The influence of pH and external H⁺ concentration on caesium toxicity and accumulation in Escherichia coli and Bacillus subtilis

Toxicity screening of Escherichia coli NCIB 9484 and Bacillus subtilis 007, NCIB 168 and NCIB 1650 has shown Cs⁺ to be the most toxic Group 1 metal cation. However, toxicity and accumulation of Cs⁺ by the bacteria was affected by two main external factors; pH and the presence of other monovalent cations, particularly K⁺. Over the pH range 6-9 both E. coli and B. subtilis showed increasing sensitivity towards caesium as the pH was raised. The presence of K⁺ and Na⁺ in the laboratory media used lowered caesium toxicity and lowered acumulation of the metal. In order to assess accurately Cs⁺ toxicity towards the bacterial strains it was therefore necessary to define the K⁺:Cs⁺ ratio in the external medium. The minimum inhibitory K⁺:Cs⁺ concentration ratio for the Bacillus strains tested was in the range 1:2-1:3 while E. coli had a minimum inhibitory K⁺:Cs⁺ concentration ratio of 1:6.