Molecular characterization of the pSinB plasmid of the arsenite oxidizing, metallotolerant Sinorhizobium sp. M14 - insight into the heavy metal resistome of sinorhizobial extrachromosomal replicons

Characteristics and Comparative Genomic Analysis of a Novel Virus, VarioGold, the First Bacteriophage of Variovorax

Variovorax represents a widespread and ecologically significant genus of soil bacteria. Despite the ecological importance of these bacteria, our knowledge about the viruses infecting Variovorax spp. is quite poor. This study describes the isolation and characterization of the mitomycin-induced phage, named VarioGold. To the best of our knowledge, VarioGold represents the first characterized virus for this genus. Comparative genomic analyses suggested that VarioGold is distinct from currently known bacteriophages at both the nucleotide and protein levels; thus, it could be considered a new virus genus. In addition, another 37 prophages were distinguished in silico within the complete genomic sequences of Variovorax spp. that are available in public databases. The similarity networking analysis highlighted their general high diversity, which, despite clustering with previously described phages, shows their unique genetic load. Therefore, the novelty of Variovorax phages warrants the great enrichment of databases, which could, in turn, improve bioinformatic strategies for finding (pro)phages.

Isolation and genomic characterization of a pathogenic Providencia rettgeri strain G0519 in turtle Trachemys scripta

Providencia rettgeri infection has occurred occasionally in aquaculture, but is rare in turtles. Here, a pathogenic P. rettgeri strain G0519 was isolated from a diseased slider turtle (Trachemys scripta) in China, and qPCR assay was established for the RTX toxin (rtxD) gene. Histopathological examination showed that many inflammatory cells were infiltrated into heart, liver and intestine, as well as the necrosis of liver, kidney and spleen. The genome consisted of one circular chromosome (4.493 Mb) and one plasmid (18.8 kb), and predicted to contain 4170 and 19 protein-coding genes, respectively. Multiple pathogenic and virulence factors (e.g., fimbria, adhesion, invasion, toxin, hemolysin, chemotaxis, secretion system), multidrug-resistant genes (e.g., ampC, per-1, oxa-1, sul1, tetR) and a novel genomic resistance island PRI519 were identified. Comparative genome analysis revealed the closest relationship was with P. rettgeri, and with P. heimbachae closer than with other Providencia spp. To our knowledge, this was first report on genomic characterization of multidrug-resistant pathogenic P. rettgeri in cultured turtles.

Enhancing the plants growth and arsenic uptake from soil using arsenite-oxidizing bacteria

Plants, that naturally inhabit arsenic-contaminated areas may be used for effective arsenic-uptake from soil. The efficiency of this process may be increased by the reducing arsenic phytotoxicity and stimulating the activity of indigenous soil microbiota. As we showed, it can be achieved by the bioaugmenting of soil with arsenite-oxidizing bacteria (AOB). This study aimed to investigate the influence of soil bioaugmentation with AOB on the structure, quantity, and activity of the indigenous soil microbiota as well as to estimate the effect of such changes on the morphology, growth rate, and arsenic-uptake efficiency of plants. Plants-microbes interactions were investigated using the effective arsenites oxidizer Ensifer sp. M14 and the native plant alfalfa. The experiments were performed both in potted garden soil enriched with arsenic and in highly arsenic polluted, natural soil. The presence of M14 strain in soil contributed to the increase both in plants growth intensity and arsenic-uptake efficiency with regard to the soil without M14. After 40 days of plants culture, their average biomass increased by about 60% compared to non-bioaugmented soil, while the arsenic accumulation increased more than two times. The soil bioaugmentation contributed also to the increase in the quantity and activity of soil microorganisms without disturbing the natural microbial community structure. In the bioaugmented soil, the noticable increase in the quantity of heterotrophic, denitrifying, nitrifying and cellulolytic bacteria as well as in the activity of dehydrogenases and cellulases were observed. Soil bioaugmentation with M14 enables the application of native and commonly occurring plant species for enhancing the treatment of arsenic-contaminated soil. This in situ strategy may constitute a valuable alternative both to the chemical and physical methods of arsenic removal from soil and to the biological ways based on the arsenic hyperaccumulating plants and/or the arsenic mobilizing bacteria.

