Genus-wide acid tolerance accounts for the biogeographical distribution of soil Burkholderia populations

Endophytic Bacterial Community, Core Taxa, and Functional Variations Within the Fruiting Bodies of Laccaria

Macrofungi do not exist in isolation but establish symbiotic relationships with microorganisms, particularly bacteria, within their fruiting bodies. Herein, we examined the fruiting bodies' bacteriome of seven species of the genus Laccaria collected from four locations in Yunnan, China. By analyzing bacterial diversity, community structure, and function through 16S rRNA sequencing, we observed the following: (1) In total, 4,840,291 high-quality bacterial sequences obtained from the fruiting bodies were grouped into 16,577 amplicon sequence variants (ASVs), and all samples comprised 23 shared bacterial ASVs. (2) The Allorhizobium-Neorhizobium-Pararhizobium-Rhizobium complex was found to be the most abundant and presumably coexisting bacterium. (3) A network analysis revealed that endophytic bacteria formed functional groups, which were dominated by the genera Allorhizobium-Neorhizobium-Pararhizobium-Rhizobium, Novosphingobium, and Variovorax. (4) The diversity, community structure, and dominance of ecological functions (chemoheterotrophy and nitrogen cycling) among endophytic bacteria were significantly shaped by geographic location, habitat, and fungal genotype, rather than fruiting body type. (5) A large number of the endophytic bacteria within Laccaria are bacteria that promote plant growth; however, some pathogenic bacteria that pose a threat to human health might also be present. This research advances our understanding of the microbial ecology of Laccaria and the factors shaping its endophytic bacterial communities.

Integrative analysis of microbiome and metabolome revealed the effect of microbial inoculant on microbial community diversity and function in rhizospheric soil under tobacco monoculture

Over-application of chemical fertilizers and continuous cropping obstacles seriously restrict the sustainable development of tobacco production. Localized fertilization of beneficial microbes has potential advantages in achieving higher productivity, but the underlying biological mechanisms of interactions between rhizospheric microorganisms and the related metabolic cycle remain poorly characterized. Here, an integrative analysis of microbiomes with non-targeted metabolomics was performed on 30 soil samples of rhizosphere, root surrounding, and bulk soils from flue-cured tobacco under continuous and non-continuous monocropping systems. The analysis was conducted using UPLC-MS/MS platforms and high-throughput amplicon sequencing targeting the bacterial 16S rRNA gene and fungal ITS gene. The microbial inoculant consisted of Bacillus subtilis, B. velezensis, and B. licheniformis at the ratio of 1:1:1 in effective microbial counts, improved the cured leaf yield and disease resistance of tobacco, and enhanced nicotine and nitrogen contents of tobacco leaves. The bacterial taxa Rhizobium, Pseudomonas, Sphingomonadaceae, and Burkholderiaceae of the phylum Proteobacteria accumulated in high relative abundance and were identified as biomarkers following the application of the microbial inoculant. Under continuous monocropping, metabolomics demonstrated that the application of the microbial inoculant significantly affected the soil metabolite spectrum, and the differential metabolites were significantly enriched to the synthesis and degradation of nicotine (nicotinate and nicotinamide metabolism and biosynthesis of alkaloids derived from nicotinic acid). In addition, microbes were closely related to the accumulation of metabolites through correlation analysis. The interactions between plant roots and rhizospheric microorganisms provide valuable information for understanding how these beneficial microbes affect complex biological processes and the adaption capacity of plants to environments.IMPORTANCEThis study elaborated on how the microbial fertilizer significantly changed overall community structures and metabolite spectrum of rhizospheric microbes, which provide insights into the process of rhizosphere microbial remolding in response to continuous monocropping. we verified the hypothesis that the application of the microbial inoculant in continuous cropping would lead to the selection of distinct microbiota communities by establishing models to correlate biomarkers. Through correlation analysis of the microbiome and metabolome, we proved that rhizospheric microbes were closely related to the accumulation of metabolites, including the synthesis and degradation of nicotine. The interactions between plant roots and rhizospheric microorganisms provide valuable information for understanding how these beneficial microbes affect complex biological processes and the adaption capacity of plants to environments.

Enhanced surface colonisation and competition during bacterial adaptation to a fungus

Bacterial-fungal interactions influence microbial community performance of most ecosystems and elicit specific microbial behaviours, including stimulating specialised metabolite production. Here, we use a co-culture experimental evolution approach to investigate bacterial adaptation to the presence of a fungus, using a simple model of bacterial-fungal interactions encompassing the bacterium Bacillus subtilis and the fungus Aspergillus niger. We find in one evolving population that B. subtilis was selected for enhanced production of the lipopeptide surfactin and accelerated surface spreading ability, leading to inhibition of fungal expansion and acidification of the environment. These phenotypes were explained by specific mutations in the DegS-DegU two-component system. In the presence of surfactin, fungal hyphae exhibited bulging cells with delocalised secretory vesicles possibly provoking an RlmA-dependent cell wall stress. Thus, our results indicate that the presence of the fungus selects for increased surfactin production, which inhibits fungal growth and facilitates the competitive success of the bacterium.

Searching for new plastic-degrading enzymes from the plastisphere of alpine soils using a metagenomic mining approach

Plastic materials, including microplastics, accumulate in all types of ecosystems, even in remote and cold environments such as the European Alps. This pollution poses a risk for the environment and humans and needs to be addressed. Using shotgun DNA metagenomics of soils collected in the eastern Swiss Alps at about 3,000 m a.s.l., we identified genes and their proteins that potentially can degrade plastics. We screened the metagenomes of the plastisphere and the bulk soil with a differential abundance analysis, conducted similarity-based screening with specific databases dedicated to putative plastic-degrading genes, and selected those genes with a high probability of signal peptides for extracellular export and a high confidence for functional domains. This procedure resulted in a final list of nine candidate genes. The lengths of the predicted proteins were between 425 and 845 amino acids, and the predicted genera producing these proteins belonged mainly to Caballeronia and Bradyrhizobium. We applied functional validation, using heterologous expression followed by enzymatic assays of the supernatant. Five of the nine proteins tested showed significantly increased activities when we used an esterase assay, and one of these five proteins from candidate genes, a hydrolase-type esterase, clearly had the highest activity, by more than double. We performed the fluorescence assays for plastic degradation of the plastic types BI-OPL and ecovio® only with proteins from the five candidate genes that were positively active in the esterase assay, but like the negative controls, these did not show any significantly increased activity. In contrast, the activity of the positive control, which contained a PLA-degrading gene insert known from the literature, was more than 20 times higher than that of the negative controls. These findings suggest that in silico screening followed by functional validation is suitable for finding new plastic-degrading enzymes. Although we only found one new esterase enzyme, our approach has the potential to be applied to any type of soil and to plastics in various ecosystems to search rapidly and efficiently for new plastic-degrading enzymes.

Acidic pH modulates Burkholderia cenocepacia antimicrobial susceptibility in the cystic fibrosis nutritional environment

IMPORTANCE

Burkholderia cenocepacia causes severe infections in cystic fibrosis (CF) patients. CF patients are prone to reoccurring infections due to the accumulation of mucus in their lungs, where bacteria can adhere and grow. Some of the antibiotics that inhibit B. cenocepacia in the laboratory are not effective for CF patients. A major contributor to poor clinical outcomes is that antibiotic testing in laboratories occurs under conditions that are different from those of sputum. CF sputum may be acidic and have increased concentrations of iron and zinc. Here, we used a medium that mimics CF sputum and found that acidic pH decreased the activity of many of the antibiotics used against B. cenocepacia. In addition, we assessed susceptibility to more than 500 antibiotics and found four active compounds against B. cenocepacia. Our findings give a better understanding of the lack of a relationship between susceptibility testing and the clinical outcome when treating B. cenocepacia infections.

