Postgate Medium Research Articles

Iron availability in mixed cultures of sulfate‐reducing bacteria

Postgate's medium C was found to be unsuitable for quantitative studies of mixed microbial cultures containing sulfate-reducing bacteria. Equilibrium calculations of iron concentration, along with the experimental observations show that precipitation of iron occurs in Postgate's medium, despite the presence of citrate and yeast extract. The often ascribed inhibitory effect of sulfide on the growth of sulfate-reducing bacteria may be largely due to the reduced availability of iron, as a result of the precipitation of ferrous sulfide. Because it is essential to clearly define the growth limiting nutrient when carrying out quantitative bacterial growth studies, yeast extract should be excluded from the culture medium. Single-phase, exponential growth was observed for mixed cultures containing sulfate-reducing bacteria, using a chemically defined medium that incorporated the metal complexing agent EDTA. When carrying out quantitative kinetic studies of sulfate-reducing bacteria, a chemically defined medium incorporating EDTA is recommended.

Purification and properties of the soluble hydrogenase from Desulfovibrio desulfuricans (strain Norway 4).

A soluble hydrogenase has been isolated from Desulfovibrio desulfuricans (strain Norway 4) grown on Postgate's medium. The enzyme differs significantly from a membrane-bound hydrogenase previously purified from the same organism grown on Starkey's medium. The enzyme consisted of two subunits of 56 kDa and 29 kDa compared with masses of 60 kDa and 27 kDa for the membrane-bound enzyme. Analysis of preparations of the soluble enzyme by various methods gave values of 5-10 iron atoms, 6 labile sulphur atoms and 0.45-0.8 nickel atom per molecule. The enzyme was unusual in that it contained selenium, in quantities equivalent to nickel. The highly purified active enzyme produced no electron-spin-resonance (ESR) signals in the oxidized state. ESR signals due to a [3Fe-xS] cluster and nickel were observed only in some of the less active fractions of the enzyme, demonstrating that neither of these ESR-detectable components is a prerequisite for hydrogenase activity. Treatment of D. desulfuricans (Norway) cells with EDTA released a minor fraction with hydrogenase activity, which might indicate the presence of a periplasmic enzyme.