To determine whether chromate reduction requires the physical presence of E. cloacae cells, cells were grown anaerobically in KSC medium with 0.5 mM K2CrO4 and then removed from the culture by filtration through membrane filters (0.45-,um pore size). The composition of KSC medium has been previously described (13). Sodium acetate was added as a carbon source and electron donor for chromate reduction, and the pH of the medium was 7.4. The cell-free filtrate did not reduce CrO42-even under anaerobic conditions (Fig. 1A). Cells were also removed by centrifugation at 13,000 x g for 10 min; no chromate reduction was observed in the supernatant. Adding fresh KSC medium to the supernatant did not cause chromate reduction. On the contrary, when washed cells were suspended in fresh KSC medium (about 2 x 107 cells per ml), chromate reduction was readily observed. They were able to reduce completely concentrations as high as 0.5 mM CrO42-within 5 h; such a high rate of chromate reduction has never been reported.This reductase activity was eliminated by aeration (Fig. iB). No chromate reduction was observed at 4 or 60°C.

Enterobacter cloacae strain (HOI) capable of reducing hexavalent chromium (chromate) was isolated from activated sludge. This bacterium was resistant to chromate under both aerobic and anaerobic conditions. Only the anaerobic culture of the E. cloacae isolate showed chromate reduction. In the anaerobic culture, yellow turned white with chromate and the turbidity increased as the reduction proceeded, suggesting that insoluble chromium hydroxide was formed. E. cloacae is likely to utilize toxic chropiate as an electron acceptor anaerobically because (i) the anaerobic growth of E. cloacae HO1 accompanied the decrease of toxic chromate in culture medium, (ii) the chromate-reducing activity was rapidly inhibited by oxygen, and (iii) the reduction occurred more rapidly in glycerolor acetate-grown cells than in glucose-grown cells. The chromate reduction in E. cloacae HO1 was observed at pH 6.0 to 8.5 (optimum pH, 7.0) and at 10 to 40°C (optimum, 30°C).

No abstract

No abstract