Simultaneous utilization of pyridine and fructose by Rhodococcus opacus UFZ B 408 without an external nitrogen source

Abstract

A bacterium classified as Rhodococcus opacus, which is able to use pyridine (a potentially growth-inhibiting substrate) as its sole source of carbon, energy and nitrogen, was isolated. In a carbon-limited chemostat culture, the kinetics was determined for growth on both pyridine and a mixture of pyridine and fructose (9 mM/22.15 mM). With growth on pyridine, stable steady states were achieved up to dilution rates of about 0.1 h⁻¹. A further increase in the dilution rate resulted in the progressive accumulation of pyridine in the culture liquid and the cells were washed out. The maximum specific growth rate (μ_max = 0.23 h⁻¹) and the K_S value (0.22 mM) for growth on pyridine were determined from the residual pyridine concentrations measured within the range of stable steady states. With growth on the substrate mixture, the specific pyridine consumption rates and the residual pyridine concentrations were lower at similar dilution rates than with growth on pyridine alone, and stable steady states were established at dilution rates of up to 0.13 h⁻¹. The maximum pyridine degradation rate was enhanced to 270 mg pyridine l⁻¹ h⁻¹ compared to 210 mg pyridine l⁻¹ h⁻¹ with growth on pyridine as a single substrate. An external nitrogen source did not need to be added in the case of growth on the substrate mixture. Fructose was assimilated by means of ammonium released from pyridine. Analysis of the nitrogen balance furnished proof that pyridine is an energy-deficient substrate; pyridine was assimilated and dissimilated at a ratio of 1 mol/0.67 mol respectively. The resulting yield coefficient was about 0.55 g dry weight/g pyridine. Moreover, it was demonstrated that, in regard to the biologically usable energy, 1 mol pyridine corresponds to 0.43 mol fructose.