ISOLATION, GROWTH, AND METABOLISM OF AN OBLIGATELY ANAEROBIC, SELENATE-RESPIRING BACTERIUM, STRAIN SES-3

A gram-negative, strictly anaerobic, motile vibrio was isolated from a selenate-respiring enrichment culture. The isolate, designated strain SES-3, grew by coupling the oxidation of lactate to acetate plus CO2, with the concomitant reduction of selenate to selenite or of nitrate to ammonium. No growth was observed on sulfate or selenite, but cell s uspensions readily reduced selenite to elemental selenium (Se-0). Henc e, SES-3 can carry out a complete reduction of selenate to Se-0. Washe d cell suspensions of selenate-grown cells did not reduce nitrate, and nitrate-grown cells did not reduce selenate, indicating that these re ductions are achieved by separate inducible enzyme systems. However, b oth nitrate-grown and selenate-grown cells have a constitutive ability to reduce selenite or nitrite. The oxidation of [C-14]lactate to (CO2 )-C-14 coupled to the reduction of selenate or nitrate by cell suspens ions was inhibited by CCCP (carbonyl cyanide m-chlorophenylhydrazone), cyanide, and azide. High concentrations of selenite (5 mM) were readi ly reduced to Se-0 by selenate-grown cells, but selenite appeared to b lock the synthesis of pyruvate dehydrogenase. Tracer experiments with [Se-75]selenite indicated that cell suspensions could achieve a rapid and quantitative reduction of selenite to Se-0. This reduction was tot ally inhibited by sulfite, partially inhibited by selenate or nitrite, but unaffected by sulfate or nitrate, Cell suspensions could reduce t hiosulfate, but not sulfite, to sulfide. These results suggest that re duction of selenite to Se-0 may proceed, in part, by some of the compo nents of a dissimilatory system for sulfur oxyanions.