High cell density culture of Rhodococcus rhodochrous by pH-stat feeding and dibenzothiophene degradation

A high cell density culture of Rhodococcus rhodochrous IGTS8 was investigated. Acetic acid was one of the most suitable carbon sources for cell growth and sulfate ion was more suitable than dibenzothiophene (DBT) as a sulfur source. Fed-batch culture was conducted in a 1-l jar fermentor with FB medium containing acetic acid and sulfate ion as carbon and sulfur sources. Cell growth was found to be inhibited when the concentrations of acetic acid and ammonium ion were above 3 g/l. To control the concentrations of the two components below 3 g/l, a mixture of acetic acid and ammonium acetate was supplied by means of pH-stat feeding. As a result, a cell concentration of 33 g dry cells/l was obtained after 28-h cultivation. When the cells obtained were incubated in a fresh medium containing DBT as a substrate, hydroxybiphenyl (HBP), which is the end-product of the DBT degradation pathway, was detected and its production rate gradually increased with incubation time. Incubation for 3 to 4 h was enough for the full induction of DBT-degrading enzymes, and the specific production rate of HBP was about 6.1 mmol/kg dry cells/h. A two-phase cultivation (cell growth phase and induction phase) is proposed in order to obtain a high cell density and full induction of DBT-degrading enzymes.