Pioneering Desmodium spp. are nodulated by natural populations of stress-tolerant alpha- and beta-rhizobia

The rhizobia-Desmodium (Leguminosae, Papilionoideae) symbiosis is generally described by its specificity with alpha-rhizobia, especially with Bradyrhizobium. Our study aimed to isolate rhizobia from root nodules of native D. barbatum, D. incanum, and D. discolor, collected in remnants of the biomes of Atlantic Forest and Cerrado in protected areas of the Paraná State, southern Brazil. Based on the 16S rRNA phylogeny, 18 out of 29 isolates were classified as Alphaproteobacteria (Bradyrhizobium and Allorhizobium/Rhizobium) and 11 as Betaproteobacteria (Paraburkholderia). Phylogeny of the recA gene of the alpha-rhizobia resulted in ten main clades, of which two did not group with any described rhizobial species. In the 16S rRNA phylogeny of the beta-rhizobia, Paraburkholderia strains from the same host and conservation unity occupied the same clade. Phenotypic characterization of representative strains revealed the ability of Desmodium rhizobia to grow under stressful conditions such as high temperature, salinity, low pH conditions, and tolerance of heavy metals and xenobiotic compounds. Contrasting with previous reports, our results revealed that Brazilian native Desmodium can exploit symbiotic interactions with stress-tolerant strains of alpha- and beta-rhizobia. Stress tolerance can highly contribute to the ecological success of Desmodium in this phytogeographic region, possibly relating to its pioneering ability in Brazil. We propose Desmodium as a promising model for studies of plant-rhizobia interactions.

Abundance and phylogenetic distribution of eight key enzymes of the phosphorus biogeochemical cycle in grassland soils

Grassland biomes provide valuable ecosystem services, including nutrient cycling. Organic phosphorus (Po) represents more than half of the total P in soils. Soil microorganisms release organic P through enzymatic processes, with alkaline phosphatases, acid phosphatases and phytases being the key P enzymes involved in the cycling of organic P. This study analysed 74 soil metagenomes from 17 different grassland biomes worldwide to evaluate the distribution and abundance of eight key P enzymes (PhoD, PhoX, PhoA, Nsap-A, Nsap-B, Nsap-C, BPP and CPhy) and their relationship with environmental factors. Our analyses showed that alkaline phosphatase phoD was the dataset's most abundant P-enzyme encoding genes, with a wide phylogenetic distribution. Followed by the acid phosphatases Nsap-A and Nsap-C showed similar abundance but a different distribution in their respective phylogenetic trees. Multivariate analyses revealed that pH, Tmax , SOC and soil moisture were associated with the abundance and diversity of all genes studied. PhoD and phoX genes strongly correlated with SOC and clay, and the phoX gene was more common in soils with low to medium SOC and neutral pH. In particular, P-enzyme genes tended to respond in a positively correlated manner among them, suggesting a complex relationship of abundance and diversity among them.

Rational design of an acidic erythritol (ACER) medium for the enhanced isolation of the environmental pathogen Burkholderia pseudomallei from soil samples

The soil bacterium Burkholderia pseudomallei causes melioidosis, a potentially fatal and greatly underdiagnosed tropical disease. Detection of B. pseudomallei in the environment is important to trace the source of infections, define risk areas for melioidosis and increase the clinical awareness. Although B. pseudomallei polymerase chain reaction (PCR)-based environmental detection provides important information, the culture of the pathogen remains essential but is still a methodological challenge. B. pseudomallei can catabolize erythritol, a metabolic pathway, which is otherwise rarely encountered among bacteria. We recently demonstrated that replacing threonine with erythritol as a single carbon source in the pH-neutral threonine-basal salt solution (TBSS-C50) historically used improved the isolation of B. pseudomallei from rice paddy soils. However, further culture medium parameters for an optimized recovery of B. pseudomallei strains from soils are still ill-defined. We, therefore, aimed to design a new erythritol-based medium by systematically optimizing parameters such as pH, buffer capacity, salt and nutrient composition. A key finding of our study is the enhanced erythritol-based growth of B. pseudomallei under acidic medium conditions. Our experiments with B. pseudomallei strains from different geographical origin led to the development of a phosphate-buffered acidic erythritol (ACER) medium with a pH of 6.3, higher erythritol concentration of 1.2%, supplemented vitamins and nitrate. This highly selective medium composition shortened the lag phase of B. pseudomallei cultures and greatly increased growth densities compared to TBSS-C50 and TBSS-C50-based erythritol medium. The ACER medium led to the highest enrichments of B. pseudomallei as determined from culture supernatants by quantitative PCR in a comparative validation with soil samples from the central part of Vietnam. Consequently, the median recovery of B. pseudomallei colony forming units on Ashdown's agar from ACER subcultures was 5.4 times higher compared to TBSS-C50-based erythritol medium (p = 0.005) and 30.7 times higher than TBSS-C50 (p < 0.001). In conclusion, our newly developed ACER medium significantly improves the isolation of viable B. pseudomallei from soils and, thereby, has the potential to reduce the rate of false-negative environmental cultures in melioidosis risk areas.

Roles of phosphate-solubilizing bacteria in mediating soil legacy phosphorus availability

Phosphorus (P), an essential macronutrient for all life on Earth, has been shown to be a vital limiting nutrient element for plant growth and yield. P deficiency is a common phenomenon in terrestrial ecosystems across the world. Chemical phosphate fertilizer has traditionally been employed to solve the problem of P deficiency in agricultural production, but its application has been limited by the non-renewability of raw materials and the adverse influence on the ecological health of the environment. Therefore, it is imperative to develop efficient, economical, environmentally friendly and highly stable alternative strategies to meet the plant P demand. Phosphate-solubilizing bacteria (PSB) are able to improve plant productivity by increasing P nutrition. Pathways to fully and effectively use PSB to mobilize unavailable forms of soil P for plants has become a hot research topic in the fields of plant nutrition and ecology. Here, the biogeochemical P cycling in soil systems are summarized, how to make full use of soil legacy P via PSB to alleviate the global P resource shortage are reviewed. We highlight the advances in multi-omics technologies that are helpful for exploring the dynamics of nutrient turnover and the genetic potential of PSB-centered microbial communities. Furthermore, the multiple roles of PSB inoculants in sustainable agricultural practices are analyzed. Finally, we project that new ideas and techniques will be continuously infused into fundamental and applied research to achieve a more integrated understanding of the interactive mechanisms of PSB and rhizosphere microbiota/plant to maximize the efficacy of PSB as P activators.

Nitrogen-Fixing Symbiotic Paraburkholderia Species: Current Knowledge and Future Perspectives

A century after the discovery of rhizobia, the first Beta-proteobacteria species (beta-rhizobia) were isolated from legume nodules in South Africa and South America. Since then, numerous species belonging to the Burkholderiaceae family have been isolated. The presence of a highly branching lineage of nodulation genes in beta-rhizobia suggests a long symbiotic history. In this review, we focus on the beta-rhizobial genus Paraburkholderia, which includes two main groups: the South American mimosoid-nodulating Paraburkholderia and the South African predominantly papilionoid-nodulating Paraburkholderia. Here, we discuss the latest knowledge on Paraburkholderia nitrogen-fixing symbionts in each step of the symbiosis, from their survival in the soil, through the first contact with the legumes until the formation of an efficient nitrogen-fixing symbiosis in root nodules. Special attention is given to the strain P. phymatum STM815T that exhibits extraordinary features, such as the ability to: (i) enter into symbiosis with more than 50 legume species, including the agriculturally important common bean, (ii) outcompete other rhizobial species for nodulation of several legumes, and (iii) endure stressful soil conditions (e.g., high salt concentration and low pH) and high temperatures.

Effects of short-interval reburns in the boreal forest on soil bacterial communities compared to long-interval reburns

Increasing fire frequency in some biomes is leading to fires burning in close succession, triggering rapid vegetation change and altering soil properties. We studied the effects of short-interval (SI) reburns on soil bacterial communities of the boreal forest of northwestern Canada using paired sites (n = 44). Both sites in each pair had burned in a recent fire; one site had burned within the previous 20 years before the recent fire (SI reburn) and the other had not. Paired sites were closely matched in prefire ecosite characteristics, prefire tree species composition, and stand structure. We hypothesized that there would be a significant effect of short vs. long fire-free intervals on community composition and that richness would not be consistently different between paired sites. We found that Blastococcus sp. was consistently enriched in SI reburns, indicating its role as a strongly ‘pyrophilous’ bacterium. Caballeronia sordidicola was consistently depleted in SI reburns. The depletion of this endophytic diazotroph raises questions about whether this is contributing to—or merely reflects—poor conifer seedling recolonization post-fire at SI reburns. While SI reburns had no significant effect on richness, dissimilarity between short- and long-interval pairs was significantly correlated with difference in soil pH, and there were small significant changes in overall community composition.

