Biodegradative potential of Burkholderia sp. DBT1in the abatement of polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) tend to accumulate in both anthropized and natural habitats as a result of incomplete combustion of fossil fuels. From the environmental point of view, the main characteristics of PAHs – namely scarce solubility in water, low bio-availability, and high persistence in soil and groundwater – are seriously troublesome. Most PAHs also exert toxic and mutagenic effects and are potentially dangerous to the biological systems. Of the PAHs occurring in soil and groundwater, about 0.04 - 5% (wt/wt) corresponds to sulfur heterocycles. Among these latter, dibenzothiophene (DBT) represents the prevailing compound, which is therefore taken as model chemical structure in studies dealing either with the biodegradation of organo-sulfur contaminants through the “Kodama pathway” or with petroleum bio-desulfurisation through the “4-S pathway”. Since several organic pollutants usually contribute together to the contamination at different sites, isolation and characterization of microorganisms able to degrade a wide range of substrates surely play an important role in the bio-reclamation of polluted areas. Burkholderia sp. DBT1 is a bacterial strain isolated from an oil refinery wastewater which can degrade DBT nearly completely through the “Kodama pathway” within three days. Molecular characterization of the strain DBT1 has shown the presence of an unusual genetic structure. Actually, strain DBT1 genes involved in dibenzothiophene transformation are harbored in two operons and show low similarity to both nah-like and phn-like genes. These particular features encouraged studies aimed at clarifying the possible role of Burkholderia DBT1 in the degradation of PAHs other than condensed thiophenes, frequently occurring in oil-contaminated sites. The growth of strain DBT1 was thus tested in minimal medium supplied with different PAHs. In this way, the strain DBT1 has been demonstrated to efficiently grow on naphthalene, fluorene and phenanthrene. Burkholderia DBT1 has been even proved to degrade 2-carboxybenzaldehyde, phthalic acid and protocatechol, which are common intermediates in the phenanthrene degradation pathway. On the other hand, 2-hydroxy-1-naphthoic acid and 1-hydroxy-2-naphthoic acid – both key intermediates within the two possible upper pathways of phenanthrene degradation – can not function as sole carbon and energy sources for the strain DBT1. RT-PCR analysis and growth tests have been performed with two mutants of Burkholderia DBT1 in order to verify the possible involvement of the two operons already identified in phenanthrene metabolism. Evidence has been gained that the transcription of both operons is activated in presence of phenanthrene. Moreover, the results so far achieved suggest that the strain DBT1 presents a versatile metabolism towards PAHs. This behavior is quite interesting for the possible exploitation of Burkholderia DBT1 in bioremediation protocols of PHA-contaminated sites.