Himanshubhai actually bacteria need metal ions that are having either +1 or +2 valance charge and vice-versa for -ve charge now Cr (VI) is having +6 charge so they are not having any general receptor to uptake it but their growth can be possible in the presence of Cr(VI) as it may works as inert material and hence Cr (VI) is not reduced.

Sulfate reducing bacteria are typically the main agent responsible for CR(VI) reduction. The favorable carbon sources for SRB's is lactate. Is lactate included in your media as the sole source for carbon? 

My question is why it should reduce Cr(VI). It involves utilization of a lot of energy for this. Instead it can easily detoxify it by bio accumulation.