Alien invasive Leucaena leucocephala successfully acquires nutrients by investing in below-ground biomass compared to native Vachellia nilotica in nutrient-amended soils in South Africa

Soils in grasslands and savannas of southern Africa are acidic and nutrient-poor. Legume plants, such as Vachellia nilotica and alien invasive Leucaena leucocephala, are a major component of the vegetation there. Vachellia nilotica can establish in drought-prone environments, and is invasive in high rainfall areas. Leucaena leucocephala is an emerging invasive in South Africa and is ranked among the world's 100 most invasive alien species. Alien plants can invade native habitats through their adaptability to low-resource soils, and thus can out-compete and displace native vegetation. We investigated the effects of phosphorus (P) deficiency and soil acidity on legume-microbe symbiosis, nitrogen (N) nutrition and carbon (C) growth costs of these two legumes in grassland soils. We used as inoculum and growth substrate soils collected from a long-term (>65 years) nutrient and lime-addition trial, the Veld Fertilizer Trial (VFT), located at Ukulinga Research Farm near Pietermaritzburg in South Africa. We used soils from three VFT treatments: soils fertilized with superphosphate (336 kg ha^-1) applied once per year (+P), soils fertilized with superphosphate (336 kg ha^-1) applied once per year with dolomitic lime (2250 kg ha^-1) applied once every 5 years (P+L) and soils with no superphosphate and no dolomitic lime applications (Control). Seeds of V. nilotica and L. leucocephala were germinated and grown independently in these soils in green house conditions and harvested after 125 days for measurement of growth, legume-microbe symbiosis, N nutrition and C growth costs. Results showed that the two legumes had different growth adaptations. Vachellia nilotica grown in control soils and +P soils nodulated with various Burkholderia spp., while L. leucocephala did not nodulate in all soil treatments. Both legumes utilized for growth both atmospheric- and soil-derived N across all treatments thereby decreasing C growth costs. Vachellia nilotica grown in +P soils accumulated the most biomass and N nutrition. Leucaena leucocephala maximized specific N assimilation rates by investing in below-ground biomass accumulation in control soils. This shows that L. leucocephala possesses traits that are successful in acquiring nutrients by investing in below-ground biomass and relying on utilization of N from both the soil and the atmosphere.

Distribution of Burkholderia pseudomallei within a 300-cm deep soil profile: implications for environmental sampling

The environmental distribution of Burkholderia pseudomallei, the causative agent of melioidosis, remains poorly understood. B. pseudomallei is known to have the ability to occupy a variety of environmental niches, particularly in soil. This paper provides novel information about a putative association of soil biogeochemical heterogeneity and the vertical distribution of B. pseudomallei. We investigated (1) the distribution of B. pseudomallei along a 300-cm deep soil profile together with the variation of a range of soil physico-chemical properties; (2) whether correlations between the distribution of B. pseudomallei and soil physico-chemical properties exist and (3) when they exist, what such correlations indicate with regards to the environmental conditions conducive to the occurrence of B. pseudomallei in soils. Unexpectedly, the highest concentrations of B. pseudomallei were observed between 100 and 200 cm below the soil surface. Our results indicate that unravelling the environmental conditions favorable to B. pseudomallei entails considering many aspects of the actual complexity of soil. Important recommendations regarding environmental sampling for B. pseudomallei can be drawn from this work, in particular that collecting samples down to the water table is of foremost importance, as groundwater persistence appears to be a controlling factor of the occurrence of B. pseudomallei in soil.

Soil bacteria protect fungi from phenazines by acting as toxin sponges

Many environmentally and clinically important fungi are sensitive to toxic, bacterially produced, redox-active molecules called phenazines. Despite being vulnerable to phenazine assault, fungi inhabit microbial communities that contain phenazine producers. Because many fungi cannot withstand phenazine challenge but some bacterial species can, we hypothesized that bacterial partners may protect fungi in phenazine-replete environments. From a single soil sample, we were able to co-isolate several such physically associated pairings. We discovered the novel species Paraburkholderia edwinii and demonstrated it can protect a co-isolated Aspergillus species from phenazine-1-carboxylic acid (PCA) by sequestering it, acting as a toxin sponge; in turn, it also gains protection. When challenged with PCA, P. edwinii changes its morphology, forming aggregates within the growing fungal colony. Further, the fungal partner triggers P. edwinii to sequester PCA and maintains conditions that limit PCA toxicity by promoting an anoxic and highly reducing environment. A mutagenic screen of P. edwinii revealed this protective program depends on the stress-inducible transcriptional repressor HrcA. We show that one relevant stressor in response to PCA challenge is fungal acidification and that acid stress causes P. edwinii to behave as though the fungus were present. Finally, we reveal this phenomenon as widespread among Paraburkholderia with moderate specificity among bacterial and fungal partners, including plant and human pathogens. Our discovery suggests a common mechanism by which fungi can gain access to phenazine-replete environments and provides a tractable model system for its study. These results have implications for how microbial communities in the rhizosphere as well as in plant and human infection sites negotiate community membership via a chemical dialectic.

Root-Associated Microbiota Response to Ecological Factors: Role of Soil Acidity in Enhancing Citrus Tolerance to Huanglongbing

The citrus orchards in southern China are widely threatened by low soil pH and Huanglongbing (HLB) prevalence. Notably, the lime application has been used to optimize soil pH, which is propitious to maintain root health and enhance HLB tolerance of citrus; however, little is known about the interactive effects of soil acidity on the soil properties and root-associated (rhizoplane and endosphere) microbial community of HLB-infected citrus orchard. In this study, the differences in microbial community structures and functions between the acidified and amended soils in the Gannan citrus orchard were investigated, which may represent the response of the host-associated microbiome in diseased roots and rhizoplane to dynamic soil acidity. Our findings demonstrated that the severity of soil acidification and aluminum toxicity was mitigated after soil improvement, accompanied by the increase in root activity and the decrease of HLB pathogen concentration in citrus roots. Additionally, the Illumina sequencing-based community analysis showed that the application of soil amendment enriched functional categories involved in host-microbe interactions and nitrogen and sulfur metabolisms in the HLB-infected citrus rhizoplane; and it also strongly altered root endophytic microbial community diversity and structure, which represented by the enrichment of beneficial microorganisms in diseased roots. These changes in rhizoplane-enriched functional properties and microbial composition may subsequently benefit the plant's health and tolerance to HLB disease. Overall, this study advances our understanding of the important role of root-associated microbiota changes and ecological factors, such as soil acidity, in delaying and alleviating HLB disease.

