Download Our Viral Companions by Dr. David L. (“Woody”) Woodland (as

Our Viral Companions
by Dr. David L. (“Woody”) Woodland
(as published in the Summit Daily News of March 16, 2015)
Some of you may have attended the free public forum on “Little Guys, Big Jobs: Microbes at
Work in Your Body” hosted by Keystone Symposia on Molecular and Cellular Biology on March
3. The event featured three leading experts on the human microbiome who discussed the
microorganisms that live in, and on, our bodies. They addressed topics ranging from the
characteristics of bacteria in our gut to the pros and cons of dietary probiotics. The speakers
noted that the human gut hosts trillions of bacteria that aid digestion, produce nutrients and
vitamins, influence the immune system, and displace pathogens.
One topic touched on was that of the human virome: the collection of all viruses that call the
human body home. Most people think of viruses as exclusively causing specific diseases, such
as the flu or Ebola, and don’t realize that we are riddled with viruses or viral remnants that
generally cause us no harm. For example, recent studies suggest that 8% of our genome is
derived from viruses, much of it the remnants of ancient viruses that are essentially dead.
Furthermore, we are all infected with multiple viruses that persist in a dormant or semi-dormant
state our entire lives. For example, virtually all of us are chronically infected with one or more of
the eight human herpes viruses, such as the chicken pox virus that can cause chicken pox and
shingles, the Epstein-Barr virus that can cause mononucleosis, and the Cytomegalovirus that
can cause disease in neonates and the immunocompromised. After the disease caused by the
initial viral infection ends, the virus can persist for life with minimal impact (although individuals
who have suffered from reactivation of the chicken pox virus and the resulting shingles might
disagree with this statement).
Viruses also play a significant role in our digestive tracts. In fact, the collection of viruses that
inhabit our guts is even more diverse than our bacterial microbiome. Interestingly, many viruses
in our bodies are bacteriophages – viruses that infect bacteria but not human cells. There are
approximately 100 bacteriophages for every bacterium in your body. This is an incredible
amount considering that we all harbor approximately 1014 (100 trillion) bacteria. These
bacteriophages constantly infect and kill host bacteria in a balance with bacterial growth and
resistance, thereby regulating bacterial density. This dynamic equilibrium is illustrated by studies
in the horse gut where it has been shown that a class of bacteriophages called coliphages
regulates the diversity and abundance of Escherichia coli bacteria.
Since bacteriophages infect and kill bacteria, they have a huge impact on the probiotics many of
us ingest. In fact, some probiotics may not elicit long-lasting effects due to the natural selection
of phages specifically infecting the ingested probiotic strains, and probiotic efficacy may quickly
decline with regular use. Consistent with this idea, recent genome sequencing studies have
revealed that many common probiotic bacterial strains are targets of bacteriophage predation.
While this may seem problematic for the probiotic industry, it is also an opportunity for a
different approach to modifying our gut microbiomes. Indeed, there is growing interest in
regulating the bacterial microbiome, not by ingesting more bacteria, but by ingesting selected
populations of bacteriophages. The approach leverages the fact that bacteriophages have
potent antibacterial activity, but in contrast to antibiotics, are very specific; i.e., they only kill
targeted bacterial species. This specificity means that bacteriophage-based probiotics can be
developed that lack the deleterious effects on beneficial gut microflora. The idea may seem farfetched, but already the Bill & Melinda Gates Foundation is encouraging research by seeking
visionary grant applications to develop bacteriophage-based approaches to improve gut health.
Bacteriophages might also be adopted by the livestock industry. A recent comparison of
probiotics and bacteriophages in pigs revealed that whereas both improve different aspects of
pig health, bacteriophages are more effective than probiotics, and both are preferable to the
current use of antibiotics. Antibiotics also change the composition of gut microbiota but act as
sledgehammers, killing many bacteria and modifying the microbiome in unhealthy ways.
So the next probiotic that you take may be teeming with viruses. In the meantime, I’ll stick to my
yogurt and contemplate the fact that my genome harbors way more viral sequences than my
laptop!
David L. Woodland
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 convening internationally renowned research conferences in Summit County and worldwide. Woody
can be reached at 970-262-1230 ext. 131 or [email protected].
For more (Petri) Dish columns, visit www.keystonesymposia.org.