Sclerotium rolfsii

Of ten actinobacterial isolates, Streptomyces cellulosae Actino 48 exhibited the strongest suppression of Sclerotium rolfsii mycelium growth and the highest chitinase enzyme production (49.2 U L^-1 min^-1). The interaction between Actino 48 and S. rolfsii was studied by scanning electron microscope (SEM), which revealed many abnormalities, malformations, and injuries of the hypha, with large loss of S. rolfsii mycelia density and mass. Three talc-based formulations with culture broth, cell-free supernatant, and cell pellet suspension of chitinase-producing Actino 48 were characterized using SEM, Fourier transform infrared spectroscopy (FTIR), and a particle size analyzer. All formulations were evaluated as biocontrol agents for reducing damping-off, root rot, and pods rot diseases of peanut caused by S. rolfsii under greenhouse and open-field conditions. The talc-based culture broth formulation was the most effective soil treatment, which decreased the percentage of peanut diseases under greenhouse and open-field conditions during two successive seasons. The culture broth formulation showed the highest increase in the dry weight of peanut shoots, root systems, and yielded pods. The transcriptional levels of three defense-related genes (PR-1, PR-3, and POD) were elevated in the culture broth formulation treatment compared with other formulations. Subsequently, the bio-friendly talc-based culture broth formulation of chitinase-producing Actino 48 could potentially be used as a biocontrol agent for controlling peanut soil-borne diseases caused by S. rolfsii.

Assessing the Involvement of Selected Phenotypes of Pseudomonas simiae PICF7 in Olive Root Colonization and Biological Control of Verticillium dahliae

Pseudomonas simiae PICF7 is an indigenous inhabitant of the olive (Olea europaea L.) rhizosphere/root endosphere and an effective biocontrol agent against Verticillium wilt of olive (VWO), caused by the soil-borne fungus Verticillium dahliae. This study aimed to evaluate the potential involvement of selected phenotypes of strain PICF7 in root colonization ability and VWO biocontrol. Therefore, a random transposon-insertion mutant bank of P. simiae PICF7 was screened for the loss of phenotypes likely involved in rhizosphere/soil persistence (copper resistance), root colonization (biofilm formation) and plant growth promotion (phytase activity). Transposon insertions in genes putatively coding for the transcriptional regulator CusR or the chemotaxis protein CheV were found to affect copper resistance, whereas an insertion in fleQ gene putatively encoding a flagellar regulatory protein hampered the ability to form a biofilm. However, these mutants displayed the same antagonistic effect against V. dahliae as the parental strain. Remarkably, two mutants impaired in biofilm formation were never found inside olive roots, whereas their ability to colonize the root exterior and to control VWO remained unaffected. Endophytic colonization of olive roots was unaltered in mutants impaired in copper resistance and phytase production. Results demonstrated that the phenotypes studied were irrelevant for VWO biocontrol.

Co-Cultivation of Fusarium, Alternaria, and Pseudomonas on Wheat-Ears Affects Microbial Growth and Mycotoxin Production

Mycotoxigenic fungal pathogens Fusarium and Alternaria are a leading cause of loss in cereal production. On wheat-ears, they are confronted by bacterial antagonists such as pseudomonads. Studies on these groups’ interactions often neglect the infection process’s temporal aspects and the associated priority effects. In the present study, the focus was on how the first colonizer affects the subsequent ones. In a climate chamber experiment, wheat-ears were successively inoculated with two different strains (Alternaria tenuissima At625, Fusarium graminearum Fg23, or Pseudomonas simiae Ps9). Over three weeks, microbial abundances and mycotoxin concentrations were analyzed and visualized via Self Organizing Maps with Sammon Mapping (SOM-SM). All three strains revealed different characteristics and strategies to deal with co-inoculation: Fg23, as the first colonizer, suppressed the establishment of At625 and Ps9. Nevertheless, primary inoculation of At625 reduced all of the Fusarium toxins and stopped Ps9 from establishing. Ps9 showed priority effects in delaying and blocking the production of the fungal mycotoxins. The SOM-SM analysis visualized the competitive strengths: Fg23 ranked first, At625 second, Ps9 third. Our findings of species-specific priority effects in a natural environment and the role of the mycotoxins involved are relevant for developing biocontrol strategies.

Identification and characterization of Pseudomonas aeruginosa derived bacteriocin for industrial applications

Drug resistance has become a major threat due to the frequent use of commercial antibiotics and there is an urgent need to combat this problem. Having this in mind, the present research was aimed at developing a novel P. aeruginosa puBac bacteriocin molecule. The bacteriocin was purified by ammonium sulfate precipitation followed by Sepharose FF and Sephadex G15 column purification and the purified bacteriocin has been reported to have the molar mass of 43 kDa. The findings of the optimization showed that 3500 AU/mL of bacteriocin was obtained at 37 °C, 3410 AU/mL of bacteriocin at 6.5 pH and 3780 AU/mL of bacteriocin at 48 h of incubation time. In addition, 3863 AU/mL of bacteriocin activity was obtained with Tween-80 followed by 3789 AU/mL with a concentration of 2% NaCl and 4200 AU/mL for Fe²⁺. PuBac bacteriocin has been shown to have a significant effect on test pathogens. For example, E. coli was found to have 3.6 μM of MIC, followed by Staphylococcus sp. with 6.15 μM of MIC and Bacillus sp. with a 7.5 μM of MIC. The remarkable properties of bacteriocin suggest that it could be used in various pharmaceutical and food industries.