Genetic diversity and transmission patterns of Burkholderia pseudomallei on Hainan island, China, revealed by a population genomics analysis

Burkholderia pseudomallei is a Gram-negative soil-dwelling bacillus that causes melioidosis, a frequently fatal infectious disease, in tropical and subtropical regions. Previous studies have identified the overall genetic and evolutionary characteristics of B. pseudomallei on a global scale, including its origin and transmission routes. However, beyond its known hyperendemicity foci in northern Australia and Southeast Asia, the distribution and genetic characteristics of B. pseudomallei in most tropical regions remain poorly understood, including in southern China. Here, we sequenced the genomes of 122 B. pseudomallei strains collected from Hainan, an island in southern China, in 2002–2018, to investigate the population structure, relationships with global strains, local epidemiology, and virulence and antimicrobial-resistance factors. A phylogenetic analysis and hierarchical clustering divided the Hainan strains into nine phylogenic groups (PGs), 80 % of which were concentrated within five major groups (group 1: corresponding to minor sequence types [STs], 12.3 %; group 3: ST46 and ST50, 31.1 %; group 9: ST58, 13.1 %; group 11: ST55, 8.2 %; group 15: mainly ST658, 15.6%). A phylogenetic analysis that included global strains suggested that B. pseudomallei in Hainan originated from Southeast Asian countries, transmitted in multiple historical importation events. We also identified several mutual transmission events between Hainan and Southeast Asian countries in recent years, including three importation events from Thailand and Singapore to Hainan and three exportation events from Hainan to Singapore, Malaysia, and Taiwan island. A statistical analysis of the temporal distribution showed that the Hainan strains of groups 3, 9, and 15 have dominated the disease epidemic locally in the last 5 years. The spatial distribution of the Hainan strains demonstrated that some PGs are distributed in different cities on Hainan island, and by combining phylogenic and geographic distribution information, we detected 21 between-city transmission events, indicating its frequent local transmission. The detection of virulence factor genes showed that 56 % of the Hainan strains in group 1 encode a B. pseudomallei-specific adherence factor, boaB, confirming the specific pathogenic characteristics of the Hainan strains in group 1. An analysis of the antimicrobial-resistance potential of B. pseudomallei showed that various kinds of alterations were identified in clinically relevant antibiotic resistance factors, such as AmrR, PenA and PBP3, etc. Our results clarify the population structure, local epidemiology, and pathogenic characteristics of B. pseudomallei in Hainan, providing further insight into its regional and global transmission networks and improving our knowledge of its global phylogeography.

Molecular diversity of rhizobia-nodulating native Mimosa of Brazilian protected areas

Symbiotic Paraburkholderia have been increasingly studied in the past 20 years, especially when associated with Mimosa; however, studies with native/endemic species are still scarce. In this study, thirty strains were isolated from root nodules of native Mimosa paranapiacabae and M. micropteris in two locations of the Campos Gerais. The BOX-PCR fingerprinting revealed high genomic diversity, and the 16S rRNA phylogeny clustered the strains in three distinct groups (GI, GII, GIII), with one strain occupying an isolated position. Phylogenetic analysis with four concatenated housekeeping genes (atpD + gltB + gyrB + recA) confirmed the same clusters of 16S rRNA, and the closest species were P. nodosa BR 3437^T and P. guartelaensis CNPSo 3008^T; this last one isolated from another Mimosa species of the Campos Gerais. The phylogenies of the symbiotic genes nodAC and nifH placed all strains in a well-supported branch with the other species of the symbiovar mimosae. The phylogenetic analyses indicated that the strains represent novel lineages of sv. mimosae and that endemic Mimosa coevolved with indigenous Paraburkholderia in their natural environments.

Bioaugmentation of acetamiprid-contaminated soil with Pigmentiphaga sp. strain D-2 and its effect on the soil microbial community

Bioaugmentation, a strategy based on microbiome engineering, has been proposed for bioremediation of pollutant-contaminated environments. However, the complex microbiome engineering processes for soil bioaugmentation, involving interactions among the exogenous inoculum, soil environment, and indigenous microbial microbiome, remain largely unknown. Acetamiprid is a widely used neonicotinoid insecticide which has caused environmental contaminations. Here, we used an acetamiprid-degrading strain, Pigmentiphaga sp. D-2, as inoculum to investigate the effects of bioaugmentation on the soil microbial community and the process of microbiome reassembly. The bioaugmentation treatment removed 94.8 and 92.5% of acetamiprid within 40 days from soils contaminated with 50 and 200 mg/kg acetamiprid, respectively. A decrease in bacterial richness and diversity was detected in bioaugmentation treatments, which later recovered with the removal of acetamiprid from soil. Moreover, the bioaugmentation treatment significantly influenced the bacterial community structure, whereas application of acetamiprid alone had little influence on the soil microbial community. Furthermore, the bioaugmentation treatment improved the growth of bacteria associated with acetamiprid degradation, and the inoculated and recruited taxa significantly influenced the keystone taxa of the indigenous microbiome, resulting in reassembly of the bacterial community yielding higher acetamiprid-degrading efficiency than that of the indigenous and acetamiprid-treated communities. Our results provide valuable insights into the mechanisms of microbiome engineering for bioaugmentation of acetamiprid-contaminated soils.

Carbonate and silicate dissolving bacteria isolated from home-made yogurt samples

In the current study, twenty-eight bacterial strains were isolated from home-made yogurt samples from Ağrı Province, Turkey. The bacterial strains were identified by conventional and molecular techniques. Among the twenty- eight isolates, seventeen isolates were identified according to the 16 S rDNA region and determined to belong to five different genus including Sphingomonas (8 isolates), Burkholderia (5 isolates), Lactobacillus (2 isolates), Lactococcus (1 isolate), Staphylococcus (1 isolate). In this study, the presence of Burkholderia in home-made yogurt samples were reported for the first time, whereas Sphingomonas was detected for the second time. We also investigated the carbonate (CaCO3 and MgCO3) and silicate (‎CaSiO3 and MgSiO3) dissolving potential of seventeen bacterial isolates. Among these seventeen bacterial isolates, fifteen bacterial isolates have CaCO3-dissolving and 10 bacterial isolates have MgCO3-dissolving potential. The silicates dissolution ability was relatively less than that of carbonates dissolving. We observed that six bacterial isolates have CaSiO3 and only two bacterial isolates have MgSiO3 dissolution abilities. In conclusion, this work clearly shows the diversity of bacteria existing in fermented cow milk samples in Ağrı Province, Turkey, which could be considered as valuable sources for lactic acid bacteria (LAB) isolation and further probiotic potential.

Twenty years of paradigm-breaking studies of taxonomy and symbiotic nitrogen fixation by beta-rhizobia, and indication of Brazil as a hotspot of Paraburkholderia diversity

Twenty years ago, the first members of the genus Burkholderia capable of nodulating and fixing N₂ during symbiosis with leguminous plants were reported. The discovery that β-proteobacteria could nodulate legumes represented a breakthrough event because, for over 100 years, it was thought that all rhizobia belonged exclusively to the α-Proteobacteria class. Over the past 20 years, efforts toward robust characterization of these bacteria with large-scale phylogenomic and taxonomic studies have led to the separation of clinically important and phytopathogenic members of Burkholderia from environmental ones, and the symbiotic nodulating species are now included in the genera Paraburkholderia and Trinickia. Paraburkholderia encompasses the vast majority of β-rhizobia and has been mostly found in South America and South Africa, presenting greater symbiotic affinity with native members of the families Mimosoideae and Papilionoideae, respectively. Being the main center of Mimosa spp. diversity, Brazil is also known as the center of symbiotic Paraburkholderia diversity. Of the 21 symbiotic Paraburkholderia species described to date, 11 have been isolated in Brazil, and others first isolated in different countries have also been found in this country. Additionally, besides the symbiotic N₂-fixation capacity of some of its members, Paraburkholderia is considered rich in other beneficial interactions with plants and can promote growth through several direct and indirect mechanisms. Therefore, these bacteria can be considered biological resources employed as environmentally friendly alternatives that could reduce the agricultural dependence on agrochemical inputs.

A Burkholderia thailandensis DedA Family Membrane Protein Is Required for Proton Motive Force Dependent Lipid A Modification

The DedA family is a conserved membrane protein family found in most organisms. A Burkholderia thailandensis DedA family protein, named DbcA, is required for high-level colistin (polymyxin E) resistance, but the mechanism awaits elucidation. Modification of lipopolysaccharide lipid A with the cationic sugar aminoarabinose (Ara4N) is required for colistin resistance and is dependent upon protonmotive force (PMF) dependent transporters. B. thailandensis ΔdbcA lipid A contains only small amounts of Ara4N, likely leading to colistin sensitivity. Two B. thailandensis operons are required for lipid A modification with Ara4N, one needed for biosynthesis of undecaprenyl-P-Ara4N and one for transport of the lipid linked sugar and subsequent lipid A modification. Here, we directed overexpression of each arn operon by genomic insertion of inducible promoters. We found that overexpression of arn operons in ΔdbcA can partially, but not completely, restore Ara4N modification of lipid A and colistin resistance. Artificially increasing the PMF by lowering the pH of the growth media also increased membrane potential, amounts of Ara4N, and colistin resistance of ΔdbcA. In addition, the products of arn operons are essential for acid tolerance, suggesting a physiological function of Ara4N modification. Finally, we show that ΔdbcA is sensitive to bacitracin and expression of a B. thailandensis UppP/BacA homolog (BTH_I1512) can partially restore resistance to bacitracin. Expression of a different UppP/BacA homolog (BTH_I2750) can partially restore colistin resistance, without changing the lipid A profile. This work suggests that maintaining optimal membrane potential at slightly alkaline pH media by DbcA is responsible for proper modification of lipid A by Ara4N and provides evidence of lipid A modification-dependent and -independent mechanisms of colistin resistance in B. thailandensis.

