A New Klebsiella planticola Strain (Cd-1) Grows Anaerobically at High Cadmium Concentrations and Precipitates Cadmium Sulfide

Heavy metal resistance by bacteria is a topic of much importance to the bioremediation of contaminated soils and sediments. We report here the isolation of a highly cadmium-resistant Klebsiella planticola strain, Cd-1, from reducing salt marsh sediments. The strain grows in up to 15 mM CdCl2 under a wide range of NaCl concentrations and at acidic or neutral pH. In growth medium amended with thiosulfate, it precipitated significant amounts of cadmium sulfide (CdS), as confirmed by x-absorption spectroscopy. In comparison with various other strains tested, Cd-1 is superior for precipitating CdS in cultures containing thiosulfate. Thus, our results suggest that Cd-1 is a good candidate for the accelerated bioremediation of systems contaminated by high levels of cadmium. Cadmium, a highly toxic metal and a group B1 human carcinogen, is commonly ranked among the top 10 priority pollutants by U.S. regulatory agencies. Significant amounts of this metal often infiltrate the groundwater at hazardous waste sites. A practical approach to minimizing Cd(II) levels in the subsurface systems is precipitating it with hydrogen sulfide as the highly insoluble cadmium sulfide (CdS). In reducing environments, sulfate-reducing bacteria (SRB) generate hydrogen sulfide through dissimilatory sulfate reduction. However, little is known about the heavy metal resistance and growth of these bacteria in contaminated systems. Recent studies by Poulson et al. (14) and White and Gadd (22) indicate that even low levels of free Cd(II), Zn(II), or Ni(II) ions, i.e., 20 to 200 mM, are toxic to SRB, such as Desulfovibrio and Desulfotomaculum. Our unpublished studies (P. K. Sharma and M. A. Vairavamurthy) also indicate that at least the SRB from coastal salt marshes from Shelter Island, New York, are sensitive to cadmium. However, a new Klebsiella planticola strain, Cd-1, from the same coastal environment grew to saturation in 15 mM Cd(II) ions in a minimal anaerobic medium, and after thiosulfate was added to the growth medium, it transformed significant amounts of the dissolved cadmium to CdS, up to 50 times higher than that reported for highly purified and continuously fed sulfate-reducing biofilms (22) or batch cultures of a genetically engineered Escherichia coli with overexpressed thiosulfate reductase (S. W. Bang, D. S. Clark, and J. D. Keasling, Abstr. 99th Gen. Meet. Am. Soc. Microbiol., abstr. Q-302, 1999). In general, facultative anaerobes, such as Cd-1, are attractive candidates for accelerated bioremediation applications, as large amounts of them can be grown rapidly with various nonhazardous substrates in aerobic reactors and can then be injected into anaerobic subsurface environments. In contrast, SRB often are slow growers and also may require expensive substrates, nutrients, or cofactors for growth. In this report, we describe the isolation and basic characteristics of strain Cd-1. We show that it can grow in high levels of Cd(II) ions under a wide range of environmental conditions, including high salinity and acidic pH. Importantly, our studies demonstrate that Cd-1 can transform high levels of cadmium to CdS in the presence of thiosulfate. We compared the growth and the ability of Cd-1 to produce CdS in media containing high levels of cadmium with those of several other metalresistant, metal-transforming, or metal-reducing ubiquitous facultative anaerobes. These results suggest that Cd-1 is a superior candidate for the anaerobic growth and transformation of cadmium, particularly in the presence of thiosulfate. Isolation of Cd-1. Strain Cd-1 was isolated from near-surface