Harnessing Rhizobia to Improve Heavy-Metal Phytoremediation by Legumes

Rhizobia are bacteria that can form symbiotic associations with plants of the Fabaceae family, during which they reduce atmospheric di-nitrogen to ammonia. The symbiosis between rhizobia and leguminous plants is a fundamental contributor to nitrogen cycling in natural and agricultural ecosystems. Rhizobial microsymbionts are a major reason why legumes can colonize marginal lands and nitrogen-deficient soils. Several leguminous species have been found in metal-contaminated areas, and they often harbor metal-tolerant rhizobia. In recent years, there have been numerous efforts and discoveries related to the genetic determinants of metal resistance by rhizobia, and on the effectiveness of such rhizobia to increase the metal tolerance of host plants. Here, we review the main findings on the metal resistance of rhizobia: the physiological role, evolution, and genetic determinants, and the potential to use native and genetically-manipulated rhizobia as inoculants for legumes in phytoremediation practices.

Genomic and Biotechnological Characterization of the Heavy-Metal Resistant, Arsenic-Oxidizing Bacterium Ensifer sp. M14

Ensifer (Sinorhizobium) sp. M14 is an efficient arsenic-oxidizing bacterium (AOB) that displays high resistance to numerous metals and various stressors. Here, we report the draft genome sequence and genome-guided characterization of Ensifer sp. M14, and we describe a pilot-scale installation applying the M14 strain for remediation of arsenic-contaminated waters. The M14 genome contains 6874 protein coding sequences, including hundreds not found in related strains. Nearly all unique genes that are associated with metal resistance and arsenic oxidation are localized within the pSinA and pSinB megaplasmids. Comparative genomics revealed that multiple copies of high-affinity phosphate transport systems are common in AOBs, possibly as an As-resistance mechanism. Genome and antibiotic sensitivity analyses further suggested that the use of Ensifer sp. M14 in biotechnology does not pose serious biosafety risks. Therefore, a novel two-stage installation for remediation of arsenic-contaminated waters was developed. It consists of a microbiological module, where M14 oxidizes As(III) to As(V) ion, followed by an adsorption module for As(V) removal using granulated bog iron ores. During a 40-day pilot-scale test in an abandoned gold mine in Zloty Stok (Poland), water leaving the microbiological module generally contained trace amounts of As(III), and dramatic decreases in total arsenic concentrations were observed after passage through the adsorption module. These results demonstrate the usefulness of Ensifer sp. M14 in arsenic removal performed in environmental settings.

Double genetically modified symbiotic system for improved Cu phytostabilization in legume roots

Excess copper (Cu) in soils has deleterious effects on plant growth and can pose a risk to human health. In the last decade, legume-rhizobium symbioses became attractive biotechnological tools for metal phytostabilization. For this technique being useful, metal-tolerant symbionts are required, which can be generated through genetic manipulation.In this work, a double symbiotic system was engineered for Cu phytostabilization: On the one hand, composite Medicago truncatula plants expressing the metallothionein gene mt4a from Arabidopsis thaliana in roots were obtained to improve plant Cu tolerance. On the other hand, a genetically modified Ensifer medicae strain, expressing copper resistance genes copAB from Pseudomonas fluorescens driven by a nodulation promoter, nifHp, was used for plant inoculation. Our results indicated that expression of mt4a in composite plants ameliorated plant growth and nodulation and enhanced Cu tolerance. Lower levels of ROS-scavenging enzymes and of thiobarbituric acid reactive substances (TBARS), such as malondialdehyde (a marker of lipid peroxidation), suggested reduced oxidative stress. Furthermore, inoculation with the genetically modified Ensifer further improved root Cu accumulation without altering metal loading to shoots, leading to diminished values of metal translocation from roots to shoots. The double modified partnership is proposed as a suitable tool for Cu rhizo-phytostabilization.