Volatile-Mediated Inhibitory Activity of Rhizobacteria as a Result of Multiple Factors Interaction: The Case of Lysobacter capsici AZ78

Plant beneficial rhizobacteria may antagonize soilborne plant pathogens by producing a vast array of volatile organic compounds (VOCs). The production of these compounds depends on the medium composition used for bacterial cell growth. Accordingly, Lysobacter capsici AZ78 (AZ78) grown on a protein-rich medium was previously found to emit volatile pyrazines with toxic activity against soilborne phypathogenic fungi and oomycetes. However, the discrepancy between the quantity of pyrazines in the gaseous phase and the minimum quantity needed to achieve inhibition of plant pathogens observed, lead us to further investigate the volatile-mediated inhibitory activity of AZ78. Here, we show that, besides VOCs, AZ78 cells produce ammonia in increased amounts when a protein-rich medium is used for bacterial growth. The production of this volatile compound caused the alkalinization of the physically separated culture medium where Rhizoctonia solani was inoculated subsequently. Results achieved in this work clearly demonstrate that VOC, ammonia and the growth medium alkalinization contribute to the overall inhibitory activity of AZ78 against R. solani. Thus, our findings suggest that the volatile-mediated inhibitory activity of rhizobacteria in protein-rich substrates can be regarded as a result of multiple factors interaction, rather than exclusively VOCs production.

Unlocking the potential of plant growth-promoting rhizobacteria on soil health and the sustainability of agricultural systems

The concept of soil health refers to specific soil properties and the ability to support and sustain crop growth and productivity, while maintaining long-term environmental quality. The key components of healthy soil are high populations of organisms that promote plant growth, such as the plant growth promoting rhizobacteria (PGPR). PGPR plays multiple beneficial and ecological roles in the rhizosphere soil. Among the roles of PGPR in agroecosystems are the nutrient cycling and uptake, inhibition of potential phytopathogens growth, stimulation of plant innate immunity, and direct enhancement of plant growth by producing phytohormones or other metabolites. Other important roles of PGPR are their environmental cleanup capacities (soil bioremediation). In this work, we review recent literature concerning the diverse mechanisms of PGPR in maintaining healthy conditions of agricultural soils, thus reducing (or eliminating) the toxic agrochemicals dependence. In conclusion, this review provides comprehensive knowledge on the current PGPR basic mechanisms and applications as biocontrol agents, plant growth stimulators and soil rhizoremediators, with the final goal of having more agroecological practices for sustainable agriculture.

Multifunctional potential of endophytic bacteria from Anacardium othonianum Rizzini in promoting in vitro and ex vitro plant growth

Anacardium othonianum Rizzini, a cashew tree native to the Brazilian Cerrado, is economically important due to its applications in the food, chemical and pharmaceutical industries. However, A. othonianum yields a crop with low productivity due to a number of factors, such as nutritionally poor soils, drought and losses due to pests and diseases. Brazil is one of the nine largest cashew nut producers worldwide, and sustainable technologies are needed to increase the productivity of this crop. In this context, the use of endophytic microorganisms could promote plant growth and provide protection against phytopathogens. In this study, the isolation of the root endophytic community of A. othonianum led to the characterization of 22 distinct bacterial strains with multifunctional traits for plant growth promotion. The results of in vitro assays to assess auxin synthesis, phosphate solubilization, phosphatase and siderophore production and biocontrol against Fusarium oxysporum led to the selection of Acinetobacter lwoffii Bac109 and Pantoea agglomerans Bac131 as the most promising strains. The reinoculation of the Bac109 and Bac131 strains onto A. othonianum seeds showed that the treatment containing a mixture of these strains was the most effective in promoting increases in the biometric parameters of early plant growth. Thus, this study highlights the biotechnological potential of a consortium of A. lwoffii Bac109 and P. agglomerans Bac131 for future applications in sustainable cashew cultivation.

Pseudomonas aeruginosa FARP72 Offers Protection Against Aeromonas hydrophila Infection in Labeo rohita

Use of probiotics as the biocontrol agent for disease prevention in aquaculture is gaining importance as an alternative to the indiscriminate use of antibiotics and other chemotherapeutics. In view of this trend, the probiotic properties of a potent antagonistic bacterium, Pseudomonas aeruginosa FARP₇₂, was characterized in terms of safety, antagonistic activities, in vitro immunomodulation, and in vivo disease resistance. Immunomodulatory activity was ascertained by measuring the production of intracellular superoxide anion, nitric oxide, total leukocyte peroxidase content, and the leukocyte proliferation in head kidney leukocytes. The bacterium isolated from the skin mucus of freshwater catfish Clarias batrachus was harmless to Labeo rohita. It showed inhibitory activity against Aeromonas caviae, A. hydrophila, Edwardsiella tarda, Pseudomonas putida, and Streptococcus agalactiae as revealed by cross and parallel streaking methods. Significantly higher superoxide anion and nitric oxide production, peroxidase content, and proliferative responses of leucocytes delineated the strains' ability to interact with immune cells to activate the immune system in vitro. Significant growth inhibition of A. hydrophila from 1.55 × 10⁵ CFU/mL was observed when co-cultured with P. aeruginosa FARP₇₂ in phosphate-buffered saline (PBS) at levels ranging from 2.61 × 10⁷ to 2.61 × 10⁹ CFU/mL in 10 days. P. aeruginosa FARP₇₂ increased the survival rate of rohu fingerlings against pathogenic A. hydrophila challenge in biocontrol study in vivo as determined by cohabitation challenge. These results suggest that P. aeruginosa FARP₇₂ is a potential probiotic strain and can be used in aquaculture to improve the health status and disease resistance of fish.

Biocontrol activity and putative mechanism of Bacillus amyloliquefaciens (SF14 and SP10), Alcaligenes faecalis ACBC1, and Pantoea agglomerans ACBP1 against brown rot disease of fruit

