Combustion of organic sulphur compounds in fossil fuels releases SO2, a major air pollutant. Dibenzothiophene (DBT) is a representative organic sulphur compound in fossil fuels, and its desulphurisation by living cells, resting cells and enzymes of a soil bacterium was investigated. The strain could grow on DBT, but its maximum growth was obtained in a medium of DBT plus thiamine (50 mg/l). GC-MS analysis reveals that the strain transformed DBT to 2-hydroxybiphenyl (2HBP), a desulphurised compound with the intact carbon skeleton of DBT. The biotransformation was via an intermediate of dibenzothiophene sulphone (DBTS). The organic sulphur was utilised for cell reproduction and little sulphur was left as sulphate in the medium. DBT desulphurisation, associated with cell growth, was promoted or depressed by inorganic sulphate depending on sulphate concentration. A high extracellular sulphate concentration (>0.1 mM) repressed the expression of DBT desulphurisation enzymes, but below this level the enzymes were expressed very well with good growth as well as a stoichiometric biotransformation of DBT to 2HBP. If no sulphate except DBT was supplied as a sulphur source, only about one-third of DBT desulphurisation and cell growth were achieved compared with those in the presence of both DBT and thiamine (or sulphate). DBT desulphurisation could also be carried out in a crude enzyme solution if NADH cofactor was added. The DBT content in the enzymic solution was reduced from 2.45 to 2.06 mM. giving 0.389 mM 2HBP in 45 min. DBT was also desulphurised by the resting cells of the bacterial strain, but NADH cofactor was not needed. Accumulation of 2HBP, the final product of DBT desulphurisation, had an inhibitory effect on both cell growth and DBT desulphurisation. (C) 2000 Elsevier Science Ltd. All rights reserved.