Benzothiophene and dibenzothiophene as the sole sulfur source in Acinetobacter: growth kinetics and oxidation products

Abstract

Acinetobacter is an aerobic bacterium able to grow on aromatic carbon sources such as phenol and benzoate when given as the only carbon and energy source. In the present research, we have investigated its ability to metabolize benzothiophene (BT) and dibenzothiophene (DBT) given as the sole source of sulfur in presence of lactate as carbon nutrient. The heterocyclic compounds have been used alone and as β-cyclodextrin (β-CD) complexes in order to enhance their solubility and consequently improve their bioavailability. The growth curves obtained in a modified Sokol medium with BT or DBT have been compared to ones obtained in the same medium having inorganic sulfur composition. Our results show that the first two metabolic steps are the same as the pathway known in other micro-organisms, evidentiating the formation of sulphone and sulphoxide products. The bacterial growth curve in DBT displays a trend quite different from the curve obtained with inorganic sulfur. The binding of BT and DBT with cyclodextrins (CD) strongly enhance their water solubility (10 and 15 folds, respectively) thus reducing the replication time of the cells ( T d from 10.2 h to 3.1 h). Growth kinetics with β-CD + DBT display a maximum velocity when the molar concentration of DBT is about 50% of the maximum complex solubility. Over this value, an inhibitory effect is observed.