Study of the biogenic potential of nanoparticle formation from selenite and tellurite by two environmental strains of Burkholderia fungorum and assessment of their resistance as planktonic cells or biofilms to polyaromatic hydrocarbons

Here we report on two strains of Burkholderia fungorum (DBT1 and 95) and their ability in both planktonic and biofilm modes to resist high concentrations of hydrocarbons in order to be exploited in bioremediation protocols. In nature, bacteria often attach to surfaces by establishing biofilms. B. fungorum DBT1 was isolated from an oil refinery drainage, while B. fungorum 95 was isolated from the inner tissues of a hybrid poplar plant cultivated in a soil contaminated by polycyclic aromatic hydrocarbons (PAHs). The hydrocarbons tested were dibenzothiophene (DBT) as a sample of thiophenes and a mixture of PAHs, namely: naphthalene, phenanthrene and pyrene. Moreover, their ability to transform toxic metalloid oxyanions (namely selenite and tellurite) to non-toxic elemental form was evaluated. This transformation not only eradicate the toxic metalloid compounds in contaminated area, but also can be utilized in order to obtain elemental form of metalloid in the form of nanoparticles with applications in technology and medicine. Our results showed that both strains degraded high concentration of dibenzothiophene and both forms of biofilm and planktonic of bacteria resisted up to 2000 mg l-1 of this compound. Moreover, B. fungorum DBT1 showed reduction in tolerance to PAHs mixture (naphthalene 2000 mg l-1, phenanthrene 800 mg l-1 and pyrene 400 mg l-1) as biofilm and planktonic forms. In contrary, formation of biofilm helped B. fungorum 95 to resist PAHs in these concentrations while planktonic form could not resist. Confocal laser scanning microscopy pictures showed that by exposing biofilm to DBT and PAHs, the structure changes. In fact, high concentration of DBT caused the formation of aggregation in biofilm. On the other hand, the result of both strains behavior in the presence of metalloids showed that strain DBT1 was able to reduce 0.5 mM selenite and 0.1 mM tellurite, while strain 95 reduced more than 1 mM selenite and 0.05 mM tellurite to elemental forms within 96 hours of aerobic incubation. B. fungorum 95 produced 1 mM selenium in the presence of 2 mM selenite. Produced selenium nanoparticles were spherical and zero charged with average hydrodynamic diameter of 170 nm (for strain 95) and 200 nm (for strain DBT1). However, produced tellurium nanoparticles were needle like and positive charged with average hydrodynamic diameter of 120 nm and 170 nm for strains 95 and DBT1 respectively. Scanning and transmission electron microscopy analyses showed both extracellular and intracellular selenium nanoparticles. Selenite reduction activity test evidenced cytoplasmic enzymatic activation by accepting electron from electron donors. Since nanoparticles occurred extracellularly but they are produced intracellularly according to selenite reduction activity test, either they exit by secretion or after cell lysis. However, tellurium nanoparticles are produced and occurred intracellularly by cytoplasmic activity. In conclusion, the findings for the resistance against hydrocarbons provide new perspectives on the efficiency of using DBT-degrading bacterial strains in bioremediation of contaminated sites containing high concentration of poly aromatic hydrocarbons and thiophenes and low concentration of metalloid oxyanions of selenium and tellurium. Production of selenium and tellurium nanoparticles under aerobic conditions by strains DBT1 and 95 could be due to intracellular reduction mechanisms. These biogenic nanoparticles of both kinds present size compatible with medical and technological applications which are currently under study.