Diversity and Geographic Distribution of Microsymbionts Associated With Invasive Mimosa Species in Southern China

In order to investigated diversity and geographic distribitution of rhizobia associated with invasive Mimosa species, Mimosa nodules and soils around the plants were sampled from five provinces in southern China. In total, 361 isolates were obtained from Mimosa pudica and Mimosa diplotricha in 25 locations. A multi-locus sequence analysis (MLSA) including 16S rRNA, atpD, dnaK, glnA, gyrB, and recA identified the isolates into eight genospecies corresponding to Paraburkhleria mimosarum, Paraburkholderia phymatum, Paraburkholeria carbensis, Cupriavidus taiwanensis, Cupriavidus sp., Rhizobium altiplani, Rhizobium mesoamericanum, and Rhizobium etli. The majority of the isolates were Cupriavidus (62.6%), followed by Paraburkholderia (33.5%) and Rhizobium (2.9%). Cupriavidus strains were more predominant in nodules of M. diplotricha (76.2) than in M. pudica (59.9%), and the distribution of P. phymatum in those two plant species was reverse (3.4:18.2%). Four symbiotypes were defined among the isolates based upon the phylogeny of nodA-nifH genes, represented by P. mimosarum, P. phymatum–P. caribensis, Cupriavidus spp., and Rhizobium spp. The species affiliation and the symbiotype division among the isolates demonstrated the multiple origins of Mimosa rhizobia in China: most were similar to those found in the original centers of Mimosa plants, but Cupriavidus sp. might have a local origin. The unbalanced distribution of symbionts between the two Mimosa species might be related to the soil pH, organic matter and available nitrogen; Cupriavidus spp. generally dominated most of the soils colonized by Mimosa in this study, but it had a particular preference for neutral-alkaline soils with low fertility whereas. While Paraburkholderia spp. preferred more acidic and fertile soils. The Rhizobium spp. tended to prefer neutral–acidic soils with high fertility soils.

Burkholderia insecticola triggers midgut closure in the bean bug Riptortus pedestris to prevent secondary bacterial infections of midgut crypts

In addition to abiotic triggers, biotic factors such as microbial symbionts can alter development of multicellular organisms. Symbiont-mediated morphogenesis is well-investigated in plants and marine invertebrates but rarely in insects despite the enormous diversity of insect-microbe symbioses. The bean bug Riptortus pedestris is associated with Burkholderia insecticola which are acquired from the environmental soil and housed in midgut crypts. To sort symbionts from soil microbiota, the bean bug develops a specific organ called the "constricted region" (CR), a narrow and symbiont-selective channel, located in the midgut immediately upstream of the crypt-bearing region. In this study, inoculation of fluorescent protein-labeled symbionts followed by spatiotemporal microscopic observations revealed that after the initial passage of symbionts through the CR, it closes within 12-18 h, blocking any potential subsequent infection events. The "midgut closure" developmental response was irreversible, even after symbiont removal from the crypts by antibiotics. It never occurred in aposymbiotic insects, nor in insects infected with nonsymbiotic bacteria or B. insecticola mutants unable to cross the CR. However, species of the genus Burkholderia and its outgroup Pandoraea that can pass the CR and partially colonize the midgut crypts induce the morphological alteration, suggesting that the molecular trigger signaling the midgut closure is conserved in this bacterial lineage. We propose that this drastic and quick alteration of the midgut morphology in response to symbiont infection is a mechanism for stabilizing the insect-microbe gut symbiosis and contributes to host-symbiont specificity in a symbiosis without vertical transmission.

Human Melioidosis

The causative agent of melioidosis, Burkholderia pseudomallei, a tier 1 select agent, is endemic in Southeast Asia and northern Australia, with increased incidence associated with high levels of rainfall. Increasing reports of this condition have occurred worldwide, with estimates of up to 165,000 cases and 89,000 deaths per year. The ecological niche of the organism has yet to be clearly defined, although the organism is associated with soil and water. The culture of appropriate clinical material remains the mainstay of laboratory diagnosis. Identification is best done by phenotypic methods, although mass spectrometric methods have been described. Serology has a limited diagnostic role. Direct molecular and antigen detection methods have limited availability and sensitivity. Clinical presentations of melioidosis range from acute bacteremic pneumonia to disseminated visceral abscesses and localized infections. Transmission is by direct inoculation, inhalation, or ingestion. Risk factors for melioidosis include male sex, diabetes mellitus, alcohol abuse, and immunosuppression. The organism is well adapted to intracellular survival, with numerous virulence mechanisms. Immunity likely requires innate and adaptive responses. The principles of management of this condition are drainage and debridement of infected material and appropriate antimicrobial therapy. Global mortality rates vary between 9% and 70%. Research into vaccine development is ongoing.

Paraburkholderia atlantica sp. nov. and Paraburkholderia franconis sp. nov., two new nitrogen-fixing nodulating species isolated from Atlantic forest soils in Brazil

A polyphasic study was conducted with 11 strains trapped by Mimosa pudica and Phaseolus vulgaris grown in soils of the Brazilian Atlantic Forest. In the phylogenetic analysis of the 16S rRNA gene, one clade of strains (Psp1) showed higher similarity with Paraburkholderia piptadeniae STM7183^T (99.6%), whereas the second (Psp6) was closely related to Paraburkholderia tuberum STM678^T (99%). An MLSA (multilocus sequence analysis) with four (recA, gyrB, trpB and gltB) housekeeping genes placed both Psp1 and Psp6 strains in new clades, and BOX-PCR profiles indicated high intraspecific genetic diversity within each clade. Values of digital DNA-DNA hybridization (dDDH) and average nucleotide identity (ANI) of the whole genome sequences were of 56.9 and 94.4% between the Psp1 strain CNPSo 3157^T and P. piptadeniae; and of 49.7% and 92.7% between the Psp6 strain CNPSo 3155^T and P. tuberum, below the threshold for species delimitation. In the nodC analysis, Psp1 strains clustered together with P. piptadeniae, while Psp6 did not group with any symbiotic Paraburkholderia. Other phenotypic, genotypic and symbiotic properties were evaluated. The polyphasic analysis supports that the strains represent two novel species, for which the names Paraburkholderia franconis sp. nov. with type strain CNPSo 3157^T (= ABIP 241, = LMG 31644) and Paraburkholderia atlantica sp. nov. with type strain CNPSo 3155^T (= ABIP 236, = LMG 31643) are proposed.

Highly competitive fungi manipulate bacterial communities in decomposing beech wood (Fagus sylvatica)

The bacterial communities in decomposing wood are receiving increased attention, but their interactions with wood-decay fungi are poorly understood. This is the first field study to test the hypothesis that fungi are responsible for driving bacterial communities in beech wood (Fagus sylvatica). A meta-genetic approach was used to characterise bacterial and fungal communities in wood that had been laboratory-colonised with known wood-decay fungi, and left for a year at six woodland sites. Alpha-, Beta- and Gammaproteobacteria and Acidobacteria were the proportionally dominant bacterial taxa, as in previous studies. Pre-colonising wood with decay fungi had a clear effect on the bacterial community, apparently via direct fungal influence; the bacterial and fungal communities present at the time of collection explained nearly 60% of their mutual covariance. Site was less important than fungal influence in determining bacterial communities, but the effects of pre-colonisation were more pronounced at some sites than at others. Wood pH was also a strong bacterial predictor, but was itself under considerable fungal influence. Burkholderiaceae and Acidobacteriaceae showed directional responses against the trend of the bacterial community as a whole.

