On the other hand, Pereira (2010) demonstrated that the ability
of Streptomyces sp. MC1 to reduce Cr(VI) was enhanced by the
presence of sulfate ions in the culture medium. Some authors have
reported that in a variety of cells (bacteria and yeast), chromate
actively crosses biological membranes by means of the sulfate
uptake pathway, which reflects the chemical analogy between
these two oxyanions. Guillén-Jiménez et al. (2008) reported that
sulfate plays an important role in tolerance to Cr(VI) as well as
Cr(VI) reduction in Candida sp. FGSFEP. However, these results
differ from other reports where no inhibitory or stimulating effects
were found for sulfate in terms of Cr(VI) reduction in aerobic
microbial cultures in Pseudomonas putida or in strains of Bacillus sp.
In Streptomyces sp. MC1 plasmids have not been found and Cr(VI)
resistance was due to the reduction of Cr(VI) to Cr(III), which does
not cross the membrane, rather than to chromium bioaccumulation.
More studies are required in order to elucidate the
mechanism of chromium resistance in this strain and how the
sulfate is involved.