by Nick Carne
There are likely more genes in the body
there are stars in the universe.
it's a lot,
according to an
US scientists have begun the daunting task of trying to work out how
many genes there are in
the human microbiome.
Even when you consider just the gut and the mouth (in itself, a
unique research double) the numbers are potentially overwhelming.
Microbiologists and bioinformaticians from Harvard Medical School
and Joslin Diabetes Centre gathered all publicly available
sequencing data on human oral and gut microbiomes and analyzed the
DNA from around 3500 samples - 1400 from mouths and 2100 from guts.
In all, there were nearly 46 million bacterial genes,
24 million in the
oral microbiome and 22 million in the gut.
That leads the team to
suggest that there may be more genes in the collective human
microbiome than there are stars in the observable universe (we're
talking trillions) - and that at least half of them may be
unique to each individual.
"Ours is a gateway
study, the first step on a what will likely be a long journey
toward understanding how differences in gene content drive
microbial behavior and modify disease risk," says Harvard's
Braden Tierney, first author of a paper (The
Landscape of Genetic Content in the Gut and Oral Human
Microbiome) published in the journal Cell Host &
Microbe, with admirable understatement.
Most research to date has
focused on mapping the types of bacteria that inhabit our bodies in
an effort to determine whether and how the presence of a given
bacterial species might affect disease risk, the researchers say.
In contrast, their work looks at the genes that make up the various
microbial species and strains.
Given that genetic content varies greatly between the same microbes,
understanding how and whether individual microbial genes affect
disease risk is important.
"Just like no two
siblings are genetically identical, no two bacterial strains are
genetically identical either," says co-senior author Chirag
Patel, also from Harvard.
"Two members of the same bacterial strain could have markedly
different genetic makeup, so information about bacterial species
alone could mask critical differences that arise from genetic
More than half of the
23 million bacterial genes in the study occurred only
once, rendering them unique to the individual.
The researchers call
them "singletons”, and they appeared to perform different
functions to the other genes.
Commonly shared genes,
appeared to be involved in more or less basic functions critical to
a microbe's day-to-day survival, such the consumption and breakdown
of enzymes, energy conversion and metabolism.
Unique genes, by contrast,
tended to carry out
more specialized functions, such as gaining resistance against
antibiotics and other pressures and helping to build a microbe's
protective cell wall, which shields it from external assaults.
This finding, the
researchers say, suggests singleton genes are key parts of a
microbe's evolutionary survival kit.
"If a microbe needs
to become resistant to an antibiotic because of exposure to
drugs or suddenly faces a new selective pressure, the singleton
genes may be the wellspring of genetic diversity the microbe can
pull from to adapt," Tierney says.