Usually I write about things that I like to imagine that I know at least a bit about. Today is an exception. I have little background in this subject, but some recent readings got me so interested that I’m writing about microbes — bacteria and viruses — which are too tiny to see without fancy equipment, but which could be said to rule the world. The implications for medical matters of some relatively new information are enormous.
There are way more kinds of microbes, with lots more genetic diversity, than all other organisms on earth. The numbers are so big that most of us can’t really comprehend them. It turns out that microbes are involved with almost everything we are and do! Mind-boggling!
For starters, each of us harbors a couple of pounds of bacteria (as well as trillions of the even-smaller viruses, of which more, presently). We are all familiar with the idea that bacteria can cause unpleasant diseases, but many bacteria are actually beneficial. Some live on our skin, where they can help keep harmful bacteria at bay. Many live in our digestive tracts, where they perform major feats of breaking down food into usable particles. Bacteria produce helpful enzymes too, such as one that allows a baby to extract nutrition from milk — reportedly, no human gene encodes an enzyme that can do this.
Having the right assortment of resident bacteria can, at least sometimes, help determine if one is obese, diabetic, has high blood pressure or multiple sclerosis, for example, or if one escapes these conditions. Bacteria that affect metabolism may even be involved with the condition of autism, or the avoidance of same. It may become possible to combat a nasty sort of bacteria with the proper application of another one, which outcompetes or destroys the first one. The previously unsuspected roles of bacteria in many human ailments — or in our avoidance of them — are just starting to be explored, and the medical implications for treatments are fascinating.
Now consider viruses for a moment. Viruses consist of a small protein shell and usually just a few genes. They make their living, so to speak, by invading the cells of other organisms and taking control of the metabolic machinery in those cells, forcing the cell to make more viruses. However, the process of replication is sloppy, with lots of mutations, and if two viruses occur together in one cell, they may exchange genetic material. So new versions of viruses are emerging all the time. Sometimes the new versions find an opportunity to invade entirely different species, jumping from bird to human, or pig to primate.
Inside the cell, reproduction of the invading virus may be extremely rapid, eventually exploding the cell and releasing viral offspring. Flu viruses and the common cold virus work this way. Other viruses don’t kill the host cell, but they speed up cell division and cause the host cell to make more cells. That’s the makings of a possible cancer. Whether or not a cancer actually develops, however, depends on many factors, including the specific kinds of cells involved and if a previous exposure has already alerted the immune system.
Surprisingly, viruses may offer a way to treat some bacterial infections. Many, perhaps most, viruses attack bacteria, and they are commonly host-specific, attacking only one kind of bacterium. These viruses are called bacteriophages, or phages, for short. So there is hope that medical treatments with phages might help control some bacterial infections. Of course, the bacteria can mutate, and some would probably become phage-resistant (just as happens with antibiotics). But phages can evolve too and might acquire a mutation that overcomes resistance by bacteria.
What really “blew me away” was learning a little about retroviruses, which insert their genes into the DNA of the host cell. These viruses can cause host DNA near the insertion to make proteins, in effect turning on host genes that had been turned off, and cancer can sometimes result. But not inevitably!
After a retrovirus inserts itself into the host DNA, the host’s immune system may inactivate it or mutations may cripple it. The inserted but inactive retrovirus can spread harmlessly through the host as the host’s body makes new cells (which it does frequently). If the retrovirus gets into an egg or sperm cell, it can be passed on to offspring and to future generations.
We now know that this has happened many, many times in the long history of life on earth. Viruses have been around for billions of years. Retroviruses are found in all kinds of vertebrates, from fishes to humans. In fact, about eight percent of DNA in humans comes from retroviruses that long ago inserted themselves into our ancestors’ DNA!
Even crippled retroviruses can be dangerous, if a mutation re-activates them, or if they insert new copies of themselves into the host genome. So, not surprisingly in this co-evolutionary arms race, humans (and probably other animals too) have evolved at least a few means of defense, by producing certain proteins that disable the retrovirus’ process of replication.
It turns out that retroviruses are essential to human (and perhaps most mammalian) reproduction. Most species of mammals, including humans, feed a growing fetus by means of a placenta. A particular retrovirus plays a critical role in the development of a placenta, without which a fetus would quickly starve to death.
So, in effect, you are not entirely what you probably thought you are. In addition to truly human DNA from your parents, you are partly an assortment of good, neutral, and bad bacteria, some resident and some transient, and good, neutral, and bad viruses, some of them buried in your DNA. In short, each of us is an ecosystem (living in an environment that itself is a gigantic ecosystem).
Just think: ecosystem management applied to medicine!
• Mary F. Willson is a retired professor of ecology.