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The pollution of the environment with toxic heavy metals is spreading throughout the world along with industrial progress. Heavy metal pollution of soil and Waste water is the most serious environmental problem and has significant implications for human health.
Soils are generally regarded as the ultimate sink for heavy metals discharged into the environment and many heavy metals are bounds to soils.
Nickel (Ni) is the 24th most abundant element in the earth crust and has been detected in different
media in all parts of the biosphere. Ni is classified as the borderline metal ion because it has both soft and hard metal properties and can bind to sulfur, nitrogen and oxygen groups. Ni has been implicated as an embryo toxin and teratogen.
1.1.2 Hexavalent chromium
Hexavalent chromium (Cr(VI)) and trivalent chromium (Cr(III)) are the most Prevalent species of chromium in the natural environment.
Major sources of chromium pollution include effluents from leather tanning, chromium electroplating, wood preservation, alloy preparation and nuclear wastes due to its use as a corrosion inhibitor in nuclear power plants.
Cadmium (Cd) is nonessential but poisonous for plants, animals, and Humans. Cadmium is one of the most toxic pollutants of the surface soil layer, released into the environment by mining and smelting activities, atmospheric deposition from metallurgical industries, incineration of plastics and batteries, land application of sewage sludge, and burning of fossil fuels.
Arsenic (As), a toxic heavy metal element, is widely distributed in nature. The sources of arsenic arised from various natural sources like weathered volcanic, marine sedimentary rocks, fossil fuels, minerals, water, air, living organisms and anthropogenic activities including mining, agricultural chemicals, wood preservatives, medicinal products, industry activities.
Lead (Pb) a major pollutant that is found in soil, water and air is a hazardous waste and is highly toxic to human, animals, plants and microbes.
Mercury is one of the most toxic metals in the environment. It has been released into environment in substantial quantities through natural events and anthropogenic activities (Kiyono and Hou, 2006).
Recent estimates calculate the annual global emissions between 4800-8300 tons per year. Mercury and its compounds when released into the environment are highly toxic to living cells because of their strong affinity for the thiol groups of proteins (Hajela et al., 2002).
Industrial use of mercury led to the pollution of environment. Consequently, mercury removal is a challenge for environmental management.
Microorganisms in contaminated environments have developed resistance to mercury and are playing a major role in natural decontamination.
An extensively studied resistance system, based on clustered genes in an operon (meroperon), allows bacteria to detoxify Hg2+ into volatile metallic mercury by enzymatic reduction. Mercury-resistance determinants have been found in a wide range of gram-negative and gram-positive bacteria isolated from different environments.
They vary in the number and identity of genes involved and is encoded by meroperons, usually located on plasmids and chromosomes they are often components of transposons and integrons.
1.2 RESISTANCE TO ANTIBIOTICS 1,2
The transfer of resistance genes between bacteria of the same and indeed of different species is of fundamental importance in the spread of antibiotic resistance.
For the transformation of resistance gene, microbial recombination places important role.
1.3 MICROBIAL RECOMBINATION 1,2
Genetic recombination is the formulation of a new genotype by re-assortment of genes following an exchange of genetic material between two different chromosomes which have similar genes at corresponding sites. These are called homologous chromosomes and are from different individuals. Progency from recombination have combinations of genes different from those that are present in the parents. In bacteria, genetic recombination results from three types of gene transfer:
Conjugation: Transfer of genes between cells that are in physical contact with one another.
Transduction: Transfer of genes from one cell to another by a bacteriophage
Transformation: Transfer of cell-free or "naked" DNA from one cell to another.
These three types of gene transfer are shown.
In bacterial recombination the cells do not fuse, and usually only a portion of the chromosome from the donor cell (male) is transferred to the recipient cell (female). The recipient cell thus becomes a merozygote, a zygote that is a partial diploid. Once merozygote transformation has occurred, recombination can take place.
The general mechanism for bacterial recombination is believed to take place as follows. Inside the recipient cell the donor DNA in such a way that homologous genes are adjacent. Enzymes act on the recipient DNA, causing nicks and excision of a fragment.
Research conducted in 1950s demonstrated conclusively that genetic recombination occurs among bacteria and may even involve viruses. The strong began with Griffith's experiments with bacteria in 1928. This experiment showed that genetic recombination were possible and led to Avery's identification of DNA as the molecule responsible for recombination.
Conjugation involves cell-to-cell contact during which chromosomal or extra chromosomal DNA is transferred from one bacterium to another, and is the main mechanism for the spread of resistance.
