Shortly after antibiotics were first mass produced for the general public, the microbes that they were designed to kill began to fight back. Since that time, many pathogenic microbes (those that cause disease) have become resistant to drug therapy. But what exactly is antibiotic resistance, and how does it happen?
* Antibiotic Resistance through Bacterial Chromosome Mutations *
When bacteria divide and multiply through binary fission, the daughter cells that result are clones; exact copies of the parent cell. Occasionally, when the bacteria’s genetic material is copied, a mistake or mutation occurs. Most genetic mutations are either harmful to the cell or neutral, not conferring any advantage or disadvantage. Very rarely a mutation will result in a beneficial trait that is advantageous to the organism.
Bacteria reproduce very quickly. Microbes such as Escherichia coli (E. coli) can, within 24 hours, increase their population from one to one million cells. With this much reproduction going on, advantageous mutations happen more often than they would in an organism that reproduces slowly, such as humans. So every once in a while, for example, a bacterial mutation results in its ability to manufacture a protein that makes that bacterium less susceptible to the action of antibiotics.
* Antibiotic Resistance through Horizontal Gene Transfer *
Bacteria also have a unique genetic trick up their sleeves. In addition to being able to pass genetic information on to their offspring (vertical gene transfer), bacteria can also share genes with their neighbors through horizontal gene transfer. All bacteria have a chromosome containing the information required to make and run the cell. Some bacteria also have an extra bit of DNA called a plasmid. The genetic material of a plasmid does not contain information necessary for the day to day functioning of the cell. Instead a plasmid contains ‘bonus DNA’ with information that confers some type of survival advantage, sometimes antibiotic resistance.
Bacteria can actually share plasmids with each other. If one bacterium has a plasmid, it can extend a projection, called a pilus, and attach that pilus to another bacterium. Once the two bacteria are connected, the donor cell sends a copy of its plasmid, though the pilus, to the recipient cell. Bacteria are even able to pick up new genes is by directly absorbing the genetic material of their dead, degraded neighbors.
* How Do Chance Mutations Result in Antibiotic Resistance? *
Shortly after the introduction of antibiotics, such as penicillin, serious problems with antibiotic resistance began to emerge. This is because the bacteria that developed a resistance to antibiotics were more likely to survive and reproduce, and therefore would eventually be present in larger numbers. The more we use antibiotics, the more we select for the bacteria that are resistant to these antibiotics.
So the overuse of antibiotics is one of the main culprits in the creation of superbugs, such as MRSA and C. diff. The acronym MRSA, stands for methicillin resistant Staphylococcus aureus; a collection of S. aureus strains that are resistant to methicillin and other antibiotics. MRSA infections are very difficult to treat. Clostridium difficile, also known as C. diff., is another type of bacteria that has become a major health issue, as resistant strains are able to cause the serious gastrointestinal distress of life-threatening pseudomembranous colitis.
* Sources *
Bauman, R. (2005) Microbiology. Pearson Benjamin Cummings. Park Talaro, K (2008) Foundations in Microbiology.