This study aimed at evaluating the antagonistic activity of 16 bacterial strains for the control of brown rot disease caused by Monilinia fructigena, and M. laxa under in vitro and a semi-commercial large-scale trial. These bacterial antagonists' belonging to the genera Alcaligenes, Bacillus, Brevibacterium, Pantoea, Pseudomonas, and Serratia were previously proven effective for control of fire blight of apple. The in vitro dual culture bioassay showed the highest inhibition rates of mycelial growth ranging from 55 to 95% and from 43 to 94% for M. fructigena and M. laxa, respectively. The in vivo bioassay showed moderate and strong inhibition for M. fructigena and M. laxa, respectively. The inhibition rates were dependent on incubation time as well as pathogen virulence. The free-cell bacterial filtrate revealed substantial mycelial growth inhibition ranging from 66 to 86%. The inhibition of conidial germination was from 32 to 78%, suggesting the involvement of metabolites in their biocontrol activity. The antifungal effect of the volatile compounds (VCOs) was observed for all bacteria with mycelial inhibition varying from 12 to 70%. Overall, their efficacy was substantially affected by the nature of the bacterial strains and the modes of action. Taken together, these results underscore that ACBC1 and SF14 for M. fructigena and SP10 and ACBP1 for M. laxa were the most effective bacterial strains. These strains were confirmed effective in a semi-commercial large-scale trial. Interestingly, their efficacies were found to be comparable to those of both commercial BCAs (B. subtilis Y1336 and P. agglomerans P10c), but slightly lower than thiophanate-methyl fungicide. The ability of most bacterial strains to produce lytic enzymes (Amylase, Protease or Cellulase) and lipopeptides (bacillomycin, fengycin, iturin and surfactin) was demonstrated by biochemical and molecular analyzes. Therefore, our findings suggest that the bacterial antagonists ACBC1, SF14, SP10 and ACBP1, have the potential to prevent brown rot disease.

Bio-fabrication of pigment-capped silver nanoparticles encountering antibiotic-resistant strains and their cytotoxic effect towards human epidermoid larynx carcinoma (HEp-2) cells

Bacterial biomolecule-mediated nanoparticle (NP) synthesis constitutes a reliable, eco-friendly approach that ameliorates green-chemistry principles. In this study, stable silver nanoparticles were synthesized by exposing aqueous silver ions to an extracellular diffusible pigment produced by Pseudomonas aeruginosa (PA6) under optimized laboratory conditions. Spectroscopic and microscopic analyses showed the typical characteristics of silver with an average size of ∼28.30 nm and spherical shape. The particles were polydispersed and showed no definite agglomeration with a zeta potential of −32.3 mV, conferring stability. Antimicrobial studies were carried out using 5, 15, 25 and 50 μg mL⁻¹ concentrations of pcAgNPs, which showed significant antibacterial activity toward clinically important pathogens at all concentrations compared to with the control sample. The bactericidal effect induced by pcAgNPs associated with cell damage was well demonstrated using electron microscopic studies. ROS production was measured using the DCFH-DA method and the oxidative stress was assessed by measuring the reduced glutathione (GSH) levels. Cytotoxicity studies on HEp-2 (Human Epidermoid Larynx Carcinoma) cells exposed to pcAgNPs showed dose-dependent cytotoxic effect with IC₅₀ of 14.8 μg mL⁻¹ compared to with IC₅₀ of 7.38 μg mL⁻¹ for the Vero cell control. Mechanistically, the pcAgNPs activated p53 that induced catalase, leading to apoptosis and DNA fragmentation via a p53 transcriptional pathway and electron transport arrest, which resulted in cell death. This synergistic efficacy of pigment-AgNPs demonstrated excellent antimicrobial and anti-proliferative activities, providing a potential lead for developing a broad-spectrum antibacterial agent and improving the therapeutic modalities targeting carcinoma cells at the gene level.

Bacterial biomolecule-mediated nanoparticle (NP) synthesis constitutes a reliable, eco-friendly approach that ameliorates green-chemistry principles.

Bioprospecting cold-adapted plant growth promoting microorganisms from mountain environments

Mountain soils are challenging environments for all kinds of living things, including plants and microorganisms. Many cold-adapted microorganisms colonizing these extreme soils play important roles as promoters of plant growth and development; for that reason, they are called collectively plant growth-promoting microorganisms (PGPM). Even though there is seldom doubt concerning the usefulness of PGPM to develop eco-friendly bioinoculants, including biofertilizers and biocontrollers, a series of aspects need to be addressed in order to make this technology field-applicable. Among these aspects, the ecological and rhizosphere competences of PGPM are of paramount importance, particularly when considering the development of bioinoculants, well suited for the intensification of mountainous agricultural production. Studies on native, cold-adapted PGPM conducted in the Indian Himalayan region (IHR) and the Tropical Andes (TA) lead nowadays the research in this field. Noticeably, some common themes are emerging. For instance, soils in these mountain environments are colonized by many cold-adapted PGPM able to mobilize soil nutrients and to inhibit growth of plant pathogens. Studies aimed at deeply characterizing the abilities of such PGPM is likely to substantially contribute towards a better crop productivity in mountainous environments. The present review focuses on the importance of this microbial resource to improve crop productivity in IHR and TA. We also present a number of successful examples, which emphasize the effectiveness of some bioinoculants-developed from naturally occurring PGPM-when applied in the field.

Antagonistic Potential of Fluorescent Pseudomonads Colonizing Wheat Heads Against Mycotoxin Producing Alternaria and Fusaria

Natural control of phytopathogenic microorganisms is assumed as a priority function of the commensal plant microbiota. In this study, the suitability of fluorescent pseudomonads in the phyllosphere of crop plants as natural control agents was evaluated. Under field conditions, ears of winter wheat were found to be colonized with high consistency and at a high density by pseudomonads at the late milk dough stage. Isolates of these bacteria were evaluated for their potential to protect the plants from phytopathogenic Alternaria and Fusarium fungi. More Pseudomonas isolates were antagonistically active against alternaria than against fusaria in the dual culture test. The alternaria responded species-specifically and more sensitively to bacterial antagonism than the strain-specific reacting fusaria. A total of 110 randomly selected Pseudomonas isolates were screened for genes involved in the biosynthesis of the antibiotics 2,4-diacetylphloroglucinol, phenazine-1-carboxylic acid, pyoluteorin, and pyrrolnitrin. The key gene for production of the phloroglucinol was found in none of these isolates. At least one of the genes, encoding the biosynthesis of the other antibiotics was detected in 81% of the isolates tested. However, the antagonistic effect found in the dual culture assay was not necessarily associated with the presence of these antibiotic genes. Wheat grains as natural substrate were inoculated with selected antagonistic Pseudomonas isolates and Alternaria and Fusarium strains, respectively. The fungal growth was only slightly delayed, but the mycotoxin production was significantly reduced in most of these approaches. In conclusion, the distribution of phytopathogenic fungi of the genera Alternaria and Fusarium in the field is unlikely to be inhibited by naturally occurring pseudomonads, also because the bacterial antagonists were not evenly distributed in the field. However, pseudomonads can reduce the production of Alternaria and Fusarium mycotoxins in wheat grains and thus have the potential to improve the crop quality.

