RAPID
METHODS
FOR
SEAFOOD
INDUSTRY
97
The colored spots correspond
to
the colonies
of
the target microorganism
on
the
master
plate.
Gene
Probes,
DNA/RNA
Hybridization
A gene
probe
is a
short length of
nucleic acid
(DNA
or
RNA)
which
consists
of
either an entire
gene
or
a fragment
of
a
gene.
Both
DNA and
RNA
probes are
used
to detect
complementary
DNA
or
RNA
of
target microorgan-
isms in foods. Gene probes
will
pair with
complementary
sequences
of
single-
stranded
DNA
extracted from bacterial cultures.
Gene
probes have regions
of
homology
with specific
sequences in
the
DNA of
the targeted organism
and
will
pair
with
these
complementary sequences
to form
a
hybrid double helix. The
gene probe carries
a label which
facilitates the detection
of
the hybridization
product.
When targeted
DNA
is
absent,
no
hybridization occurs.
DNA
target
sequences most
often are
double-stranded
and must
be denatured before the
hybridization process.
RNA
target sequences
are
usually
single-stranded
and
do
not
need
to
be
denatured. Additionally, probes
can
be targeted
against
either
DNA
or
RNA
of
bacteria.
A
number
of
commercially
available gene probes are
RNA probes,
enzymatically
or
radioactively labeled and
are
targeted to bacterial
ribosomal RNA
(rRNA).
Ribosomal RNA is present
in multiple copies
(about
1,OOO)
in
the bacterial cell
whereas
DNA
is present
in
a
single copy.
By
using
rRNA
as
a
target, the sensitivity
of
the
assays
increases.