Growings On: Understanding viruses

Roger Gates

Chicken pox, Chikungunya, Ebola, H1N1, hepatitis, Heartland, HIV, influenza, measles, rabies, rubella, SARS, smallpox, West Nile, Zika — the list includes names both familiar and exotic. Most are, at best, unpleasant terms. The list now includes another: coronavirus (COVID-19). All these terms are labels that identify a human disease associated with a virus, many highly infectious.

The virus first demonstrated to cause disease was responsible for tobacco mosaic. In the 1890s scientists found that extract from an infected tobacco plant could cause the disease in a healthy plant, even after passing through a fine filter know to exclude bacteria. Since that discovery, more than a century ago, scientists have disagreed about whether viruses are alive. Compared to familiar organisms, viruses are very simple — not much more than a capsule containing genetic material, DNA or RNA. A frequently used criteria to denote something as being alive is the ability to reproduce. This, viruses cannot do — on their own. They must infect a living cell (plant, animal or bacteria) and stimulate that cell to reproduce the virus’ genetic information. With pathogenic viruses, this reproduction is followed by the lysis of the cell and release of multiplied viruses. Lysis or rupture of the host cell results in its death.

Alive or not, viruses are an indisputable component of our planet’s interdependent life systems. While virologists, scientists who study viruses, have worked diligently to understand, prevent and manage diseases caused by viruses, efforts have also been underway to understand viruses that are not responsible for disease. The notion that some microorganisms may be beneficial is more and more evident. Health practitioners are aware that the use of antibiotics to control pathogenic bacteria may lead to reduction or even elimination of microflora that are critical to normal function of the digestive system. Probiotics is a general term for products sold with the intention of stimulating the “good bugs” in the gut.

Virologist Dr. Marilyn Roossnick, who studies beneficial viruses at Penn State, says that all of us have “a population of viruses in the gut, the skin and even in the blood. This viral collection even has its own name to counter the bacteria-centric focus of the microbiome; it's called the virome.” The virus population is present soon after birth and fluctuates rapidly at first. Composition of an individual’s virome eventually stabilizes and is unique for everyone.

A large fraction of the normal viral population are bacteriophages, which target bacteria. One particular virus, which is present in about half of the human population targets the most common gut bacteria, bacteroides. When are why certain viruses are present remains uncertain, but it is clear that they are present in healthy individuals.

Another group of virologists at San Diego State University are examining the role of viruses found in mucus. This slimy material lines most of the surfaces that line internal organs and are exposed to the external environment, notably the lungs and the digestive tract. Mucus is composed of mucin, molecules consist of strings of sugars extending from a protein axis. A solution of nutrients and other compounds surrounds the mucin. Bacteria are attracted to the surface of the mucus, but the lowest layer of the mucus, adjacent to the tissue being protected is essentially sterile. The mucus surface is also populated by bacteriophage in high numbers. These viruses kill the bacteria attracted to the mucus layer.

Mucus-covered surfaces occur in organisms other than humans. It protects fish, worms and corals and protective phages appear to be evident is these other animals as well. According to microbiologist Jeremy Barr, "It's a novel immune system that we think is applicable to all mucosal surfaces, and it's one of the first examples of a direct symbiosis between phages and an animal host.”

Another application of beneficial viruses is in the treatment of cancer. Typical treatments of chemotherapy, radiation therapy and surgery have improved survival rates but often lead to undesirable side effects because healthy cells are also targeted by the treatment. Current research is employing viruses to kill cancer cells selectively. By identifying a virus that targets a cancer cell, a cancer patient may be treated by introducing the virus. The virus infects the cancerous cells, reproduces and results in the lysis and death of the cancerous cell, while healthy cells are not affected.

While the effects of coronavirus (COVID-19) range from inconvenience to devastation, a growing understanding of viruses and their importance to living systems provides compelling evidence of the complexity and intricacy of the design in nature.

Roger Gates is the agricultural and natural resources agent for University of Georgia Extension, Whitfield County. Contact him at roger.gates@uga.edu.

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