The LuxR Regulators PcoR and RfiA Co-regulate Antimicrobial Peptide and Alginate Production in Pseudomonas corrugata

Cyclic lipopeptides (CLPs) are considered as some of the most important secondary metabolites in different plant-associated bacteria, thanks to their antimicrobial, cytotoxic, and surfactant properties. In this study, our aim was to investigate the role of the Quorum Sensing (QS) system, PcoI/PcoR, and the LuxR-type transcriptional regulator RfiA in CLP production in the phytopatogenic bacterium, Pseudomonas corrugata based on our previous work where we reported that the pcoR and rfiA mutants were devoid of the CLPs cormycin and corpeptin production. Due to the close genetic link between the QS system and the RfiA (rfiA is co-transcribed with pcoI), it was difficult to ascertain the specific regulatory role in the expression of target genes. A transcriptional approach was undertaken to identify the specific role of the PcoR and RfiA transcriptional regulators for the expression of genes involved in CLP production. The RNA-seq-based transcriptional analysis of the wild-type (WT) strain CFBP 5454 in comparison with GL2 (pcoR mutant) and GLRFIA (rfiA mutant) was performed in cultural conditions favoring CLP production. Differential gene expression revealed that 152 and 130 genes have significantly different levels of expression in the pcoR and rfiA mutants, respectively. Of these, the genes linked to the biosynthesis of CLPs and alginate were positively controlled by both PcoR and RfiA. Blast homology analysis showed that 19 genes in a large CLP biosynthetic cluster involved in the production of three antimicrobial peptides, which span approximately 3.5% of the genome, are strongly over-expressed in the WT strain. Thus, PcoR and RfiA function mainly as activators in the production of bioactive CLPs, in agreement with phenotype analysis of mutants. RNA-seq also revealed that almost all the genes in the structural/biosynthetic cluster of alginate exopolysaccharide (EPS) are under the control of the PcoR-RfiA regulon, as supported by the 10-fold reduction in total EPS yield isolated in both mutants in comparison to the parent strain. A total of 68 and 38 gene expressions was independently regulated by PcoR or RfiA proteins, respectively, but at low level. qPCR experiments suggest that growth medium and plant environment influence the expression of CLP and alginate genes.

Indigenous Pseudomonas spp. Strains from the Olive (Olea europaea L.) Rhizosphere as Effective Biocontrol Agents against Verticillium dahliae: From the Host Roots to the Bacterial Genomes

The use of biological control agents (BCA), alone or in combination with other management measures, has gained attention over the past decades, driven by the need to seek for sustainable and eco-friendly alternatives to confront plant pathogens. The rhizosphere of olive (Olea europaea L.) plants is a source of bacteria with potential as biocontrol tools against Verticillium wilt of olive (VWO) caused by Verticillium dahliae Kleb. A collection of bacterial isolates from healthy nursery-produced olive (cultivar Picual, susceptible to VWO) plants was generated based on morphological, biochemical and metabolic characteristics, chemical sensitivities, and on their in vitro antagonistic activity against several olive pathogens. Three strains (PIC25, PIC105, and PICF141) showing high in vitro inhibition ability of pathogens' growth, particularly against V. dahliae, were eventually selected. Their effectiveness against VWO caused by the defoliating pathotype of V. dahliae was also demonstrated, strain PICF141 being the rhizobacteria showing the best performance as BCA. Genotypic and phenotypic traits traditionally associated with plant growth promotion and/or biocontrol abilities were evaluated as well (e.g., phytase, xylanase, catalase, cellulase, chitinase, glucanase activities, and siderophore and HCN production). Multi-locus sequence analyses of conserved genes enabled the identification of these strains as Pseudomonas spp. Strain PICF141 was affiliated to the “Pseudomonas mandelii subgroup,” within the “Pseudomonas fluorescens group,” Pseudomonas lini being the closest species. Strains PIC25 and PIC105 were affiliated to the “Pseudomonas aeruginosa group,” Pseudomonas indica being the closest relative. Moreover, we identified P. indica (PIC105) for the first time as a BCA. Genome sequencing and in silico analyses allowed the identification of traits commonly associated with plant-bacteria interactions. Finally, the root colonization ability of these olive rhizobacteria was assessed, providing valuable information for the future development of formulations based on these strains. A set of actions, from rhizosphere isolation to genome analysis, is proposed and discussed for selecting indigenous rhizobacteria as effective BCAs.

Antifungical Activity of Autochthonous Bacillus subtilis Isolated from Prosopis juliflora against Phytopathogenic Fungi

The ability of Bacillus subtilis, strain ALICA to produce three mycolytic enzymes (chitinase, β-1,3-glucanase, and protease), was carried out by the chemical standard methods. Bacillus subtilis ALICA was screened based on their antifungal activity in dual plate assay and cell-free culture filtrate (25%) against five different phytopathogenic fungi Alternaria alternata, Macrophomina sp., Colletotrichum gloeosporioides, Botrytis cinerea, and Sclerotium rolfesii. The B. subtilis ALICA detected positive for chitinase, β-1,3-glucanase and protease enzymes. Fungal growth inhibition by both strain ALICA and its cell-free culture filtrate ranged from 51.36% to 86.3% and 38.43% to 68.6%, respectively. Moreover, hyphal morphological changes like damage, broken, swelling, distortions abnormal morphology were observed. Genes expression of protease, β-1,3-glucanase, and lipopeptides (subtilosin and subtilisin) were confirmed their presence in the supernatant of strain ALICA. Our findings indicated that strain ALICA provided a broad spectrum of antifungal activities against various phytopathogenic fungi and may be a potential effective alternative to chemical fungicides.

