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Plasmid antibiotic resistance is a form of bacterial antibiotic resistance mediated by plasmids, small chunks of DNA that exist independently of the cell. Bacteria can pass plasmids between each other, facilitating the growth of antibiotic resistance in a given colony of organisms. Combating this form of resistance requires developing new classes of medications that are able to kill cells carrying plasmids with resistance to common antibiotic drugs.
Plasmids function by infecting bacterial cells and hijacking their cellular processes to reproduce. They are essentially parasitic, using the cell for survival because they cannot live independently, but they are not an integral part of the cellular DNA. Some plasmids carry genes for antibiotic resistance and pass this on to the cells they infiltrate. With plasmid antibiotic resistance, a cell can acquire plasmids from the surrounding environment or other cells, and will also reproduce them when it divides, perpetuating the plasmids and the antibiotic resistance.
Bacteria in nature usually do not have resistance to antibiotic medications, as they do not encounter the drugs and thus do not need to evolve responses to them. As bacteria start to colonize human and animal populations, the presence of antibiotics pressures them to evolve and exposes them to other bacteria carrying resistance plasmids, creating plasmid antibiotic resistance. A cell can carry multiple resistance genes and pass them on, creating a situation where a patient may develop an infection that does not respond to several different classes of antibiotics.
Existing as separate DNA, plasmids confer immense advantages on bacteria. They can spread through a bacterial colony and grow in numbers as parasitized bacteria reproduce and create more plasmids. For these small chunks of DNA, transferring plasmid antibiotic resistance carries a genetic advantage, as it will ensure that the bacteria they colonize will continue to reproduce, spreading the plasmids throughout a bacterial population.
Treating patients with antibiotic-resistant bacterial infections can be challenging. Plasmid antibiotic resistance is only one form, and it is possible for bacteria to have several lines of defense against antibiotics. Fully completing courses of medication is important to reduce the risk of accidentally breeding antibiotic resistance by killing off the most susceptible cells and leaving those with some resistance behind to breed and create more resistant bacteria. If a doctor suspects a patient has a resistant infection, she may order a culture to determine which antibiotic would be most suitable for treatment. In the culture, a technician will plate samples of the bacteria on gels treated with various antibiotics, and see which gels the bacteria grow on to determine susceptibility.