Viruses and mucus team up to ward off bacteria

The last thing most people would
want in their bodies is mucus laden
with viruses. But a new study
suggests that viruses called
bacteriophages, or phages, grab
onto mucus and then infect and kill
invasive bacteria. The finding,
reported May 20 in the
Proceedings of the National
Academy of Sciences by Forest
Rohwer of San Diego State
University and colleagues, could
mean that some viruses partner
with animals and humans to stave
off bacterial infections and control
the composition of friendly
microbes in the body.
Bacteriophages are viruses that
break open bacteria, killing them.
Researchers have studied
bacteriophages for decades, and
some disease therapies take
advantage of the viruses’ bacteria-
slaying abilities, says microbiologist
Frederic Bushman of the University
of Pennsylvania medical school.
But the study provides what
Bushman says is a revelation that
should have been obvious; phage
may be a natural part of the
immune system. “It’s new in a way
that is sort of common-sensey,” he
says.
Previously, researchers thought of
mucus mainly as a physical barrier
to keep invading organisms from
entering the body. The slimy
substance made by our noses,
intestines and other organs also
fights invaders with antimicrobial
molecules. Some researchers had
found bacteriophages stuck in
mucus, but they figured that the
mucus had stopped or slowed the
viruses. No one realized that the
viruses are part of the body’s
defense, says study coauthor
Jeremy Barr, who works in
Rohwer’s lab. “This is a natural use
of phage therapy that has probably
been around since mucosal
surfaces evolved,” Barr says.
Rohwer, who studies corals, had
noticed that phages tend to
concentrate in mucus. To find out
why, the researchers collected
mucus from human gums, sea
anemones, fish, corals and mouse
intestines. Mucus layers had more
phages and fewer bacteria than the
surrounding environment,
suggesting that the viruses helped
to limit the number of bacteria
allowed into the mucus.
Phages are coated in proteins that
latch onto sugars called glycans,
anchoring the viruses in the
mucus, the team discovered. From
there the phages can ambush
encroaching bacteria.
So far, the researchers have
demonstrated that mucus and
phages can work together to
protect cells in a dish. The next
step, Bushman says, would be
determining what happens inside
an organism, an experiment the
researchers are already planning.