Synthesis, nature and utility of universal iron chelator - Siderophore: A review

Siderophores, the secondary metabolite of various microorganisms are ferric ion specific chelators secreted under iron stressed condition. These non-ribosomal peptides have been classified as catecholate, hydroxamate, carboxylate and mixed types. Recent studies focus on discovery of possible mammalian siderophores. The biosynthesis pathway including non-ribosomal dependent as well as non-ribosomal independent pathways are of great interest now a days. Many significant roles of siderophores such as virulence in pathogens, oxidative stress tolerance, classification of organisms etc. are being discovered. Studies on siderophore utilization in bioremediation and other heavy metal chelation have increased in past decade. The iron chelation ability of siderophores is being recently studied with regards to malignant cancerous cells. Not only this, it has been found that they possess antimicrobial properties which can be utilized against number of microbes. This review covers all recent aspects of siderophore and its applications.

Biocontrol of Bacterial Leaf Blight of Rice and Profiling of Secondary Metabolites Produced by Rhizospheric Pseudomonas aeruginosa BRp3

Xanthomonas oryzae pv. oryzae (Xoo) is widely prevalent and causes Bacterial Leaf Blight (BLB) in Basmati rice grown in different areas of Pakistan. There is a need to use environmentally safe approaches to overcome the loss of grain yield in rice due to this disease. The present study aimed to develop inocula, based on native antagonistic bacteria for biocontrol of BLB and to increase the yield of Super Basmati rice variety. Out of 512 bacteria isolated from the rice rhizosphere and screened for plant growth promoting determinants, the isolate BRp3 was found to be the best as it solubilized 97 μg/ mL phosphorus, produced 30 μg/mL phytohormone indole acetic acid and 15 mg/ L siderophores in vitro. The isolate BRp3 was found to be a Pseudomonas aeruginosa based on 16S rRNA gene sequencing (accession no. HQ840693). This bacterium showed antagonism in vitro against different phytopathogens including Xoo and Fusarium spp. Strain BRp3 showed consistent pathogen suppression of different strains of BLB pathogen in rice. Mass spectrometric analysis detected the production of siderophores (1-hydroxy-phenazine, pyocyanin, and pyochellin), rhamnolipids and a series of already characterized 4-hydroxy-2-alkylquinolines (HAQs) as well as novel 2,3,4-trihydroxy-2-alkylquinolines and 1,2,3,4-tetrahydroxy-2-alkylquinolines in crude extract of BRp3. These secondary metabolites might be responsible for the profound antibacterial activity of BRp3 against Xoo pathogen. Another contributing factor toward the suppression of the pathogen was the induction of defense related enzymes in the rice plant by the inoculated strain BRp3. When used as an inoculant in a field trial, this strain enhanced the grain and straw yields by 51 and 55%, respectively, over non-inoculated control. Confocal Laser Scanning Microscopy (CLSM) used in combination with immunofluorescence marker confirmed P. aeruginosa BRp3 in the rice rhizosphere under sterilized as well as field conditions. The results provide evidence that novel secondary metabolites produced by BRp3 may contribute to its activity as a biological control agent against Xoo and its potential to promote the growth and yield of Super Basmati rice.

Mining the Volatilomes of Plant-Associated Microbiota for New Biocontrol Solutions

Microbial lifeforms associated with land plants represent a rich source for crop growth- and health-promoting microorganisms and biocontrol agents. Volatile organic compounds (VOCs) produced by the plant microbiota have been demonstrated to elicit plant defenses and inhibit the growth and development of numerous plant pathogens. Therefore, these molecules are prospective alternatives to synthetic pesticides and the determination of their bioactivities against plant threats could contribute to the development of control strategies for sustainable agriculture. In our previous study we investigated the inhibitory impact of volatiles emitted by Pseudomonas species isolated from a potato field against the late blight-causing agent Phytophthora infestans. Besides the well-documented emission of hydrogen cyanide, other Pseudomonas VOCs impeded P. infestans mycelial growth and sporangia germination. Current advances in the field support the emerging concept that the microbial volatilome contains unexploited, eco-friendly chemical resources that could help select for efficient biocontrol strategies and lead to a greener chemical disease management in the field.

Synergistic effect of Chryseobacterium gleum sp. SUK with ACC deaminase activity in alleviation of salt stress and plant growth promotion in Triticum aestivum L

Soil salinity is major abiotic stresses affecting morphological, biochemical and physiological processes of plant growth. Chryseobacterium gleum sp. SUK isolated from salt-stressed soil exhibited ACC (1-aminocyclopropane-1-carboxylate) deaminase activity with IAA (indole acetic acid), siderophore, ammonia, hydrogen cyanide production, 2% salt tolerance and fungal cell wall degrading enzyme production (cellulase, protease). The isolate also showed a poultry feather degrading activity which is the main waste material of poultry industry and opulent source of proteins, amino acids, nitrogen, phosphorous, calcium, potassium, manganese, zinc and copper. Application of feather-degraded lysate with the degrading isolate, C. gleum sp. SUK denotes triple role of bioformulation to surmount salinity stress, management of poultry waste disposal and utilization of feathers degraded products as a biostimulant for better growth of plants as well as strain SUK having multifarious plant growth promoting traits. Wheat crops exposed to salt stressor were inoculated with studied bioformulation. Results of plant analysis showed improvement in root and shoot length, fresh and dry weight, chlorophyll, proteins, amino acids, phenolics, flavonoids content and decreased level of proline. In addition, Na⁺ uptake was decreased and K⁺ uptake was increased. Therefore, application of novel bioformulation could increase the yield of crops by ameliorating growth of plants and alleviating the salinity stress.