Host-symbiont specificity determined by microbe-microbe competition in an insect gut

Despite the omnipresence of specific host-symbiont associations with acquisition of the microbial symbiont from the environment, little is known about how the specificity of the interaction evolved and is maintained. The bean bug Riptortus pedestris acquires a specific bacterial symbiont of the genus Burkholderia from environmental soil and harbors it in midgut crypts. The genus Burkholderia consists of over 100 species, showing ecologically diverse lifestyles, and including serious human pathogens, plant pathogens, and nodule-forming plant mutualists, as well as insect mutualists. Through infection tests of 34 Burkholderia species and 18 taxonomically diverse bacterial species, we demonstrate here that nonsymbiotic Burkholderia and even its outgroup Pandoraea could stably colonize the gut symbiotic organ and provide beneficial effects to the bean bug when inoculated on aposymbiotic hosts. However, coinoculation revealed that the native symbiont always outcompeted the nonnative bacteria inside the gut symbiotic organ, explaining the predominance of the native Burkholderia symbiont in natural bean bug populations. Hence, the abilities for colonization and cooperation, usually thought of as specific traits of mutualists, are not unique to the native Burkholderia symbiont but, to the contrary, competitiveness inside the gut is a derived trait of the native symbiont lineage only and was thus critical in the evolution of the insect gut symbiont.

Contrasting environmental factors drive bacterial and eukaryotic community successions in freshly deglaciated soils

Glacier retreats expose deglaciated soils to microbial colonization and succession; however, the differences in drivers of bacterial and eukaryotic succession remain largely elusive. We explored soil bacterial and eukaryotic colonization and yearly community succession along a deglaciation chronosequence (10 years) on the Tibetan Plateau using qPCR, terminal restriction fragment length polymorphism (T-RFLP) and sequencing of clone libraries. The results exhibited that bacteria and eukaryotes rapidly colonized the soils in the first year of deglaciation, thereafter slowly increasing from 107 up to 1010 and 1011 gene copies g-1 soil, respectively. Bacterial and eukaryotic community changes were observed to group into distinct stages, including early (0-2 year old), transition (3-5 year old) and late stages (6-10 year old). Bacterial community succession was dominantly driven by soil factors (47.7%), among which soil moisture played a key role by explaining 26.9% of the variation. In contrast, eukaryotic community succession was dominantly driven by deglaciation age (22.2%). The dominant bacterial lineage was Cyanobacteria, which rapidly decreased from the early to the transition stage. Eukaryotes were dominated by glacier-originated Cercozoa in early stage soils, while green algae Chlorophyta substantially increased in late stage soils. Our findings revealed contrasting environmental factors driving bacterial and eukaryotic community successions.

Burkholderia pseudomallei, the causative agent of melioidosis, is rare but ecologically established and widely dispersed in the environment in Puerto Rico

BACKGROUND

Burkholderia pseudomallei is a soil-dwelling bacterium and the causative agent of melioidosis. The global burden and distribution of melioidosis is poorly understood, including in the Caribbean. B. pseudomallei was previously isolated from humans and soil in eastern Puerto Rico but the abundance and distribution of B. pseudomallei in Puerto Rico as a whole has not been thoroughly investigated.

METHODOLOGY/PRINCIPAL FINDINGS

We collected 600 environmental samples (500 soil and 100 water) from 60 sites around Puerto Rico. We identified B. pseudomallei by isolating it via culturing and/or using PCR to detect its DNA within complex DNA extracts. Only three adjacent soil samples from one site were positive for B. pseudomallei with PCR; we obtained 55 isolates from two of these samples. The 55 B. pseudomallei isolates exhibited fine-scale variation in the core genome and contained four novel genomic islands. Phylogenetic analyses grouped Puerto Rico B. pseudomallei isolates into a monophyletic clade containing other Caribbean isolates, which was nested inside a larger clade containing all isolates from Central/South America. Other Burkholderia species were commonly observed in Puerto Rico; we cultured 129 isolates from multiple soil and water samples collected at numerous sites around Puerto Rico, including representatives of B. anthina, B. cenocepacia, B. cepacia, B. contaminans, B. glumae, B. seminalis, B. stagnalis, B. ubonensis, and several unidentified novel Burkholderia spp.

CONCLUSIONS/SIGNIFICANCE

B. pseudomallei was only detected in three soil samples collected at one site in north central Puerto Rico with only two of those samples yielding isolates. All previous human and environmental B. pseudomallei isolates were obtained from eastern Puerto Rico. These findings suggest B. pseudomallei is ecologically established and widely dispersed in the environment in Puerto Rico but rare. Phylogeographic patterns suggest the source of B. pseudomallei populations in Puerto Rico and elsewhere in the Caribbean may have been Central or South America.

Deciphering rhizosphere microbiome assembly of wild and modern common bean (Phaseolus vulgaris) in native and agricultural soils from Colombia

BACKGROUND

Modern crop varieties are typically cultivated in agriculturally well-managed soils far from the centers of origin of their wild relatives. How this habitat expansion impacted plant microbiome assembly is not well understood.

RESULTS

Here, we investigated if the transition from a native to an agricultural soil affected rhizobacterial community assembly of wild and modern common bean (Phaseolus vulgaris) and if this led to a depletion of rhizobacterial diversity. The impact of the bean genotype on rhizobacterial assembly was more prominent in the agricultural soil than in the native soil. Although only 113 operational taxonomic units (OTUs) out of a total of 15,925 were shared by all eight bean accessions grown in native and agricultural soils, this core microbiome represented a large fraction (25.9%) of all sequence reads. More OTUs were exclusively found in the rhizosphere of common bean in the agricultural soil as compared to the native soil and in the rhizosphere of modern bean accessions as compared to wild accessions. Co-occurrence analyses further showed a reduction in complexity of the interactions in the bean rhizosphere microbiome in the agricultural soil as compared to the native soil.

CONCLUSIONS

Collectively, these results suggest that habitat expansion of common bean from its native soil environment to an agricultural context had an unexpected overall positive effect on rhizobacterial diversity and led to a stronger bean genotype-dependent effect on rhizosphere microbiome assembly.

The Eno Gene of Burkholderia cenocepacia Strain 71-2 is Involved in Phosphate Solubilization

Bacterial strain 71-2 with phosphate-solubilizing activity was isolated from tobacco rhizosphere and classified as Burkholderia cenocepacia based on sequence analyses of 16S rRNA and recA genes. To learn phosphate-solubilizing mechanisms of 71-2, mutants showing reduced solubilizing phosphate activity were obtained using the EZ-Tn5 transposon. Mutant 71-2-MT51 was reduced in the solubilizing phosphate content to 34.36% as compared with the wild-type strain 71-2. The disrupted gene in 71-2-MT51 was cloned and sequenced, and the putative protein from the gene shared 65.26% identity to protein sequences of enolase from Escherichia coli, which suggests the gene encodes an enzyme of enolase. Complementation analyzing showed that Eno was responsible for phosphate solubilizing for B. cenocepacia strain 71-2. To our knowledge, this is the first report of Eno involved in phosphate solubilizing in B. cenocepacia as well as in other bacteria.

