New analysis reveals human
mouth carries more germs than expected
BY KRISTIN WEIDENBACH
The human mouth is awash
with bacteria. Mostly neighborly bugs, they live on our
teeth and gums, helping to digest food and to ward off
attack by less friendly, disease-causing bacteria that
can steal their way in.
Stanford researchers have
now shown that more of these oral inhabitants exist than
previously thought. Using a combination of old and new
scientific methods to study a scraping of plaque from a
healthy human mouth, the researchers found evidence of 37
unique bacteria that microbiologists had never before
recorded. Some were closely related to bacteria that
scientists are familiar with, but others were very
different.
Knowing more about the
bacteria that reside in a normal, healthy mouth may help
physicians understand changes in the bacterial population
that can lead to gingivitis, periodontitis and tooth
decay.
"Our data suggest
that a significant proportion of the resident human
bacterial flora remain poorly characterized, even within
this well-studied and familiar microbial
environment," said David Relman, MD, assistant
professor of medicine and of microbiology and immunology
at Stanford, and lead author of the study published in
the December 7 issue of the Proceedings of the National
Academy of Sciences. Relman and colleagues conducted the
research in his lab at the Veterans Affairs Palo Alto
Health Care System.
According to Relman, the
subgingival crevice -- the deep gum pocket cradling each
tooth -- has been repeatedly scrutinized in the search
for microbes. Even though almost 500 bacterial strains
have been identified already, Relman believes this may be
only a fraction of the bacteria living in this oral
groove.
Oral bacteria have
traditionally been studied by taking a scraping or sample
from inside the mouth, growing the bugs in the laboratory
and then identifying different species according to
biochemical tests and the type of food source that each
bacteria prefers. Using this method, the Relman team
identified bacteria found in a sample of plaque taken
from the subgingival crevice.
They also searched the
same sample using molecular techniques. Instead of
nurturing the bacteria in the lab, they prepared DNA
directly from the plaque and studied each genetic
sequence that had a bacterial signature. Comparing the
results, they found that the molecular method yielded
many new bacteria. Not only did the method reveal bugs
that had never before been found in the mouth, many were
bugs that had not yet been documented by microbiologists.
The team discovered 31
bacteria using the molecular method. In contrast, the
traditional approach, which only identifies bacteria that
can be cultivated in the lab, uncovered only six new
bugs.
"Sequence-based
environmental microbial surveys have taught us that
cultivation methods woefully underrepresent the true
extent of bacterial diversity," said Relman.
Although the cultivation
method traditionally used by clinical microbiology labs
did not uncover the bacterial diversity revealed by the
molecular technique, the researchers found that some
bacteria were more readily recovered the old way,
indicating that DNA analysis and cultivation both have a
role in the comprehensive study of human microbial
populations.
Characterization of
previously undocumented oral bacteria is part of the
Relman team's larger effort to identify rare and unusual
microbes that make their home in the human body -- many
cohabiting benignly but some causing mysterious human
disease.
Ian Kroes, MD, lead author
of the study, was a medical student in Relman's lab at
the time the research was conducted. Paul Lepp, PhD, a
postdoctoral fellow in the lab and the third member of
the research team, conducted much of the DNA analysis.
The research was funded by
the Donald E. and Delia B. Baxter Foundation and the
Lucille P. Markey Charitable Trust. SR
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