Assessment of plant growth promoting bacterial populations in the rhizosphere of metallophytes from the Kettara mine, Marrakech

Soil heavy metal contamination resulting from mining activities constitutes a major environmental problem worldwide. The spread of heavy metals is often facilitated by scarce vegetation cover, so there is an urgent need to improve plant survival and establishment in these metalliferous areas. This study is aimed at the isolation and analysis of the phylogenetic relationship of culturable bacteria from the rhizosphere of metallophyte plants growing in the Kettara mine, in Marrakech, in order to select plant growth-promoting rhizobacteria (PGPR), which could be used in assisted-phytoremediation. Bacterial isolates were grouped by random amplified polymorphic DNA analysis and identified by 16S rRNA gene sequencing. Strains were further characterized for the production of plant growth-promoting (PGP) substances, such as NH₃, siderophores, indol-3-acetic acid (IAA), hydrogen cyanide, and extracellular enzymes, for ACC-deaminase activity, their capacity to solubilize phosphate, and for their tolerance to heavy metals and acidic pH. Rhizosphere soils were highly contaminated with Cu and Zn and presented low fertility. Phylogenetic analysis showed that the rhizobacteria were affiliated to three major groups: γ-Proteobacteria (48 %), β-Proteobacteria (17 %), and Bacilli (17 %). The most represented genera were Pseudomonas (38 %), Bacillus (10 %), Streptomyces (10 %), and Tetrathiobacter (10 %). Overall, rhizobacterial strains showed an ability to produce multiple, important PGP traits, which may be helpful when applied as plant growth promoter agents in contaminated soils. PGPR were also able to withstand high levels of metals (up to 2615.2 mg Zn l^-1, 953.29 mg Cu l^-1, and 1124.6 mg Cd l^-1) and the order of metal toxicity was Cd > Cu > Zn. The rhizobacterial strains isolated in the present study have the potential to be used as efficient bioinoculants in phytoremediation strategies for the recovery of Kettara mine soils.

Harnessing Host-Vector Microbiome for Sustainable Plant Disease Management of Phloem-Limited Bacteria

Plant health and productivity is strongly influenced by their intimate interaction with deleterious and beneficial organisms, including microbes, and insects. Of the various plant diseases, insect-vectored diseases are of particular interest, including those caused by obligate parasites affecting plant phloem such as Candidatus (Ca.) Phytoplasma species and several species of Ca. Liberibacter. Recent studies on plant-microbe and plant-insect interactions of these pathogens have demonstrated that plant-microbe-insect interactions have far reaching consequences for the functioning and evolution of the organisms involved. These interactions take place within complex pathosystems and are shaped by a myriad of biotic and abiotic factors. However, our current understanding of these processes and their implications for the establishment and spread of insect-borne diseases remains limited. This article highlights the molecular, ecological, and evolutionary aspects of interactions among insects, plants, and their associated microbial communities with a focus on insect vectored and phloem-limited pathogens belonging to Ca. Phytoplasma and Ca. Liberibacter species. We propose that innovative and interdisciplinary research aimed at linking scales from the cellular to the community level will be vital for increasing our understanding of the mechanisms underpinning plant-insect-microbe interactions. Examination of such interactions could lead us to applied solutions for sustainable disease and pest management.

Wide pH range tolerance in extremophiles: towards understanding an important phenomenon for future biotechnology

Microorganisms that inhabit the extreme pH environments are classified as acidophiles and alkaliphiles. A number of studies emerged from extreme high (hot springs, hydrothermal vents) as well as low temperature (arctic and antarctic regions, sea water, ice shelf, marine sediments, cold deserts, glaciers, temperate forests, and plantations) environments have highlighted the occurrence of microorganisms (thermophiles/psychrophiles) with the ability to tolerate wide pH range, from acidic to alkaline (1.5-14.0 in some cases), under laboratory conditions. However, the sampling source (soil/sediment) of these microorganisms showed the pH to be neutral or slightly acidic/alkaline. The aim of the present review is to discuss the phenomenon of wide pH range tolerance possessed by these microorganisms as a hidden character in perspective of their habitats, possible mechanisms, phylogeny, ecological and biotechnological relevance, and future perspectives. It is believed that the genome is a probable reservoir of the hidden variations. The extremophiles have the ability to adapt against the environmental change that is probably through the expression/regulation of the specific genes that were already present in the genome. The phenomenon is likely to have broad implications in biotechnology, including both environmental (such as bioremediation, biodegradation, and biocontrol), and industrial applications (as a source of novel extremozymes and many other useful bioactive compounds with wide pH range tolerance).

Control of Wilt and Rot Pathogens of Tomato by Antagonistic Pink Pigmented Facultative Methylotrophic Delftia lacustris and Bacillus spp

The studies on the biocontrol potential of pink pigmented facultative methylotrophic (PPFM) bacteria other than the genus Methylobacterium are scarce. In the present study, we report three facultative methylotrophic isolates; PPO-1, PPT-1, and PPB-1, respectively, identified as Delftia lacustris, Bacillus subtilis, and Bacillus cereus by 16S rRNA gene sequence analysis. Hemolytic activity was tested to investigate the potential pathogenicity of isolates to plants and humans, the results indicates that the isolates PPO-1, PPT-1, and PPB-1 are not pathogenic strains. Under in vitro conditions, D. lacustris PPO-1, B. subtilis PPT-1, and B. cereus PPB-1 showed direct antagonistic effect by inhibiting the mycelial growth of fungal pathogens; Fusarium oxysporum f. sp. lycopersici (2.15, 2.05, and 1.95 cm), Sclerotium rolfsii (2.14, 2.04, and 1.94 cm), Pythium ultimum (2.12, 2.02, and 1.92 cm), and Rhizoctonia solani (2.18, 2.08, and 1.98 cm) and also produced volatile inhibitory compounds. Under plant growth chamber condition methylotrophic bacterial isolates; D. lacustris PPO-1, B. subtilis PPT-1, and B. cereus PPB-1 significantly reduced the disease incidence of tomato. Under greenhouse condition, D. lacustris PPO-1, B. subtilis PPT-1, and B. cereus PPB-1 inoculated tomato plants, when challenged with F. oxysporum f. sp. lycopersici, S. rolfsii, P. ultimum, and R. solani, increased the pathogenesis related proteins (β-1,3-glucanase and chitinase) and defense enzymes (phenylalanine ammonia lyase, peroxidase, polyphenol oxidase, and catalase) on day 5 after inoculation. In the current study, we first report the facultative methylotrophy in pink pigmented D. lacustris, B. subtilis, and B. cereus and their antagonistic potential against fungal pathogens. Direct antagonistic and ISR effects of these isolates against fungal pathogens of tomato evidenced their possible use as a biocontrol agent.

