Cadmium and zinc sensitivity and tolerance in Bacillus subtilis subsp. niger and in a Pseudomonas sp.

The action of Cd2+ and Zn2+ on Bacillus subtilis subsp. niger ATCC 9372, and on a Pseudomonas sp. (possibly Pseudomonas fluorescens), isolated from cadmium-polluted soil has been determined and compared with results obtained previously with Klebsiella aerogenes. In liquid medium the lag and the mean generation time of Bacillus subtilis subsp, niger increased with increasing Cd2+ or Zn2+ concentrations whereas only the total biomass of the Pseudomonas sp. was affected. Nevertheless, the responses of both species indicated a specific action at low concentrations and a more general toxic action at high concentrations. The survival on Cd2+ - or Zn2+ - agar depended on the state of the metal ions with regard to chelating ligands and on the nutritional stage of the organisms. In admixture, the metal ions acted synergistically, particularly on the Pseudomonas sp. Resistance to both metal ions developed. It was graded to the training concentration and reciprocal cross-resistance occurred with Bacillus subtilis. subsp. niger but not with the Pseudomonas sp.