Journey

We compulsively wash
our hands, spray our countertops and grimace when someone sneezes near us—in
fact, we do everything we can to avoid unnecessary encounters with the germ
world. But the truth is we are practically walking petri
dishes, rife with bacterial colonies from our skin to the deepest recesses
of our guts.

All the bacteria living inside you would fill a half-gallon jug; there are 10
times more bacterial cells in your body than human cells, according to Carolyn
Bohach, a microbiologist at the University of Idaho (U.I.), along with other
estimates from scientific studies. (Despite their vast numbers, bacteria don’t
take up that much space because bacteria are far smaller than human cells.)
Although that sounds pretty gross, it’s actually a very good thing.

The infestation begins at birth: Babies ingest mouthfuls of bacteria during
birthing and pick up plenty more from their mother’s skin and milk—during breast-feeding,
the mammary glands become colonized with bacteria. "Our interaction with our
mother is the biggest burst of microbes that we get," says Gary Huffnagle, a
microbiologist and internist at the University of Michigan at Ann Arbor. And
that’s just for starters: Throughout our lives, we consume bacteria in our food
and water "and who knows where else," Huffnagle says.

Starting in the mouth, nose or other orifices, these microbes travel through
the esophagus, stomach and / or intestines—locations where most of them set up
camp. Although there are estimated to be more than 500 species living at any one
time in an adult intestine, the majority belong to two phyla, the Firmicutes
(which include Streptococcus,
Clostridium and Staphylococcus), and the Bacteroidetes (which
include Flavobacterium).

For a long time, scientists assumed that these bacteria, despite their
numbers, neither did us much harm nor much good. But in the past decade or so,
researchers have changed their tune.

For one thing, bacteria produce chemicals that help us harness energy and
nutrients from our food, Huffnagle explains. Germ-free rodents have to consume
nearly a third more calories than normal rodents to maintain their body weight,
and when the same animals were later given a dose of bacteria, their body fat
levels spiked, even if they didn’t eat any more than they had before.

Intestinal
bacteria also appear to keep our immune systems healthy. Several studies
suggest that microbes regulate the population and density of intestinal immune
cells by aiding in the development of gut-associated lymphoid tissues that
mediate a variety of immune functions.

The bacteria also appear to influence the function of immune cells like
dendritic cells, T cells
and B cells, although scientists don’t know the precise mechanisms yet. And one
chemical released by the bacterium Bacteroides fragilis is capable of
directing how the developing immune system matures.

Further, probiotics—dietary
supplements containing potentially beneficial microbes—have been shown to boost
immunity. Not only do gut bacteria "help protect against other disease-causing
bacteria that might come from your food and water," Huffnagle says, "they truly
represent another arm of the immune system."

Of course, they can’t protect against every onslaught, which is why we still
have to depend on antibiotics to rid us of some disease-causing infections. But
antibiotics don’t just kill off the "bad" microbes, they wipe out the "good"
ones, too. That’s why antibiotic use can cause diarrhea and upset stomach: The
drugs interfere with the balance of our bacterial flora, making us sick,
Huffnagle explains.

But the bacterial body has made another contribution to our humanity—genes.
Soon after the Human Genome
Project published its preliminary results in 2001, a group of scientists
announced that a handful of human genes—the consensus today is around 40—appear
to be bacterial in origin.

The question that remains, however, is how exactly they got there. Some
scientists argue that the genes must have been transferred to humans from
bacteria fairly recently in evolutionary history, because the genes aren’t found
in our closest animal ancestors. Others argue that they may be ancient relics
from evolutionary events that took place early in our species’s history and, for
reasons unknown, the genes were lost in these ancestors. It’s impossible to know
for sure at this point.

"There remain to my knowledge no clear cases of human genes recently acquired
from bacteria," says Cédric Feschotte, a biologist at the University of Texas at
Arlington. "It doesn’t mean there are none, but they are not well
documented."

One thing is for sure: our lives and even our identities are more closely
linked to the microbial world than we may think. Bacteria do a lot to keep us
healthy, and scientists are just beginning to uncover their valuable secrets. As
U.I.’s Bohach says: "We do not completely understand the full impact of our
bacterial flora on our health and physiology."

Reference:

http://www.sciam.com/article.cfm?id=strange-but-true-humans-carry-more-bacterial-cells-than-human-ones&page=2

This entry was posted on Thursday, December 6th, 2007 at 9:02 pm and is filed under Uncategorized. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.