Saturday, March 13, 2010

The "Superbug"

On page 130, Jerry Coyne shares one side of evolution that may be harming humans: antibiotic resistance. Explain how antibiotic resistance relates to evolution. Fossils suggest that species take millions of years to evolve, so how is it that penicillin, which was produced in 1940, could have already evolved? If bacteria continue to evolve, what will happen if the medical world is unable to keep up with these "superbugs"? What else can we potentially use to fight infection if we have no antibiotics? (Hint: think viruses) What effect does the use of antibacterial hand sanitizers and soaps have on this phenomenon?


  1. Antiobiotic resistance shows evolution at work over a short amount of time. Bacteria who have traits that allows them to survive exposure to antiobiotics have a selective advantage that allows them to reproduce and pass on their genes more often than bacteria who do not have the antibiotic resistant traits. Over time, the antiobiotic resistant traits show up in more and more of the population, showing a population evolving to meet the environmental conditions it lives in.
    Bacteria are able to reproduce much quicker than other species due to their simple structure (they are prokaryotes with no organelles) and their use of binary fission to reproduce. According to Tomar's Online Book of Evolution, bacteria can reproduce from beginning to end in as little as 15 minutes. This quick generation rate allows humans to see evolution in bacteria in a much shorter period of time than other species. Another thing that allows bacteria to evolve quickly is that binary fission creates a large amount of bacteria in a short amount of time since every division by a population doubles that population. With so many divisions, it becomes much more likely that a mutation will arise that makes bacteria resistant to an antiobiotic. Ac Coyne referenced, the large number of divisions therefore creates "a few individuals with lucky mutations that allow them to surive and reproduce, quickly evolving a sensitive population into a resistant one" (132).
    If bacteria contine to evolve quicker than the medical field can keep up, superbugs that arise will kill of those with a weak immune system including children, the elderly, and the sick. A superbug that could be spread by the air or by contact could end up killing millions. If we have no antiobiotics that can kill off bacteria, then we must turn to bacteriophages as an alternative. According to Campbell 8th Edition, bacteriophages work by injecting their DNA into a host cell(bacteria in this case), which then directs the bacteria to create phage proteins and duplicates of the phage DNA which are used to build more bacteriophages. The bacteriophages that are made then cause the bacteria cell to lyse, releasing the bacteriophages that then go on totakeover more bacteria cells. This cycle where a bacteriphage that reproduces leads to the death of the host cell(bacteria) is called a lytic cycle and the bacteirophage is called a virulent phage. This treatment may not always work because some bacteria may have restriction enzymes that restrict the ability of the phage to enter the bacteria by cutting up the DNA of the bacteriophage.
    Antibacterial hand sanitizers and soaps only speeds up the evolution of superbugs by making it a selective advantage to be resistant to the antibacterial soap or sanitizer. This leads to bacteria evolving to become resistant to the antibacterial soap, creating a population of resistant bacteria who could end up becoming the superbugs society fears. Antibacterial soap is also bad because it eliminates "good bacteria" on our skin that protects us from infection by killing off other bacteria that try to invade. Without these good bacteria, we are more likely to get sick as well as have an increase in the population size of resistant bacteria.

  2. Bacteria have a great evolutionary advantage in that they can reproduce at an extremely rapid rate, with some creating new generations by binary fission every 20 minutes. This rapid rate of reproduction allows bacteria to experience natural selection quickly and undergo adaptive evolution in response to environmental stresses. When humans develop an antibiotic to combat an infectious bacteria, such as staphylococcus, the antibiotic initially affects a large proportion of the bacteria population, either by actually killing the bacterial cells or by preventing bacterial growth. However, a small number of the bacteria will carry mutated DNA that allows them to resist the effects of the antibiotic. These bacteria then survive and reproduce, and when they reproduce, each offspring has identical DNA to its parent. Therefore, after relatively few generations, all of the surviving bacteria may contain genes for antibiotic resistance, and then continue to proliferate. An example of this is MRSA, a staph infection which has acquired resistance to methicillin, a common antibiotic used to fight such infections. Penicillin used to be used to fight staph infections, but now more than 95% of staph infections are resistant to it (131). Methicillin is no longer effective in fighting the bacteria, and as scientists struggle to develop a new class of antibiotics, new strains of MRSA have appeared that are resistant to virtually every known antibiotic (Wikipedia).
    One strategy to counter this bacterial resistance to antibiotics would be the use of bacteriophages to fight the infectious bacteria. However, this strategy has been extensively described by the first responder to this prompt, so I will bring up an alternative: natural selection. If humans can no longer stop bacteria with antibiotics and bacteriophages fail to work, we will eventually have to accept that unstoppable infections may invade our bodies. However, due to the immense amount of DNA in the human body, some people are bound to carry mutated DNA that gives them resistance to the bacteria. Perhaps by discovering people that are naturally resistant to these infections, we could make all humans resistant to the bacteria. We could accomplish this by extracting the resistance genes from the resistant humans, multiplying the DNA using PCR, and then injecting the DNA into nonresistant humans or human embryos. In this way, all humans would become resistant to the infection. If this strategy does not work, then nonresistant humans would die off naturally, and eventually the only survivors of the bacterial infection would be those who carry resistance. These humans would reproduce and repopulate the earth with humans that are resistant to the infection.
    Antibacterial soaps have essentially the same effect as antibiotics, albeit on a lesser scale. Excessive use of antibacterial soap will eventually kill off all bacteria except those that are resistant to the substance; these organisms will reproduce and eventually all the remaining bacteria will be unaffected by the substance.