Acquiring genes through gene transfer provides new genetic information to microorganisms, which may allow them to survive changing environments. The major source of variation within a bacterial species is mutation. In mutations, usually only a single gene changes at any one time. In contrast, gene transfer results in many genes being transferred simultaneously, giving the recipient cell much more additional genetic information. For example, in a hospital environment in which a large variety of antimicrobials are constantly used, a mutation usually confers a selective advantage for resistance to only a single antimicrobial medication. If other antimicrobials are in the environment, the mutant will still die. In contrast, transfer of a resistance plasmid confers resistance simultaneously to many different antimicrobials. Thus, the organism is more likely to survive.
Transposons greatly expand the opportunity for gene movement. Any gene or genes flanked by insertion sequences can move to different sites in the genome. A chromosomal gene flanked by insertion sequences can move to a broad host range plasmid in the same cell, which can then be transferred to a large number of different genera of bacteria, both Gram-positive and Gram-negative. This explains why the same antimicrobial-resistance genes occur in E. coli (Gram-negative) and Clostridium (Gram-positive).
It is not surprising that bacteria have evolved mechanisms for gene transfer and also for spontaneous gene mutation. Both mechanisms play important roles in altering the genotype, which allows bacteria to adapt to changing environmental conditions through the process of natural selection.
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