Burkholderia and Paraburkholderia are Predominant Soybean Rhizobial Genera in Venezuelan Soils in Different Climatic and Topographical Regions

The climate, topography, fauna, and flora of Venezuela are highly diverse. However, limited information is currently available on the characterization of soybean rhizobia in Venezuela. To clarify the physiological and genetic diversities of soybean rhizobia in Venezuela, soybean root nodules were collected from 11 soil types located in different topographical regions. A total of 395 root nodules were collected and 120 isolates were obtained. All isolates were classified in terms of stress tolerance under different concentrations of NaCl and Al3+. The tolerance levels of isolates to NaCl and Al3+ varied. Based on sampling origins and stress tolerance levels, 44 isolates were selected for further characterization. An inoculation test indicated that all isolates showed the capacity for root nodulation on soybean. Based on multilocus sequence typing (MLST), 20 isolates were classified into the genera Rhizobium and Bradyrhizobium. The remaining 24 isolates were classified into the genus Burkholderia or Paraburkholderia. There is currently no evidence to demonstrate that the genera Burkholderia and Paraburkholderia are the predominant soybean rhizobia in agricultural fields. Of the 24 isolates classified in (Para) Burkholderia, the nodD-nodB intergenic spacer regions of 10 isolates and the nifH gene sequences of 17 isolates were closely related to the genera Rhizobium and Bradyrhizobium, respectively. The root nodulation numbers of five (Para) Burkholderia isolates were higher than those of the 20 α-rhizobia. Furthermore, among the 44 isolates tested, one Paraburkholderia isolate exhibited the highest nitrogen-fixation activity in root nodules.

A selective genome-guided method for environmental Burkholderia isolation

The genus Burkholderia is an emerging source of novel natural products chemistry, yet to date few methods exist for the selective isolation of strains of this genus from the environment. More broadly, tools to efficiently design selection media for any given genus would be of significant value to the natural products and microbiology communities. Using a modification of the recently published SMART protocol, we have developed a two-stage isolation protocol for strains from the genus Burkholderia. This method uses a combination of selective agar isolation media and multiplexed PCR profiling to derive Burkholderia strains from environmental samples with 95% efficiency. Creation of this new method paves the way for the systematic exploration of natural products chemistry from this important genus and offers new insight into potential methods for selective isolation method development for other priority genera.

Seed-born Burkholderia glumae Infects Rice Seedling and Maintains Bacterial Population during Vegetative and Reproductive Growth Stage

Rice world production is affected due to the growing impact of diseases such as bacterial panicle blight, produced by Burkholderia glumae. The pathogen-induced symptoms include seedling rot, grain rot and leaf-sheath browning in rice plants. It is currently recognized the entrance of this pathogen to the plant, from infected seeds and from environmental sources of the microorganism. However, it is still not fully elucidated the dynamics and permanence of the pathogen in the plant, from its entry until the development of disease symptoms in seedlings or panicles. In this work it was evaluated the infection of B. glumae rice plants, starting from inoculated seeds and substrates, and its subsequent monitoring after infection. Various organs of the plant during the vegetative stage and until the beginning of the reproductive stage, were evaluated. In both inoculation models, the bacteria was maintained in the plant as an endophyte between 1 × 101 and 1 × 105 cfu of B. glumae.g-1 of plant throughout the vegetative stage. An increase of bacterial population towards initiation of the panicle was observed, and in the maturity of the grain, an endophyte population was identified in the flag leaf at 1 × 106 cfu of B. glumae.g-1 fresh weight of rice plant, conducting towards the symptoms of bacterial panicle blight. The results found, suggest that B. glumae in rice plants developed from infected seeds or from the substrate, can colonize seedlings, establishing and maintaining a bacterial population over time, using rice plants as habitat to survive endophyticly until formation of bacterial panicle blight symptoms.

Biodiversity of cultivable Burkholderia species in Argentinean soils under no-till agricultural practices

No-tillage crop production has revolutionized the agriculture worldwide. In our country more than 30 Mha are currently cultivated under no-till schemes, stressing the importance of this management system for crop production. It is widely recognized that soil microbiota is altered under different soil managements. In this regard the structure of Burkholderia populations is affected by soils management practices such as tillage, fertilization, or crop rotation. The stability of these structures, however, has not been evaluated under sustainable schemes where the impact of land practices could be less deleterious to physicochemical soils characteristics. In order to assess the structure of Burkholderia spp. populations in no-till schemes, culturable Burkholderia spp. strains were quantified and their biodiversity evaluated. Results showed that Burkholderia spp. biodiversity, but not their abundance, clearly displayed a dependence on agricultural managements. We also showed that biodiversity was mainly influenced by two soil factors: Total Organic Carbon and Total Nitrogen. Results showed that no-till schemes are not per se sufficient to maintain a richer Burkholderia spp. soil microbiota, and additional traits should be considered when sustainability of productive soils is a goal to fulfil productive agricultural schemes.

Differential Preference of Burkholderia and Mesorhizobium to pH and Soil Types in the Core Cape Subregion, South Africa

Over 760 legume species occur in the ecologically-heterogeneous Core Cape Subregion (CCR) of South Africa. This study tested whether the main symbionts of CCR legumes (Burkholderia and Mesorhizobium) are phylogenetically structured by altitude, pH and soil types. Rhizobial strains were isolated from field nodules of diverse CCR legumes and sequenced for 16S ribosomic RNA (rRNA), recombinase A (recA) and N-acyltransferase (nodA). Phylogenetic analyses were performed using Bayesian and maximum likelihood techniques. Phylogenetic signals were determined using the D statistic for soil types and Pagel’s λ for altitude and pH. Phylogenetic relationships between symbionts of the narrowly-distributed Indigofera superba and those of some widespread CCR legumes were also determined. Results showed that Burkholderia is restricted to acidic soils, while Mesorhizobium occurs in both acidic and alkaline soils. Both genera showed significant phylogenetic clustering for pH and most soil types, but not for altitude. Therefore, pH and soil types influence the distribution of Burkholderia and Mesorhizobium in the CCR. All strains of Indigofera superba were identified as Burkholderia, and they were nested within various clades containing strains from outside its distribution range. It is, therefore, hypothesized that I. superba does not exhibit rhizobial specificity at the intragenic level. Implications for CCR legume distributions are discussed.

Soil microbial community responses to acid exposure and neutralization treatment

Changes in microbial community induced by acid shock were studied in the context of potential release of acids to the environment due to chemical accidents. The responses of microbial communities in three different soils to the exposure to sulfuric or hydrofluoric acid and to the subsequent neutralization treatment were investigated as functions of acid concentration and exposure time by using 16S-rRNA gene based pyrosequencing and DGGE (Denaturing Gradient Gel Electrophoresis). Measurements of soil pH and dissolved ion concentrations revealed that the added acids were neutralized to different degrees, depending on the mineral composition and soil texture. Hydrofluoric acid was more effectively neutralized by the soils, compared with sulfuric acid at the same normality. Gram-negative ß-Proteobacteria were shown to be the most acid-sensitive bacterial strains, while spore-forming Gram-positive Bacilli were the most acid-tolerant. The results of this study suggest that the Gram-positive to Gram-negative bacterial ratio may serve as an effective bio-indicator in assessing the impact of the acid shock on the microbial community. Neutralization treatments helped recover the ratio closer to their original values. The findings of this study show that microbial community changes as well as geochemical changes such as pH and dissolved ion concentrations need to be considered in estimating the impact of an acid spill, in selecting an optimal remediation strategy, and in deciding when to end remedial actions at the acid spill impacted site.

Parallelized, Aerobic, Single Carbon-Source Enrichments from Different Natural Environments Contain Divergent Microbial Communities

Microbial communities that inhabit environments such as soil can contain thousands of distinct taxa, yet little is known about how this diversity is maintained in response to environmental perturbations such as changes in the availability of carbon. By utilizing aerobic substrate arrays to examine the effect of carbon amendment on microbial communities taken from six distinct environments (soil from a temperate prairie and forest, tropical forest soil, subalpine forest soil, and surface water and soil from a palustrine emergent wetland), we examined how carbon amendment and inoculum source shape the composition of the community in each enrichment. Dilute subsamples from each environment were used to inoculate 96-well microtiter plates containing triplicate wells amended with one of 31 carbon sources from six different classes of organic compounds (phenols, polymers, carbohydrates, carboxylic acids, amines, amino acids). After incubating each well aerobically in the dark for 72 h, we analyzed the composition of the microbial communities on the substrate arrays as well as the initial inocula by sequencing 16S rRNA gene amplicons using the Illumina MiSeq platform. Comparisons of alpha and beta diversity in these systems showed that, while the composition of the communities that grow to inhabit the wells in each substrate array diverges sharply from that of the original community in the inoculum, these enrichment communities are still strongly affected by the inoculum source. We found most enrichments were dominated by one or several OTUs most closely related to aerobes or facultative anaerobes from the Proteobacteria (e.g., Pseudomonas, Burkholderia, and Ralstonia) or Bacteroidetes (e.g., Chryseobacterium). Comparisons within each substrate array based on the class of carbon source further show that the communities inhabiting wells amended with a carbohydrate differ significantly from those enriched with a phenolic compound. Selection therefore seems to play a role in shaping the communities in the substrate arrays, although some stochasticity is also seen whereby several replicate wells within a single substrate array display strongly divergent community compositions. Overall, the use of highly parallel substrate arrays offers a promising path forward to study the response of microbial communities to perturbations in a changing environment.

