Course: SAT 170 Introductory Biotechnology

Unit 9 Learning Module

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Antibiotic Resistance

Figure 1

Figure 2
As mentioned previously, one of the factors that biotechnology is trying to address with respect to microbes is the increasing prevalence of antibiotic resistance amongst microbes.  Antibiotics are a type of antimicrobial, which as we mentioned previously, are chemicals that can be used to destroy or inhibit the growth of microbes. Some of these antibiotics are known to attack a variety of different microbes and are known as broad spectrum antibiotics. Narrow spectrum antibiotics, on the other hand, are those which target only specific organisms. As a survival mechanism, many microbes have developed resistance to these drugs as they acquire antibiotic resistance genes. An example of an antibiotic resistance gene is one that codes for a membrane protein that will pump the antibiotic out of the microbial cell as soon as it enters. If the antibiotic is not allowed into the cell, it cannot carry out its job of killing the microorganism. These genes are often acquired through genetic mutation. Since most microbes divide very rapidly, their mutation rate is also very high. In some instances, a mutation may provide a benefit to the microbe, such as one that would allow that organism to be resistant to an antimicrobial. One of the major problems with this is that microbes, especially bacteria, can easily share genes with other microbes, including these antimicrobial resistance genes. In general, the more exposure that microorganisms receive to a particular antibiotic, the more likely they are to develop a resistance to these drugs. Furthermore, the misuse of antimicrobials by humans can lead to selection of the microorganisms that have these resistance genes. The idea is that when you use an antibiotic you are likely to wipe out the microbes that are sensitive to that antibiotic, while the resistant microbes remain unaffected. Now that these resistant microbes no longer have the competition from the non-resistant strains, they can thrive. Such misuse of antibiotics is exemplified by individuals who are prescribed an antibiotic for an infection but fail to complete the entire course of the antibiotic. This typically occurs because the individual begins to feel better and, therefore, sees no benefit from continuing the course of antibiotics. This type of action could apply the selective pressures that I mentioned previously, where only those microbes with the resistance genes survive. While this particular individual will feel better because there are few microbes present, they can pass the resistant microbes to other individuals or may get sick again themselves. Because this round of illness is caused by the resistant microbes, the antibiotic that was originally used is completely ineffective at treating the infection. One other factor that compounds our inability to treat infections is the ineffective process for diagnosing certain infections. This leads to the prescription of an antibiotic in circumstances where it is not appropriate. For example, viral infections are not effectively treated with most antimicrobials; so if a person is given an antibiotic to treat a viral infection it will do no good to treat the infection but will expose the microbes in their body to the antibiotic unnecessarily. Also, in many cases a broad spectrum antibiotic is used when a narrow spectrum would be more effective, but without the identification of the microbe causing the infection, it is difficult to prescribe narrow spectrum antibiotics. The field of agriculture is also thought to be involved in this problem of antibiotic resistance as more than half of the antibiotics used in the United States are used for the purposes of reducing the spread of infectious diseases in animal husbandry. Much of this use is prophylactic, and many consider it to be unnecessary. Figure 1: Notice the natural variation in resistance among a group of microbes (shown in here in different shades of yellow, orange and red). After a selection pressure is applied (such as the misuse of an antibiotic) observe how the population of microbes becomes increasingly resistant.

Using methods, such as polymerase chain reaction (PCR), DNA microarrays, and modified cell cultures we can begin to quickly identify the presence of these antibiotic resistance genes in different organisms. Many researchers believe that identification is the first step to tackling the problem of antibiotic resistance. Figure 2: The resistance that microbes exhibit to a particular antibiotic can be evaluated by looking for the presence of particular resistance genes using DNA analysis techniques such as the polymerase chain reaction (PCR) discussed previously. Another mechanism is to plate the bacteria in a lawn across a petri dish and add antibiotic disks to that bacterial lawn. The presence of growth directly around the disk indicates that the bacterium is resistant to that particular antibiotic.

Aside from the role that antibiotic resistance genes play in the development of drug resistant microbes, these genes are also used in the development of certain cell cultures. For example, by incorporating antibiotic resistance genes into a plasmid that will be used to create a genetically modified bacterium, we can use the expression of the antibiotic resistance gene as a selectable marker to allow us to identify the cells that have been transformed. The way that this is accomplished is by growing those organisms on media containing the antibiotic; only those that have incorporated the gene will survive. This is particularly helpful if the gene that we are intending to introduce to these organisms does not change the physical appearance of these modified cultures. This use of incorporating antibiotic resistance genes into genetically modified organisms as a selectable marker is one that is commonly practiced in the field of biotechnology. For example, if you insert a plasmid containing the gene for human insulin into a culture of bacteria and then plate those bacteria on nutrient agar alone, you will likely get the growth of the transformed bacteria contaminated with other non-transformed bacteria. If you add an antibiotic resistance gene along with the insulin gene to the plasmid, the bacteria that are successfully transformed will be resistant to that particular antibiotic. When you plate such bacteria on media containing that antibiotic they will survive, while other possible contaminants will not be able to survive and grow on the media.