Quantification of bacterial lead resistance via activity assays

The level of microbial resistance to heavy metals is an important issue for the microbial ecology of heavy metal-contaminated habitats. However, assays based upon growth in nutrient media will overestimate the resistance level due to metal ion interactions with inorganic and organic components. The analysis of Pb-resistant bacteria isolated from soils containing up to 38 mmol total Pb x kg(-1) indicated that PYT80B medium which did not contain inorganic salts, contained low amounts of organic matter, and was buffered with a molecule that did not interact with metal ions (2-N-morpholinoethanesulfonic acid (MES)) provided the lowest estimates of lead resistance. However, better results were obtained by assaying metabolic activity (aerobic respiration) of resting cells suspended in 10 mM MES. By this criterion, 50% inhibition of Arthrobacter JS7 was found at 37 microM Pb(NO3)2. The effects of Pb+2 concentrations upon respiration of resting cells and growth rate in PYT80B medium were similar. The activity assay also showed that metal resistance was induced to higher levels when Arthrobacter JS7 was grown in the presence of Pb.