Volatile Organic Compounds from Native Potato-associated Pseudomonas as Potential Anti-oomycete Agents

The plant kingdom represents a prominent biodiversity island for microbes that associate with the below- or aboveground organs of vegetal species. Both the root and the leaf represent interfaces where dynamic biological interactions influence plant life. Beside well-studied communication strategies based on soluble compounds and protein effectors, bacteria were recently shown to interact both with host plants and other microbial species through the emissions of volatile organic compounds (VOCs). Focusing on the potato late blight-causing agent Phytophthora infestans, this work addresses the potential role of the bacterial volatilome in suppressing plant diseases. In a previous study, we isolated and identified a large collection of strains with anti-Phytophthora potential from both the phyllosphere and the rhizosphere of potato. Here we report the characterization and quantification of their emissions of biogenic volatiles, comparing 16 Pseudomonas strains differing in (i) origin of isolation (phyllosphere vs. rhizosphere), (ii) in vitro inhibition of P. infestans growth and sporulation behavior, and (iii) protective effects against late blight on potato leaf disks. We systematically tested the pharmacological inhibitory activity of core and strain-specific single compounds against P. infestans mycelial growth and sporangial behavior in order to identify key effective candidate molecules present in the complex natural VOCs blends. We envisage the plant bacterial microbiome as a reservoir for functional VOCs and establish the basis for finding the primary enzymatic toolset that enables the production of active components of the volatile bouquet in plant-associated bacteria. Comprehension of these functional interspecies interactions will open perspectives for the sustainable control of plant diseases in forthcoming agriculture.

Specific rhizobacterial resources: characterization and comparative analysis from contrasting coastal environments of Korea

This study analyzed the rhizobacterial distribution from two coasts, which show contrasting climates and geographical and geological characteristics, to secure specific microbial resources. Furthermore, rhizobacteria were characterized and the results were comparatively analyzed with reference to the characteristics of two coastal environments. For this purpose, three representative halophyte species communities native to the Dokdo Islands and the East Sea coast of Korea were selected. Partial identification of rhizobacteria showed a clear difference between each sampling site and halophyte. Furthermore, isolates were characterized by their growth properties under NaCl or pH gradients related with previous geographical, geological, and climatic studies of the Dokdo Islands and the East Sea coast. A high proportion of the East Sea isolates showed halotolerance, but a high proportion of Dokdo isolates shared halophilic traits. Meanwhile, a higher proportion of East Sea isolates grew at a wider range of pH values than those of the Dokdo Islands. The results of our study suggest that unique rhizobacterial resources developed under specific rhizospheric conditions derived from halophytes interacting with their specific environment, even within the same coastal halophytic species. Therefore, this study proposes the necessity of securing characterized and unique microbial resources to apply to specific environments for the purpose of recovering and restoring sand dunes or salt-damaged agricultural lands.

Control of postharvest fungal pathogens by antifungal compounds from Penicillium expansum

The fungicidal effects of secondary metabolites produced by a strain of Penicillium expansum (R82) in culture filtrate and in a double petri dish assay were tested against one isolate each of Botrytis cinerea, Colletotrichum acutatum, and Monilinia laxa and six isolates of P. expansum, revealing inhibitory activity against every pathogen tested. The characterization of volatile organic compounds released by the R82 strain was performed by solid-phase microextraction-gas chromatographic techniques, and several compounds were detected, one of them identified as phenethyl alcohol (PEA). Synthetic PEA, tested in vitro on fungal pathogens, showed strong inhibition at a concentration of 1,230 μg/ml of airspace, and mycelium appeared more sensitive than conidia; nevertheless, at the concentration naturally emitted by the fungus (0.726 ± 0.16 m g/ml), commercial PEA did not show any antifungal activity. Therefore, a combined effect between different volatile organic compounds produced collectively by R82 can be hypothesized. This aspect suggests further investigation into the possibility of exploiting R82 as a nonchemical alternative in the control of some plant pathogenic fungi.

Trichoderma gamsii (NFCCI 2177): a newly isolated endophytic, psychrotolerant, plant growth promoting, and antagonistic fungal strain

An endophytic fungus has been isolated from the lateral roots of lentil (Lens esculenta Moench), growing under mountain ecosystem of Indian Himalayan Region (IHR). While the fungus was observed as fast growing with white scanty mycelium turning to turmeric brown in 5 days of incubation at 25 °C, it also produced a unique odor. The fungus exhibited growth between 4 and 30 °C (optimum 25 °C) and tolerated pH between 2.0 and 13.5 (optimum 4-6). Based on phenotypic (colony morphology and microscopy) and genotypic (18S rRNA analysis) characters, the fungus was identified as Trichoderma gamsii (99% similarity). The fungus was evaluated for its plant growth promotion and biocontrol properties. The fungus was found to be positive for phosphate solubilization, chitinase activity, and production of ammonia and salicylic acid, while the results for production of IAA, HCN, and siderophores were negative. Out of the seven phytopathogenic fungi tested, it showed antagonism against six. Bioassays conducted under green house using four test crops (two cereals and two legumes) showed its potential in plant growth promotion. The fungus has potential to be developed as a bioformulation for application under mountain ecosystem.

