Evidence of indigenous sulphate-reducing bacteria in commercial Wyoming bentonite MX-80

The presence of sulphate-reducing bacteria (SRB) indigenous to the Wyoming bentonite MX-80 was investigated. Bentonite was used as an inoculum for enrichment cultures with a medium selective for SRB. The enrichment cultures were re-inoculated to achieve pure cultures, and DNA was extracted from these. The phylogenetic analysis demonstrated that 16S rDNA gene sequences in all the pure cultures were similar to that of Desulfovibrio africanus. Further experiments revealed that SRB from the enrichment cultures could grow in temperatures of up to 40 °C and sulphide production was detected in the enrichment cultures growing in salt concentrations from 0.7% to 4.0%. These results, combined with a sigmoid morphology of cells in the pure cultures, supported our conclusion that D. africanus was present in the bentonite. In addition, dry bentonite was treated for 20 h in 100 °C dry heat before incubation in the growth medium. SRB in the bentonite survived and were viable after this treatment. The results indicate that SRB are present in commercial bentonite and that they can survive in a state of desiccation in bentonite at high temperatures and salinity.