Navel Gazing ■ 277The gambit worked. Mathematician Sharon
Berwick at the University of Maryland took a
new approach to the data. Instead of focusing
on characteristics of the participants, such as
gender or location, she looked at the charac-
teristics of the microbes and found that people
tend to be dominated by one of two types of belly
button bacteria: aerobic or anaerobic. Aerobes
are prokaryotes that need oxygen gas to survive.
Micrococcus species, which Dunn found in the
navel, are aerobes; they need oxygen. Therefore,
species of Micrococcus are unlikely to do well
deep inside a belly button, says Dunn, but they
appear to thrive on the surface.
Another type of navel resident, species of Clos-
tridia, do not use oxygen. They are anaerobes,
prokaryotes that survive without oxygen. In fact,
some anaerobes may actually be poisoned by
oxygen. Among the anaerobic archaeans are several
species of methanogens, which feed on hydrogen
and produce methane gas as a by-product of their
metabolism. The ability to exist in both oxygen-rich
and oxygen-free environments is another reason
why prokaryotes can be found in most habitats.
Some prokaryotes can even switch between
the two. One of the most common bacteria
Dunn’s team found in belly buttons was Staph-
ylococcus. Although it is best known as a patho-
gen, staph on your skin is typically a good guy. On
the skin, it is beneficial for your health, fighting
off other pathogens that want to crowd in. Some
species of staph, such as Staphylococcus epider-
midis, are typically aerobes, but when oxygen is
in short supply, they switch over to a special type
of anaerobic metabolism known as fermenta-
tion, involving the breakdown of sugars (yes, the
same process that is used to ferment beer and
wine; to review fermentation, see Figure 5.12).
In other words, as Dunn wrote in one descrip-
tion of S. epidermidis, “Right now you might be
making a teeny tiny bit of navel wine.”
Like staph, Bacillus subtilis, a rod-shaped,
spore-forming bacterium, can grow in both
aerobic and anaerobic conditions. B. subtilis istypically reproduce by splitting in two in a process
called binary fission, a form of asexual reproduc-
tion (see Figure 6.3). The DNA in the parent cell
is copied before fission, and one copy is trans-
ferred to each of the resulting daughter cells. The
genetic information in the daughter cells is virtu-
ally identical to that of the parent cell, as is invari-
ably the case in asexual reproduction.
Although sexual reproduction has not been
seen in prokaryotes, they are adept DNA pick-
pockets: microbes can capture bits of DNA from
their environment or other bacteria and incorpo-
rate them into their own genetic material. The
transfer of genetic material between microbes is
known as horizontal gene transfer and it involves
plasmids, loops of extra DNA in the cytoplasm of
prokaryotes (see Figure 12.12). (Horizontal gene
transfer has been seen in some eukaryotes as well.
Bdelloid rotifers, for example, a type of micro-
scopic freshwater animal, stole about 8 percent
of the DNA in their genome from bacteria.) A
bacterium can directly trade DNA with another
bacterium through a process known as bacterial
conjugation. Alternatively, when a bacterium
dies, the cell may burst open and another bacte-
rium may simply take up the released DNA.
Some types of bacteria—but no archaeans—
can undergo sporulation, the formation of
thick-walled dormant structures called spores.
Spores are the bomb shelters of microbes:
they can survive boiling and freezing, thereby
allowing the microbes to hang out for a long
period of time until the conditions are again
favorable to reproduce. In many cases, even
antibiotics can’t kill bacterial spores.
All Hands on Deck
Dunn, Menninger, and the team analyzed the addi-
tional samples. Still, they were unable to explain
the differences among belly button microbes. They
could not identify any factors affecting the number
and types of species a person might host.
So they decided to take a step that more and
more scientists are taking—they made all their
data freely available online for other researchers
to use (with all identifying information about
the participants removed). “We asked other
researchers to let us know what their insights
are, because we’re all in this together,” says
Menninger. “We need all hands on deck.”
Holly Menninger is the director of public science at North
Carolina State University, where she works to engage
the public in citizen science and science education.HOLLY MENNINGER