The ability to conjugate is encoded in conjugate plasmids; these are plasmids that contain transfer genes, which in colifom bacteria, code for the production by the host bacterium of proteinaceous surface tubules, termed sex pili that connect the two cells.
The conjugative plasmid then passes across from one bacterial cell to another (generally of the same species). Many Gram-negative and some Gram-positive bacteria can conjugate. Some promiscuous plasmids can the species barrier, accepting one host as readily as another.
Many R plasmids are conjugative. Non-conjugative plasmids, if they coexist in a 'donor' cell with conjugative plasmids, can hitch-hike from one bacterium to the other with the conjugative plasmids.
The transfer of resistance by conjugation is significant in populations of bacteria that are normally found high densities, as in the gut.
The genetic recombination process in which one live bacterium acquires fragments of DNA from another live bacterium and express the proteins encoded by the acquired DNA.
Transduction is a process by which plasmid DNA is enclosed in a bacterial virus (or phage) and transferred to another bacterium of the same species. It is a relatively in effective means of transfer of genetic material but is clinically important in the transmission of resistance genes between staphylococci and of streptococci.
The genetic recombination process in which bacterial viruses acquire fragments
of bacterial DNA during viral replication and transport those fragments into another
live bacterium, where the DNA is expressed.
Generalized Transduction: If all fragments of bacterial DNA (i.e., from any region of the bacterial chromosomes) have a chance to enter a transducing phage, the process is called generalised transduction.
Specialized Transduction: Bacterial genes can also be transduced by bacteriophage in another process called specialised transduction.
GENERALIZED TRANSDUCTION SPECIALIZED TRANSDUCTION
One of the more remarkable observations of 1950s was that it is not unusual for bacteria to take up genetic material from molecular debris in the environment.
The phenomenon came to be known as transformation. Transformation takes place in less than one percent of a bacterial population and it can bring about profound genetic changes.
During transformation a number of donor bacteria break apart and their DNA explode out of the cells into fragments should recipient bacteria be present segment of double-stranded DNA containing about 10 to 20 genes pass through their cell walls and membranes.
Enzymes dissolve one strand of the DNA and the remaining second strand displaces a segment of single stranded DNA in the recipient chromosome. To make the transformation complete, the foreign genes express themselves during protein synthesis under natural conditions, transformation take place in organisms whose DNAs are very similar one of the effects is to increase the pathogenicity of the Recipient organism (as in Griffth's pneumo cocci).
Another effect may be the development of drug resistance indeed, scientists believe that a major reason for increasing drug resistance among bacterial pathogens is the uptake of genes from the environment.
The genetic recombination process in which one live bacterium acquires fragments of DNA from the local environment and express the proteins encoded by the genes in those fragments.
A few species of bacteria can, under natural condition, undergo transformation by taking up DNA from the environment and incorporating it into the genome by normal homologous recombination.
1.4 THE TRANSFER OF RESISTANCE GENES BETWEEN GENETIC ELEMENTSWITHIN THE BACTERIUM
GENE CASSETTES AND INTEGRONS
1.4.1 TRANSPOSONS 1,2
Some stretches of DNA are readily transferred (transposed) from one plasmid to another and also from plasmid to chromosome or vice versa. This is because integration of these segments of DNA, which are called transposons, into the acceptor DNA can occur independently of the normal mechanism of homologous genetic recombination. Unlike plasmids, transposons are not able to replicate independently, although some may replicate during the process of integration, resulting in a copy in both the donor and the acceptor DNA molecules.
Transposons may carry one or more resistance genes and can 'hitchi-hike' on a plasmid to a new species of bacterium. Even if the plasmid is unable to replicate in the new host, the transposons may integrate into the new host's chromosome or into its indigenous plasmids. This probably accounts for the widespread distribution of certain of the resistance genes n different R plasmids and among unrelated bacteria.
1.4.2 GENE CASSETTES AND INTEGRONS 1,2
Plasmids and transposons do not complete the tally of mechanism that natural selection has provided to confound the hopes of the microbiologist/chemotherapist. Resistance in fact, multidrug resistance can also be spread by another mobile element, the gene cassette, which consists of a resistance gene attached to a small recognition site. Several cassettes may be packaged together in a multicassette array, which can in turn, be integrated into a larger mobile DNA unit termed an integron.
The integron (which may be located on a transposon) contains a gene for an enzyme, integrase (recombinase), which inserts the cassette(s) at unique sites on the integron. This system trasposon/integron/multiresistance cassette array allows particularly rapid and efficient transfer of multi resistance between genetic elements both within and between bacteria.