The (Petri) Dish: The overlooked value of viruses
April 3, 2013
I have frequently written about viruses in The (Petri) Dish column. These miniscule life forms are parasites that infect the cells of our bodies and cause a range of diseases from AIDS to the common cold. Naturally, we consider viruses to be bad and to be avoided at all costs. However, viruses come in many forms and, surprisingly, many of them might even be considered good.
It may amaze you to learn that the most ubiquitous life form on earth is a type of virus called a bacteriophage which infects and kills bacteria. It is estimated that there are approximately 1031 (one followed by thirty-one zeros) bacteriophages on the planet, which means that they outnumber their bacterial hosts by at least 10-fold. Impressively, the introduction of a single bacteriophage into a bacterial culture can lead to the complete elimination of the bacteria in just a few hours. These efficient killing machines are able to land on the surface of a bacterium, inject their genetic material and hijack the bacterium’s metabolism to produce many copies of themselves before destroying the bacterium.
Bacteriophages can be found wherever there are concentrations of bacteria, including the soil, seawater and our own intestines. Their ubiquity may lead one to ask: How have bacteria managed to be so successful in the face of the bacteriophage challenge? It turns out that bacteria are in a constant “arms race” against bacteriophages, just as we humans are in an arms race against pathogenic viruses. As the bacteria evolve protective mechanisms to neutralize the viruses, the bacteriophages evolve countermeasures. But by staying one step ahead of the bacteriophages, bacteria are able to survive and be successful despite the constantly lurking threat.
A beautiful example of this arms race was recently discovered by scientists interested in the evolutionary dynamics of bacteriophages. It has been known for some time that bacteria have a sophisticated mechanism for disabling bacteriophages. The bacteria grab stretches of genetic material from the infecting bacteriophage, make copies of it and then insert it into an enzyme complex designed to destroy genetic material. The captured genetic material then acts as a template to guide the enzyme complex onto the bacteriophage genes and destroy them. In other words, the bacteria use the bacteriophages’ own genetic material to target them with a gene-seeking missile. This is a highly sophisticated and efficient protective mechanism. But it has now emerged that some bacteriophages have hijacked the idea and developed a similar mechanism, one that is targeted against their bacterial hosts. In particular, it was found that bacteriophages which infect cholera-causing bacteria have acquired the technology to re-target the degrading enzyme complex back onto bacterial genes responsible for controlling the anti-viral response. And so the race continues.
The potent anti-bacterial activity of bacteriophages has naturally led scientists to consider using them as “natural” antibiotics. As early 1896, this idea arose when Ernest Hanbury Hankin observed that water from the Ganges and Yamuna rivers in India was active against the cholera bacteria. While those early attempts to use river water to cure disease were a little premature, modern science offers tremendous hope for harnessing these viruses for good. It is important to note here that bacteriophages are not able to infect human cells since the physiologies of bacterial and human cells are very different. Therefore, bacteriophages would be very safe to use in a clinical setting. Indeed, bacteriophages offer considerable hope in our own arms race against the bacterial world, especially as we begin to run out of antibiotics against some types of pathogenic bacteria. In addition to clinical applications, bacteriophages have been used to control bacterial infections in a variety of food products, including meat and cheese, and they have been considered as potential antidotes to bio-terror weapons containing the bacteria that cause diseases such as anthrax. While the potential benefits of bacteriophages are many, there are some negative aspects. Biotechnology companies that rely on the mass culture of bacteria to produce products are frequently impacted by bacteriophage infections. Even the dairy fermentation industry is impacted, with bacteriophages causing significant problems in yogurt production. In an evolutionary war, it seems you can’t win them all.
David L. “Woody” Woodland, Ph.D. is the chief scientific officer of Silverthorne-based Keystone Symposia on Molecular and Cellular Biology, a nonprofit dedicated to accelerating life science discovery by
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