15.2: Antibiotic Selection
Researchers use antibiotic resistance genes to identify bacteria that possess a plasmid containing their gene of interest. Antibiotic resistance naturally occurs when a spontaneous DNA mutation creates changes in bacterial genes that eliminate antibiotic activity. Bacteria can share these new resistance genes with their offspring and other bacteria. The overuse and misuse of antibiotics have created a public health crisis, as resistant and multi-resistant bacteria continue to develop.
Antibiotic Resistance is an Essential Tool in Genetic Engineering
Antibiotics, such as penicillin, are drugs that kill or stop bacterial growth. Bacteria that naturally or artificially acquired antibiotic resistance genes do not respond to antibiotics. Scientists exploit this by designing plasmids—small, self-replicating pieces of DNA—that carry both an antibiotic resistance gene and a gene of interest. Antibiotic resistance is an integral part of DNA cloning that allows a researcher to identify cells that absorbed a DNA of interest.
The researcher’s DNA of interest is introduced into bacterial cells using a process called transformation. Bacterial transformation involves temporarily creating small holes in the bacterial cell wall to permit the uptake of external DNA such as a plasmid. Only some bacterial cells absorb new DNA. Since the plasmid includes both the DNA of interest and a gene that confers resistance to a specific antibiotic, applying the antibiotic to the bacterial cells (i.e., antibiotic selection) can help determine which cells were genetically modified.
The researcher spreads the bacterial cells onto a culture plate containing a chosen antibiotic. Only bacteria containing the antibiotic resistance gene survive and grow on the plate. After a few days, the researcher can select a bacterial colony to culture for further experiments—such as gene expression studies. Following antibiotic selection, the researcher will further test the bacteria using other methods (e.g., PCR) to confirm that DNA of interest is correct. Errors often occur such as the plasmid not containing the gene of interest at all.
Bacteria Can Acquire Antibiotic Resistance Naturally
Bacteria can acquire antibiotic resistance through spontaneous DNA mutations that alter the proteins produced by the cell. Resistant bacteria may produce proteins that cause the antibiotic to either be degraded, pumped out of the cell, or prevented from interacting with its target. For example, the antibiotic vancomycin inhibits synthesis of the bacterial cell wall. Some bacteria have developed resistance to this antibiotic by changing the types of protein subunits—amino acids—used in the assembly of their cell wall to ones that are unaffected by vancomycin.
Once antibiotic resistance genes emerge, bacteria can pass them on to their offspring. Bacteria can also acquire antibiotic resistance genes from other bacteria of the same or different species through a process called horizontal gene transfer (HGT). There are three mechanisms of HGT: transformation, conjugation, and transduction. Antibiotic resistance genes are often found in plasmids or transposons—pieces of DNA that are easily transferred between bacteria—that are exchanged during HGT. As a result, new types of antibiotic resistance can rapidly spread to multiple types of infectious bacteria.
Clinical Overuse and Misuse of Antibiotics Produces “Superbugs”
Antibiotics are a critical treatment for bacterial infections. However, their use can cause bacteria to become resistant and render the antibiotic ineffective, leading to untreatable and potentially deadly infections. Antibiotic overuse and misuse—for instance, using antibiotics to treat viral (rather than bacterial) infections or to increase livestock growth—is problematic because it promotes resistance.
Antibiotics cause resistance to evolve because they kill susceptible bacteria and leave only the resistant individuals. The surviving bacteria divide rapidly, producing offspring with the same antibiotic resistance. When antibiotics are overused, this selection pressure causes the number of resistant bacteria in the population to rise quickly. This is a major public health concern because it increases antibiotic resistance and creates “superbugs” that are resistant to multiple antibiotics. Continued overuse and misuse of antibiotics may eventually exhaust treatment options for bacterial infections.