Cadmium and zinc sensitivity and tolerance in Klebsiella (Aerobacter) aerogenes

The resistance of strains of K. aerogenes NCIB 418 'trained' to Cd2+ or Zn2+ in liquid medium is graded to the training concentration. Training to Cd2+ increased the sensitivity to Zn2+ but training to Zn2+ reduced the sensitivity to Cd2+. The trained organisms, particularly those trained to Cd2+, grow slowly in medium containing the metals and the growth rates after 20 and 200 subcultures were not significantly different. The survival of untrained organisms on Cd2+-agar decreased progressively as the Cd2+ concentration was increased, but a threshold concentration of Zn2+ was necessary before any decrease set in. Very low concentrations of Zn2+ potentiated the lethal action of Cd2+ and vice versa, whereas chelating the metal ions with citrate, aspartate or gluconate eliminated it. Nutrient-limitation, as might occur in natural environments, also had a profound effect. Mg2+-limited organisms were particularly resistant to both metals whereas glucose-, NH+4 and PO3-4-limited organisms were very sensitive to Cd2+, and glucose- and K+-limited organisms to Zn2+.