64 PRINCIPLES OFFOODSANITATION
The reaction of the motile Salmonellaewith
the flagellar antibodies results in an immobi-
lized precipitation band 8 to 14 hours after
inoculation of the 1-2 test unit.
DNA-Based Microarray Assays
The emergence of new DNA-based
microarray assays permits a look at DNA
sequences of microorganisms, including
strains within an organism, for very pre-
cise identification. DNA microarrays are
considered to be a revolutionary concept in
the evolution of food microbiology tests
because in a single or small number of
assays, one can screen for a large number of
microorganisms. Following the standard
PCR protocol that amplifies the DNA for
detection of a microbe, an analyst can use a
single DNA chip to identify 40 to 100
species or strains of microorganisms in a
single test. DNA chip technology also
changes the way to approach and unknown
organism in a food matrix. With conven-
tional tests, one can only detect one
pathogen per single test. Knowledge of what
organisms may be in the food matrix is
essential before choosing an appropriate
test. DNA microarrays permit one to iden-
tify what microbe is in the food matrix
(McNamara and Williams, 2003).
IDEXX Bind
The IDEXX Bind for Salmonellais based
on the use of genetically engineered bacte-
riophages. The modified bacteriophages
attach to Salmonellareceptors and insert
DNA into the bacterial cells. During incuba-
tion, the modified DNA causes Salmonella
to produce ice nucleation proteins. At a spec-
ified temperature, the ice nucleation proteins
promote the formation of ice crystals. Posi-
tive samples will freeze and turn orange at
this temperature; whereas negative samples
will not freeze.
Random Amplified Polymorphic DNA
The random amplified polymorphic DNA
(RAPD) method has achieved promising
results, especially to trace L. monocytogenes
infections in humans. Advantages are the
low cost of the multiple DNA primers,
discriminating nature of the test, and, the
ability to trace small amounts ofL. monocy-
togenes. Since this assay is time consuming,
it has more utility as a research tool than as
a diagnostic test for industry use.Immunomagnetic Separation and Flow
Cytometry
This technique can be used to detect less
than 10 E. coliO157:H7 cells/g of ground beef
after enrichment for 6 hours. The immuno-
magnetic beads concentrate cells, making it
easier to detect, using flow cytometry. Detec-
tion limit is not significantly influenced by the
presence of other microorganisms. During
the past, this method has been used more as a
research tool than as a diagnostic tool in the
food industry.Diagnostic Identification Kits
These kits were developed for human clin-
ical medicine but can aid in the identification
of various microorganisms. Most of these
tests are for use with isolated colonies, which
require 1 to 3 days to obtain.CAMP Test
In this test, a bacterial isolate that is sus-
pected to be L. monocytogenesis streaked
adjacent to or across a streak of a second,
known bacterium on a blood agar plate. At
the juncture of the two streaks, the metabolic
by-products of the two bacteria diffuse and
result in an augmented hemolytic reaction.
Hemolysis of blood cells is an important
characteristic of pathogenic bacteria such as
L. monocytogenesbecause it appears to be