Production of DAPG and HCN by Pseudomonas sp. LBUM300 contributes to the biological control of bacterial canker of tomato

Bacterial canker caused by Clavibacter michiganensis subsp. michiganensis is known to cause significant economic losses to tomato production worldwide. Biological control has been proposed as an alternative to current chemical containment methods, which are often inefficient and may leave adverse effects on the environment. However, only little headway has so far been made in developing biocontrol strategies against C. michiganensis subsp. michiganensis. To address this knowledge gap, we investigated the antagonistic capacity of PCA, produced by Pseudomonas sp. LBUM223, and DAPG and HCN, both produced by Pseudomonas sp. LBUM300, on C. michiganensis subsp. michiganensis under in vitro and in planta conditions. Nonsynthesizing isogenic mutants of the producer strains were also developed to further dissect the role of each individual metabolite on C. michiganensis subsp. michiganensis biological control. Novel specific quantitative polymerase chain reaction TaqMan assays allowed quantification of C. michiganensis subsp. michiganensis in tomato plants and rhizospheric soil. Pseudomonas spp. LBUM223 and LBUM300 significantly repressed C. michiganensis subsp. michiganensis growth in vitro, while their respective nonproducing mutants showed less or no significant antagonistic activity. In planta, only Pseudomonas sp. LBUM300 was capable of significantly reducing disease development and C. michiganensis subsp. michiganensis rhizospheric population, suggesting that the production of both DAPG and HCN was involved. In summary, simultaneous DAPG/HCN production by Pseudomonas sp. LBUM300 shows great potential for controlling bacterial canker of tomato.

Isolation and characterization of beneficial bacteria associated with citrus roots in Florida

Cultivable diversity of bacteria associated with citrus was investigated as part of a larger study to understand the roles of beneficial bacteria and utilize them to increase the productive capacity and sustainability of agro-ecosystems. Citrus roots from Huanglongbing (HLB) diseased symptomatic and asymptomatic citrus were used in this study. A total of 227 and 125 morphologically distinct colonies were isolated and characterized from HLB asymptomatic and symptomatic trees, respectively. We observed that the frequency of bacterial isolates possessing various plant beneficial properties was significantly higher in the asymptomatic samples. A total of 39 bacterial isolates showing a minimum of five beneficial traits related to mineral nutrition [phosphate (P) solubilization, siderophore production, nitrogen (N) fixation], development [indole acetic acid (IAA) synthesis], health [production of antibiotic and lytic enzymes (chitinase)], induction of systemic resistance [salicylic acid (SA) production], stress relief [production of 1-amino-cyclopropane-1-carboxylate deaminase] and production of quorum sensing [N-acyl homoserine lactones] signals were characterized. A bioassay using ethidium monoazide (EMA)-qPCR was developed to select bacteria antagonistic to Candidatus Liberibacter asiaticus. Using the modified EMA-qPCR assay, we found six bacterial isolates showing maximum similarity to Paenibacillus validus, Lysinibacillus fusiformis, Bacillus licheniformis, Pseudomonas putida, Microbacterium oleivorans, and Serratia plymutica could significantly reduce the population of viable Ca. L. asiaticus in HLB symptomatic leaf samples. In conclusion, we have isolated and characterized multiple beneficial bacterial strains from citrus roots which have the potential to enhance plant growth and suppress diseases.

Combined treatment of Pseudomonas aeruginosa biofilm with lactoferrin and xylitol inhibits the ability of bacteria to respond to damage resulting from lactoferrin iron chelation

With an ageing and ever more obese population, chronic wounds such as diabetic ulcers, pressure ulcers and venous leg ulcers are an increasingly relevant medical concern. Identification of bacterial biofilm contamination as a major contributor to non-healing wounds demands biofilm-targeted strategies to manage chronic wounds. Pseudomonas aeruginosa has been identified as a principal biofilm-forming opportunistic pathogen in chronic wounds. The innate immune molecule lactoferrin and the rare sugar alcohol xylitol have been demonstrated to be co-operatively efficacious against P. aeruginosa biofilms in vitro. Data presented here propose a model for the molecular mechanism behind this co-operative antimicrobial effect. Lactoferrin iron chelation was identified as the primary means by which lactoferrin destabilises the bacterial membrane. By microarray analysis, 183 differentially expressed genes of ≥ 1.5-fold difference were detected. Interestingly, differentially expressed transcripts included the operon encoding components of the pyochelin biosynthesis pathway. Furthermore, siderophore detection verified that xylitol is the component of this novel synergistic treatment that inhibits the ability of the bacteria to produce siderophores under conditions of iron restriction. The findings presented here demonstrate that whilst lactoferrin treatment of P. aeruginosa biofilms results in destabilisation of the bacterial cell membrane though iron chelation, combined treatment with lactoferrin and xylitol inhibits the ability of P. aeruginosa biofilms to respond to environmental iron restriction.

Plant growth promoting rhizobacteria and endophytes accelerate phytoremediation of metalliferous soils

Technogenic activities (industrial-plastic, textiles, microelectronics, wood preservatives; mining-mine refuse, tailings, smelting; agrochemicals-chemical fertilizers, farm yard manure, pesticides; aerosols-pyrometallurgical and automobile exhausts; biosolids-sewage sludge, domestic waste; fly ash-coal combustion products) are the primary sources of heavy metal contamination and pollution in the environment in addition to geogenic sources. During the last two decades, bioremediation has emerged as a potential tool to clean up the metal-contaminated/polluted environment. Exclusively derived processes by plants alone (phytoremediation) are time-consuming. Further, high levels of pollutants pose toxicity to the remediating plants. This situation could be ameliorated and accelerated by exploring the partnership of plant-microbe, which would improve the plant growth by facilitating the sequestration of toxic heavy metals. Plants can bioconcentrate (phytoextraction) as well as bioimmobilize or inactivate (phytostabilization) toxic heavy metals through in situ rhizospheric processes. The mobility and bioavailability of heavy metal in the soil, particularly at the rhizosphere where root uptake or exclusion takes place, are critical factors that affect phytoextraction and phytostabilization. Developing new methods for either enhancing (phytoextraction) or reducing the bioavailability of metal contaminants in the rhizosphere (phytostabilization) as well as improving plant establishment, growth, and health could significantly speed up the process of bioremediation techniques. In this review, we have highlighted the role of plant growth promoting rhizo- and/or endophytic bacteria in accelerating phytoremediation derived benefits in extensive tables and elaborate schematic sketches.