Binding of Mercuric and Other Heavy Metal Ions by Microbial Growth Media

Ion-specific electrodes were used to study the binding of Hg(2+), Pb(2+), Cu(2+), and Cd(2+) ions to widely used bacterial growth media (Nutrient broth, trypticase soy broth, the medium of Foot and Taylor [6] and of Nelson et al. [12]) and to media components [yeast extract, peptone, tryptone, proteose peptone, and casamino acids (acid hydrolyzed casein)]. Volatilization of Hg(2+) from aqueous solutions could be prevented by any of the growth media or their components. All media bound large amounts of Hg(2+), Pb(2+), and Cu(2+), but much less Cd(2+). Of the media components, casamino acids showed the most binding activity for all metal ions; the relative affinity of other media components to different ions varied with the cation studied. In general, the Irving-Williams [8] series for cation affinity to organic ligands was followed: Hg(2+) > Pb(2+) >> Cu(2+) >> Cd(2+). After adding 20 ppm of Hg(2+), Pb(2+), or Cu(2+) (concentrations inhibitory to the growth of most microorganisms) to the growth media, 80 ppb or less remained as free cations in the solution. This might suggest that such ions enter bacterial cells as organic complexes, or that bacterial cells can compete successfully with growth media for the bound ions.