Lead resistance in two bacterial isolates from heavy metal-contaminated soils

Microorganisms have developed mechanisms of coping with a variety of toxic metals; however, few studies have explored microbial resistance to lead. In this study, the overall mechanisms of a lead-resistant Pseudomonas marginalis and a lead-resistant Bacillus megaterium isolated from two different metal-contaminated soils were investigated. The P. marginalis had a higher lead resistance level at 2.5 mM total lead as compared to 0.6 mM for B. megaterium. Resistance to soluble lead was much lower, 0.3 and 0.1 mM, respectively. The degree of lead resistance and the mechanism of lead resistance for these two isolates corresponded with their environmental lead exposure. When viewed with transmission electron microscopy, P. marginalis, isolated from a soil contaminated with high total but undetectable soluble lead, showed extracellular lead exclusion. B. megaterium, from a soil with both high total and soluble lead levels, was less resistant with an intracellular cytoplasmic accumulation of lead as observed with TEM. Polarization microscopy indicated that while P. marginalis produced a high amount of an extracellular polymer implicated in the organism's mechanism of lead resistance, B. megaterium produced no discernable extracellular polymeric substances. The study of these two organisms demonstrated differences in how soil microorganisms respond to environmental lead exposure, including the novel mechanism of intracellular sequestration of lead.