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Unformatted text preview: n instantaneous dose 3,000- Gy of γ-irradiation with no loss in viability. In
contrast, a single dose of 8 Gy will kill a human. Based on the amount of radiation it
can handle, it has been estimated that D. radiodurans could even survive the amount
of radiation released in an atomic explosion. The name of the bacterium means “strange
or marvelous” (dein) berry (coccus) that withstands radiation. Fortunately, there is little
need to worry about its being a super pathogen: the organism does not cause disease and
it is susceptible to antibiotics.
D. radiodurans has been isolated from the water surrounding the cores of nuclear
reactors and environments as diverse as Antarctic dry valleys, elephant dung, rainwater
and the upper atmosphere. Ignoring reactor cooling water, no natural environment on
Earth comes even close to providing the radiation doses D. radioduransis resistant to,
which begs the question: Why has this bacterium evolved hyperradiation resistance?
Although the D. radiodurans chromosome is fragmented by irradiation, the
organism can reassemble the pieces of its DNA in the correct order to re-form a
functional chromosome. The D. radiodurans protein that mediates DNA repair is RecA
but it is slightly different in amino acid sequence and function from the RecAs of other
bacteria. Most RecAs first coat single-stranded DNA, then take up homologous doublestranded DNA (Chapter 5). The D. radiodurans RecA does exactly the opposite. It
forms a filament on double-stranded DNA and then incorporates a homologous singlestranded molecule. D. radiodurans RecA is thought to bind the double-stranded
fragments of irradiated DNA and pair the homologous ends together. With the help of
other enzymes, the chromosome is reconstructed. This mechanism can only work if
there are at least two chromosome copies present per cell so overlapping fragments are
generated during irradiation and this is the case. Unlike most bacteria, cell division in D.
radiodurans keeps several copies of each chromosome, making it more likely that it will
find an intact copy of any given sequence of nucleotide (The repair process will be
described in more detail in class). It is a remarkable evolutionary adaptation to
environments where chromosome fragmentation may be a problem. In the case of D.
radiodurans, whatever these environments are remains a mystery. 9.7 The human body as an environment
Even for non cannibals, one source of bacteria in food is the human body. The
microorganisms which normally colonize the human body are called the normal flora.
The term flora is a historical one, microorganisms used to be classified as plants. The skin The human adult, on average, posses 2 m2 of skin. This organ is normally populated by
1012 microbes most of which are Bacteria, fungi and yeasts; so far Archaea have not been
found there. As far as bacteria go, about different 200 species have been detected both by
pure culture techniques and SSU rRNA gene analysis of microbial DNA recovered from
Most of th...
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- Fall '12