Metal resistance in bacteria

Abstract

Bacteria have been examined that have developed very efficient and different mechanisms for tolerating heavy metals. Often normal toxic levels of metals have no effect on cell growth of resistant strains. In many organisms, the genes controlling metal resistance are carried on plasmids, which provide the bacteria with a competitive advantage over other organisms when metals are present. Not all metal-resistant bacteria contain plasmids. For example, a Pseudomonas sp. (research at the Canada Centre for Inland Waters and the University of Guelph) resistant to 1000 μg/ml lead, also displays multiple antibiotic resistance. Vertical and horizontal agarose gel electrophoresis of cleared lysates revealed that plasmids were not present, even though the organism was tolerant to extremely high concentrations of lead. Plasmid-encoded resistance may provide organisms with efflux and bypass mechanisms, enzymes which catalyze the transformation of metals to volatile forms, or make the bacterial cell wall impermeable to the metal(s). A problem still remains in actually defining the concentrations that distinguish metal-resistant from metal-sensitive bacteria. Some researchers have proposed mathematical equations, and used statistical analysis to solve this problem. Nevertheless, standard concentrations have not been universally proposed and/or accepted by the scientific community. This task is complicated by the various forms of metals used, the effect of media components, pH, and culture conditions, which have the capability of influencing the toxicity of the metal. Research on metal–bacteria interactions is in some ways still in its early stages of development. Nevertheless, significant advances in scientific knowledge have been achieved over the last 20 years. To fill the existing gaps in knowledge, research will need to be conducted on the extent of metal resistance in bacteria, the relationship between metal and antibiotic resistance, mechanisms specified by the plasmids, and the ecology, physiology, and genetics of gene transfer in the natural environment. Since some microorganisms are responsible for environmental metal transformations, they may also serve as bioassay indicator organisms in polluted and non-polluted environments. Moreover, there may be potential biotechnological applications for metal-resistant bacteria in the area of toxic metal control in waste-water treatment.