Uptake of Selenite by Rahnella aquatilis HX2 Involves the Aquaporin AqpZ and Na+/H+ Antiporter NhaA.

Microbial transformation of selenite [Se(IV)] to elemental selenium nanoparticles (SeNPs) is known to be an important process for removing toxic soluble selenium (Se) oxyanions and recovery of Se from the environment as valuable nanoparticles. However, the mechanism of selenite uptake by microorganisms, the first step through which Se exerts its cellular function, remains not well studied. In this study, the effects of selenite concentration, time, pH, metabolic inhibitors, and anionic analogues on selenite uptake in Rahnella aquatilis HX2 were investigated. Selenite uptake by R. aquatilis HX2 was concentration- and time-dependent, and its transport activity was significantly dependent on pH. In addition, selenite uptake in R. aquatilis HX2 was significantly inhibited by the aquaporin inhibitor AgNO3 and sulfite (SO32-), and partially inhibited by carbonyl cyanide m-chlorophenyl hydrazone (CCCP) and 2,4-dinitrophenol (2,4-DNP) treatments. Three mutants with in-frame deletions of aqpZ, glpF, and nhaA genes were constructed. The transport assay showed that the water channel protein AqpZ, and not GlpF, was a key channel of selenite uptake by R. aquatilis HX2, and sulfite and selenite had a common uptake pathway. In addition, the Na+/H+ antiporter NhaA is also involved in selenite uptake in R. aquatilis HX2.

The small RNA chaperone Hfq is a critical regulator for bacterial biosynthesis of selenium nanoparticles and motility in Rahnella aquatilis.

The RNA chaperone, Hfq, is a global post-transcriptional regulator that plays an important role in regulating pleiotropic functions, such as cell growth and motility, stress tolerance, and virulence to host, in many Gram-negative bacteria. This study examined the functional roles of Hfq in Rahnella aquatilis HX2, a plant beneficial, selenium nanoparticles (SeNPs)-producing soil bacterium. A mutant HX2∆hfq with an in-frame deletion within the hfq gene in R. aquatilis HX2 was constructed and tested for various phenotypic features. Bacterial growth, motility, selenite reduction, and SeNPs production were compared between the mutant, the wild-type, and the complementation strain. The hfq gene deletion delayed the growth of strain HX2, with a lower bacterial population during the stationary phase, and significantly impaired the swimming motility of the bacterium, showing a smaller motility ring on the plate. The hfq mutation also dramatically declined microbial-induced reduction of selenite and SeNPs production in HX2, which was independent of cell growth. The introduction of a trans-expressed hfq gene into HX2∆hfq for complementation completely restored impacted phenotypes. In addition, reverse transcription real-time quantitative PCR (RT-qPCR) analysis revealed that the expression of ten genes involved in bacterial growth and survival, motility and chemotaxis, and selenite or seleno-compound metabolism were influenced by Hfq loss-of-function by at least two-fold. Six genes including two involved in SeNPs production were positively regulated by hfq, while other four genes were negatively regulated. Homolog search suggested that the rprA gene might encode a small RNA regulated by Hfq in R. aquatilis HX2. Overall, the present study provides novel information about the function of Hfq and the regulation of bacterial biosynthesis of SeNPs.

Rahnella spp. are commonly isolated from onion (Allium cepa) bulbs and are weakly pathogenic.

AIMS: Bacterial decays of onion bulbs have serious economic consequences for growers, but the aetiologies of these diseases are often unclear. We aimed to determine the role of Rahnella, which we commonly isolated from bulbs in the United States and Norway, in onion disease. METHODS AND RESULTS: Isolated bacteria were identified by sequencing of housekeeping genes and/or fatty acid methyl ester analysis. A subset of Rahnella spp. strains was also assessed by multilocus sequence analysis (MLSA); most onion strains belonged to two clades that appear closely related to R. aquatilis. All tested strains from both countries caused mild symptoms in onion bulbs but not leaves. Polymerase chain reaction primers were designed and tested against strains from known species of Rahnella. Amplicons were produced from strains of R. aquatilis, R. victoriana, R. variigena, R. inusitata and R. bruchi, and from one of the two strains of R. woolbedingensis. CONCLUSIONS: Based on binational testing, strains of Rahnella are commonly associated with onions, and they are capable of causing mild symptoms in bulbs. SIGNIFICANCE AND IMPACT OF THE STUDY: While Rahnella strains are commonly found within field-grown onions and they are able to cause mild symptoms, the economic impact of Rahnella-associated symptoms remains unclear.

Cold-adapted and rhizosphere-competent strain of Rahnella sp. with broad-spectrum plant growth-promotion potential.

A phosphate-solubilizing bacterial strain isolated from Hippophae rhamnoides rhizosphere was identified as Rahnella sp. based on its phenotypic features and 16S rRNA gene sequence. The bacterial strain showed the growth characteristics of a cold-adapted psychrotroph, with the multiple plant growth-promoting traits of inorganic and organic phosphate solubilization, 1-aminocyclopropane-1- carboxylate-deaminase activity, ammonia generation, and siderophore production. The strain also produced indole- 3-acetic acid, indole-3-acetaldehyde, indole-3-acetamide, indole-3-acetonitrile, indole-3-lactic acid, and indole-3- pyruvic acid in tryptophan-supplemented nutrient broth. Gluconic, citric and isocitric acids were the major organic acids detected during tricalcium phosphate solubilization. A rifampicin-resistant mutant of the strain exhibited high rhizosphere competence without disturbance to the resident microbial populations in pea rhizosphere. Seed bacterization with a charcoal-based inoculum significantly increased growth in barley, chickpea, pea, and maize under the controlled environment. Microplot testing of the inoculum at two different locations in pea also showed significant increase in growth and yield. The attributes of coldtolerance, high rhizosphere competence, and broad-spectrum plant growth-promoting activity exhibited the potential of Rahnella sp. BIHB 783 for increasing agriculture productivity.

Aerobic uranium (VI) bioprecipitation by metal-resistant bacteria isolated from radionuclide- and metal-contaminated subsurface soils.

In this study, the immobilization of toxic uranium [U(VI)] mediated by the intrinsic phosphatase activities of naturally occurring bacteria isolated from contaminated subsurface soils was examined. The phosphatase phenotypes of strains belonging to the genera, Arthrobacter, Bacillus and Rahnella, previously isolated from subsurface soils at the US Department of Energy's (DOE) Oak Ridge Field Research Center (ORFRC), were determined. The ORFRC represents a unique, extreme environment consisting of highly acidic soils with co-occurring heavy metals, radionuclides and high nitrate concentrations. Isolates exhibiting phosphatase-positive phenotypes indicative of constitutive phosphatase activity were subsequently tested in U(VI) bioprecipitation assays. When aerobically grown in synthetic groundwater (pH 5.5) amended with 10 mM glycerol-3-phosphate (G3P), phosphatase-positive Bacillus and Rahnella spp. strains Y9-2 and Y9602 liberated sufficient phosphate to precipitate 73% and 95% of total soluble U added as 200 microM uranyl acetate respectively. In contrast, an Arthrobacter sp. X34 exhibiting a phosphatase-negative phenotype did not liberate phosphate from G3P or promote U(VI) precipitation. This study provides the first evidence of U(VI) precipitation via the phosphatase activity of naturally occurring Bacillus and Rahnella spp. isolated from the acidic subsurface at the DOE ORFRC.