Bacterial resistance to antibiotics is a growing threat to public health.
In recent years, bacteria resistant to antibiotics have become a growing problem for many countries, the United States included. Pesticide resistance among insects is sometimes a problem for farmers, and it can also be a challenge for health officials trying to control the spread of diseases carried by mosquitoes, like malaria and dengue fever. Both bacterial resistance to antibiotics and insect resistance to pesticides are examples of evolution in action.
Diversity
DNA replication is a very accurate process, but occasionally errors occur; these errors create changes in the genetic code called mutations. Environmental insults like ultraviolet light and certain chemicals can also cause mutations. Mutation creates genetic diversity in a population. Among sexually-reproducing organisms like insects, sexual reproduction also creates more genetic diversity by combining genes from two parents and "reshuffling the deck" during meiosis, the process that generates sperm and eggs. In bacteria, genes can also be transferred between individuals through various processes, collectively called horizontal gene transfer (HGT).
Artificial Selection
Given genetic diversity in populations of insects and bacteria, it's likely that some individuals in the population are more resistant to a given antibiotic or pesticide than others. Applying a pesticide or an antibiotic is basically artificial selection; it favors genes for resistance by ensuring that more resistant organisms are the ones most likely to pass on their genes. Over time, repeating this process can gradually lead to a population much more resistant than the original.
Considerations
Overuse of antibiotics is a major contributing factor, because it brings more populations of bacteria in contact with the antibiotics, thereby selecting for resistant organisms. Applying insufficient doses of antibiotics (or pesticides) can also drive the emergence of resistance, because it allows the individuals at the more resistant end of the spectrum to survive and pass on their genes, whereas a larger dose would wipe out the more resistant individuals as well.
Biochemistry
The genetic and biochemical changes involved in resistance are complex and varied, although several common themes often recur when scientists study these organisms. Sometimes resistance results from a change in the structure of the protein targeted by the antibiotic or pesticide; often it results from an improved ability to break down the pesticide or antibiotic or export it from the organism or cell. Many bacteria resistant to penicillin, for example, feature an enzyme called beta-lactamase, which breaks up the penicillin molecule, thereby rendering it ineffective.
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