Gold(III) reduction by the rhizobacterium Azospirillum brasilense with the formation of gold nanoparticles

The Collection of Rhizosphere Microorganisms: its importance for the study of associative plant-bacterium interactions

Microbial culture collections are very important components of biological science. They provide researchers with material for studies and preserve biological resources. One such collection is the Collection of Rhizosphere Microorganisms, kept at the Institute of Biochemistry and Physiology of Plants and Microorganisms of the Russian Academy of Sciences, Saratov (IBPPM). Its activity is primarily directed toward the isolation and preservation of microorganisms from the plant root zone. The international research interest in microorganisms from this ecological niche is not waning, because they are very important for plant growth and development and, consequently, for plant breeding. The group of bacteria with properties of significance for plants has been given the name "plant-growth-promoting rhizobacteria" (PGPR). This group includes nitrogen-fixing soil alpha-proteobacteria of the genus Azospirillum, which form the core of the IBPPM collection. First discovered by Brazilian scientists in the 1970s, azospirilla are now a universally recognized model object for studying the molecular mechanisms underlying plant-bacterium interactions. The broad range of useful properties found in these microorganisms, including the fixation of atmospheric nitrogen, production of phytohormones, solubilization of phosphates, control of pathogens, and formation of induced systemic resistance in the colonized plants, make these bacteria an all-purpose tool that has been used for several decades in basic and applied research. This article reviews the current state of Azospirillum research, with emphasis on the results obtained at the IBPPM. Scientific expeditions across the Saratov region undertaken by IBPPM microbiologists in the early 1980s formed the basis for the unique collection of members of this bacterial taxon. Currently, the collection has more than 160 Azospirillum strains and is one of the largest collections in Europe. The research conducted at the IBPPM is centered mostly on the Azospirillum structures involved in associative symbiosis with plants, primarily extracellular polysaccharide-containing complexes and lectins. The development of immunochemical methods contributed much to our understanding of the overall organization of the surface of rhizosphere bacteria. The extensive studies of the Azospirillum genome largely deepened our understanding of the role of the aforesaid bacterial structures, motility, and biofilms in the colonization of host plant roots. Of interest are also applied studies focusing on agricultural and environmental technologies and on the "green" synthesis of Au, Ag, and Se nanoparticles. The Collection of Rhizosphere Microorganisms continues to grow, being continually supplemented with newly isolated strains. The data presented in this article show the great importance of specialized microbial culture repositories, such as the IBPPM collection, for the development and maintenance of the microbial research base and for the effective solution of basic and applied tasks in microbiology.

Selenite reduction by the rhizobacterium Azospirillum brasilense, synthesis of extracellular selenium nanoparticles and their characterisation

Microbial reduction of selenium oxyanions has attracted attention in recent years. In this study, an original and simple method for the synthesis of extracellular selenium nanoparticles (Se NPs) of relatively uniform size has been developed using strains Sp7 and Sp245 of the ubiquitous plant-growth promoting rhizobacterium Azospirillum brasilense, both capable of selenite (SeO₃^2-) reduction. In addition, a reliable purification protocol for the recovery of the Se NPs has been perfected, which could be applied with minor modifications to cultures of other microbial species. Importantly, it was found that, by changing the conditions of bacterial reduction of selenite, extracellularly localised Se NPs can be obtained using bacteria which would otherwise produce intracellular Se NPs. In particular, bacterial cultures grown up to the end of the logarithmic growth phase, washed free of culture medium and then incubated with selenite, were used to obtain extracellular Se NPs. Their sizes depended on the initial selenite concentration (∼25-80 nm in diameter at 50-10 mM selenite, respectively). The Se NPs obtained were characterised by transmission electron microscopy (TEM), dynamic light scattering, as well as Raman and UV-vis spectroscopies. Their zeta potential was found to be negative (ca. minus 21-24 mV). Bacterial selenite reduction was also studied in the presence of the efflux pump inhibitor carbonyl cyanide m-chlorophenylhydrazone (CCCP). In this case, TEM indicated the formation only of intracellular selenium crystallites. The data show that the formation of extracellular Se NPs requires normal bacterial metabolic activity, while CCCP evidently blocks the membrane export of Se⁰ nuclei.

Reduction of selenite by Azospirillum brasilense with the formation of selenium nanoparticles

The ability to reduce selenite (SeO(3)(2-)) ions with the formation of selenium nanoparticles was demonstrated in Azospirillum brasilense for the first time. The influence of selenite ions on the growth of A. brasilense Sp7 and Sp245, two widely studied wild-type strains, was investigated. Growth of cultures on both liquid and solid (2 % agar) media in the presence of SeO(3)(2-) was found to be accompanied by the appearance of the typical red colouration. By means of transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS) and X-ray fluorescence analysis (XFA), intracellular accumulation of elementary selenium in the form of nanoparticles (50 to 400 nm in diameter) was demonstrated for both strains. The proposed mechanism of selenite-to-selenium (0) reduction could involve SeO(3)(2-) in the denitrification process, which has been well studied in azospirilla, rather than a selenite detoxification strategy. The results obtained point to the possibility of using Azospirillum strains as endophytic or rhizospheric bacteria to assist phytoremediation of, and cereal cultivation on, selenium-contaminated soils. The ability of A. brasilense to synthesise selenium nanoparticles may be of interest to nanobiotechnology for "green synthesis" of bioavailable amorphous red selenium nanostructures.