If plasmids are not an essential part of the cell's genome, why do cells allow them to persist? Some plasmids are indeed useless and, as discussed above, some plasmids possess special mechanisms to protect their own survival at the expense of the host cell. Nonetheless, most plasmids do in fact provide useful properties to their host cells. In principle any gene can be plasmid borne and plasmids are indeed widely used in genetic engineering to move genes between organisms. In practice certain properties are widespread among naturally occurring plasmids. A selection of these are given in Table 16.01.
Plasmids often carry genes for resistance to antibiotics. This protects bacteria both from human medicine and from antibiotics produced naturally in the soil. Plasmids with genes for resistance to toxic heavy metals such as mercury, lead or cadmium protect bacteria from industrial pollution and from natural deposits of toxic mineral. Other plasmids provide genes that allow bacteria to grow by breaking down various industrial chemicals, including herbicides, or the components of petroleum. From the human perspective, such bacteria may be a nuisance or may be useful in cleaning up oil spills or other chemical pollution. Finally, some plasmids provide virulence or colonization factors needed by infectious bacteria to invade their victims and survive the countermeasures taken by the host immune system.
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