|Van Berkum, Peter|
Submitted to: Chemosphere
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/19/2006
Publication Date: 3/30/2007
Citation: Van Berkum, P.B., Abou-Shanab, R.A., Angle, J.S. 2007. Heavy metal resistance and genotypic analysis of metal resistance genes in gram-positive and gram-negative bacteria present in serpentine soil and in the rhizosphere of alyssum murale. Chemosphere. v. 68, p. 360-367. Interpretive Summary: Bacteria in soils occupy every small micro-environment. A micro-environment that is most favored by bacteria is the soil that closely surrounds plant roots, which is known as the rhizosphere. The rhizosphere is favored because it is nutrient-rich and, therefore, many important biological processes occur in the root environment. One such process is bacterial detoxification of heavy metals permitting their removal by plants. Besides having application in the restoration of contaminated soils, this process when managed is financially worthwhile because the metals can be bio-mined by harvesting the plants. The bacteria that detoxify heavy metals are highly diverse and include rhizobia. All these bacteria have resistance to high levels of different heavy metals, which was shown to be mediated by the same genes irrespective of genus and species affiliation. The most likely explanation for this finding is that heavy metal resistance genes in bacteria are exchanged much in the same way that genes conferring antibiotic resistance have been shared among bacterial populations. The information provided will be useful to bacteriologists who investigate mechanisms and evolution of resistance in bacteria. Also, the information is useful to ecologists involved with studies of bioremediation.
Technical Abstract: Forty-six bacterial cultures, including one culture collection strain, thirty from the rhizosphere of Alyssum murale and fifteen from Ni-rich soil, were tested for their ability to tolerate arsenate, cadmium, chromium, zinc, mercury, lead, cobalt, copper, and nickel in their growth medium. The resistance patterns, expressed as Minimum Inhibitory Concentrations (MICs), for all cultures to the nine different metal ions were surveyed by using the agar dilution method. A large number of the cultures were resistant to Ni (100%), Pb (100%), Zn (100%), Cu (98%), and Co (93%). However, 82, 71, 58 and 47% were sensitive to As, Hg, Cd and Cr(VI), respectively. All cultures had multiple metal-resistant, with heptametal resistance as the major pattern (28.8%). Five of the cultures (about of 11.2% of the total), specifically Arthrobacter rhombi AY509239, Clavibacter xyli AY509235, Microbacterium arabinogalactanolyticum AY509226, Rhizobium mongolense AY509209 and Variovorax paradoxus AY512828 were tolerant to nine different metals. The Polymerase Chain Reaction in combination with DNA sequence analysis was used to investigate the genetic mechanism responsible for the metal resistance in some of these Gram positive and Gram negative bacteria that were, highly resistant to Hg, Zn, Cr and Ni. The czc, chr, ncc and mer genes that are responsible for resistance to Zn, Cr, Ni and Hg, respectively were shown to be present in these bacteria by using PCR. In the case of Microbacterium arabinogalactanolyticum AY509226 these genes were shown to have highly homology to the czcD, chrB, nccA, and mer genes of Ralstonia metallidurans CH34. Therefore, Hg, Zn, Cr and Ni resistance genes are widely distributed in both Gram-positive and Gram-negative isolates obtained from Alyssum murale rhizosphere and Ni-rich soils