A novel Burkholderia ambifaria strain able to degrade the mycotoxin fusaric acid and to inhibit Fusarium spp. growth

Fusaric acid (FA) is a fungal metabolite produced by several Fusarium species responsible for wilts and root rot diseases of a great variety of plants. Bacillus spp. and Pseudomonas spp. have been considered as promising biocontrol agents against phytopathogenic Fusarium spp., however it has been demonstrated that FA negatively affects growth and production of some antibiotics in these bacteria. Thus, the capability to degrade FA would be a desirable characteristic in bacterial biocontrol agents of Fusarium wilt. Taking this into account, bacteria isolated from the rhizosphere of barley were screened for their ability to use FA as sole carbon and energy source. One strain that fulfilled this requirement was identified according to sequence analysis of 16S rRNA, gyrB and recA genes as Burkholderia ambifaria. This strain, designated T16, was able to grow with FA as sole carbon, nitrogen and energy source and also showed the ability to detoxify FA in barley seedlings. This bacterium also exhibited higher growth rate, higher cell densities, longer survival, higher levels of indole-3-acetic acid (IAA) production, enhanced biofilm formation and increased resistance to different antibiotics when cultivated in Luria Bertani medium at pH 5.3 compared to pH 7.3. Furthermore, B. ambifaria T16 showed distinctive plant growth-promoting features, such as siderophore production, phosphate-solubilization, 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity, in vitro antagonism against Fusarium spp. and improvement of grain yield when inoculated to barley plants grown under greenhouse conditions. This strain might serve as a new source of metabolites or genes for the development of novel FA-detoxification systems.

Vaccines for the Prevention of Melioidosis and Glanders

Purpose of review

Burkholderia pseudomallei's and Burkholderia mallei's high rate of infectivity, limited treatment options, and potential use as biological warfare agents underscore the need for development of effective vaccines against these bacteria. Research efforts focused on vaccines against these bacteria are in pre-clinical stages, with no approved formulations currently on the market.

Recent findings

Several live attenuated and subunit vaccine formulations have been evaluated in animal studies, with no reports of significant long term survival after lethal challenge.

Summary

This review encompasses the most current vaccine strategies to prevent B. pseudomallei and B. mallei infections while providing insight for successful vaccines moving forward.

Burkholderia pseudomallei in a lowland rice paddy: seasonal changes and influence of soil depth and physico-chemical properties

Melioidosis, a severe infection with the environmental bacterium Burkholderia pseudomallei, is being recognised increasingly frequently. What determines its uneven distribution within endemic areas is poorly understood. We cultured soil from a rice field in Laos for B. pseudomallei at different depths on 4 occasions over a 13-month period. We also measured physical and chemical parameters in order to identify associated characteristics. Overall, 195 of 653 samples (29.7%) yielded B. pseudomallei. A higher prevalence of B. pseudomallei was found at soil depths greater than the 30 cm currently recommended for B. pseudomallei environmental sampling. B. pseudomallei was associated with a high soil water content and low total nitrogen, carbon and organic matter content. Our results suggested that a sampling grid of 25 five metre square quadrats (i.e. 25 × 25 m) should be sufficient to detect B. pseudomallei at a given location if samples are taken at a soil depth of at least 60 cm. However, culture of B. pseudomallei in environmental samples is difficult and liable to variation. Future studies should both rely on molecular approaches and address the micro-heterogeneity of soil when investigating physico-chemical associations with the presence of B. pseudomallei.

High-quality draft genome sequence of Rhizobium mesoamericanum strain STM6155, a Mimosa pudica microsymbiont from New Caledonia

Rhizobium mesoamericanum STM6155 (INSCD = ATYY01000000) is an aerobic, motile, Gram-negative, non-spore-forming rod that can exist as a soil saprophyte or as an effective nitrogen fixing microsymbiont of the legume Mimosa pudica L.. STM6155 was isolated in 2009 from a nodule of the trap host M. pudica grown in nickel-rich soil collected near Mont Dore, New Caledonia. R. mesoamericanum STM6155 was selected as part of the DOE Joint Genome Institute 2010 Genomic Encyclopedia for Bacteria and Archaea-Root Nodule Bacteria (GEBA-RNB) genome sequencing project. Here we describe the symbiotic properties of R. mesoamericanum STM6155, together with its genome sequence information and annotation. The 6,927,906 bp high-quality draft genome is arranged into 147 scaffolds of 152 contigs containing 6855 protein-coding genes and 71 RNA-only encoding genes. Strain STM6155 forms an ANI clique (ID 2435) with the sequenced R. mesoamericanum strain STM3625, and the nodulation genes are highly conserved in these strains and the type strain of Rhizobium grahamii CCGE501T. Within the STM6155 genome, we have identified a chr chromate efflux gene cluster of six genes arranged into two putative operons and we postulate that this cluster is important for the survival of STM6155 in ultramafic soils containing high concentrations of chromate.

Rhizobacterial Community Structures Associated with Native Plants Grown in Chilean Extreme Environments

Chile is topographically and climatically diverse, with a wide array of diverse undisturbed ecosystems that include native plants that are highly adapted to local conditions. However, our understanding of the diversity, activity, and role of rhizobacteria associated with natural vegetation in undisturbed Chilean extreme ecosystems is very poor. In the present study, the combination of denaturing gradient gel electrophoresis and 454-pyrosequencing approaches was used to describe the rhizobacterial community structures of native plants grown in three representative Chilean extreme environments: Atacama Desert (ATA), Andes Mountains (AND), and Antarctic (ANT). Both molecular approaches revealed the presence of Proteobacteria, Bacteroidetes, and Actinobacteria as the dominant phyla in the rhizospheres of native plants. Lower numbers of operational taxonomic units (OTUs) were observed in rhizosphere soils from ATA compared with AND and ANT. Both approaches also showed differences in rhizobacterial community structures between extreme environments and between plant species. The differences among plant species grown in the same environment were attributed to the higher relative abundance of classes Gammaproteobacteria and Alphaproteobacteria. However, further studies are needed to determine which environmental factors regulate the structures of rhizobacterial communities, and how (or if) specific bacterial groups may contribute to the growth and survival of native plants in each Chilean extreme environments.

PCR detection of Burkholderia multivorans in water and soil samples

Background

Although semi-selective growth media have been developed for the isolation of Burkholderia cepacia complex bacteria from the environment, thus far Burkholderia multivorans has rarely been isolated from such samples. Because environmental B. multivorans isolates mainly originate from water samples, we hypothesized that water rather than soil is its most likely environmental niche. The aim of the present study was to assess the occurrence of B. multivorans in water samples from Flanders (Belgium) using a fast, culture-independent PCR assay.

Results

A nested PCR approach was used to achieve high sensitivity, and specificity was confirmed by sequencing the resulting amplicons. B. multivorans was detected in 11 % of the water samples (n = 112) and 92 % of the soil samples (n = 25) tested. The percentage of false positives was higher for water samples compared to soil samples, showing that the presently available B. multivorans recA primers lack specificity when applied to the analysis of water samples.

Conclusions

The results of the present study demonstrate that B. multivorans DNA is commonly present in soil samples and to a lesser extent in water samples in Flanders (Belgium).

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-016-0801-9) contains supplementary material, which is available to authorized users.