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842 Part 8: Food Safety and Food Allergensproduction of by-products of metabolism that have a negative
effect on the target organism such as acids and other inhibitory
substances; (2) the state of the organism in the sample (injury
status); studies have demonstrated that injured cells may need
additional time to recover prior to any standard enrichment as
they may not be able to grow in the presence of a selective
agent, and so on, therefore, there is a risk in missing detection of
the target; (3) selectivity of the enrichment media used–-authors
have noted that not all standard protocols may be adequate for
detection of the target organism, there is evidence to suggest
that some selective enrichment media may not be appropriate
for detection of all target populations and that some strain types
may be inhibited or missed as a result of the enrichment process,
thus there is a risk that the protocol may not truly be reflective
of the population in a sample. Wu et al. (2001) demonstrated
some of the challenges posed in the detection and enumeration
ofE. coliO157:H7 in alfalfa seeds, an unusual food product that
has been linked to outbreaks of human illness (Mohle-Boetani
et al. 2001). Regardless of the approach taken, readers of this
article should consider carefully the type of food matrix that
requires testing and how it in itself may pose unique challenges
to detection of the target pathogen.
In recent years, methods to automate and significantly shorten
the detection time required have been implemented for some
protocols used in the detection of pathogens and these can sig-
nificantly reduce labor and time of detection. They do, however,
come at a cost in terms of technology or specialized systems
for detection, which may not be appropriate for all laboratory or
production facilities.Automated Methods to Detect PathogensCommercial companies have considerably expanded their of-
ferings in automated and rapid methods for the detection and
identification of food-borne pathogens, while in some instances
the methods are proprietary, the basic principle of the meth-
ods are relatively similar—examples of some of the more com-
mon commercial detection systems currently available include
the VIDAS©R
system (BioM ́erieux), which has application for
the detection of pathogens such asE. coliO157:H7,Campy-
lobacter,Listeria,Salmonella, and so on in food systems. The
VIDAS system is based on an immunoassay type reaction that
can detect the pathogen in an enrichment culture in as little
time as 45 minutes. The protocol does, however, require en-
richment prior to detection and this can entail time frames of
6–24 hours.
The BAX©R
system (DuPont and Qualcon) is another auto-
mated system for the detection of pathogens such asCampy-
lobacter,E. coliO157:H7,Salmonella,Listeria, S. aureus,and
so on in foodstuffs using a real-time PCR-based assay. The
BAX system can detect the pathogen directly in a sample (if
heavy contamination is suspected) or following an enrichment
phase. The BAX system uses a PCR-based protocol to amplify
genes specific to the species of interest and detect presence
of the organism through fluorescence of the amplified product.The system is also capable of assessing the number of cells in
a sample—the minimum detection limit for the technology is
about 10^4 colony forming units (CFU)/mL, and can detect target
pathogens in as little as 90 minutes. This technology has re-
cently received AOAC approval for application in the detection
of a range of pathogens.
Another rapid technology for the detection of pathogens in-
cludes the simple and relatively low-cost alternative called lat-
eral flow kits. Lateral flow kits (Dipstick, DuPont) work on the
principle of an antigen antibody reaction and are designed to
detect the presence of antigens in a sample. The system uses
specific enrichment media to increase the population of the tar-
get pathogen and then a test strip is added to an aliquot of the
enriched sample, the strip employs a combination of antibodies
specific to the target pathogen and gold colloidal particles on the
surface of the membrane. The liquid sample wicks up the test
strip by capillary action and if the target pathogen is present it
reacts with the antibodies present to result in a color change and
a visible line in the strip. This method has been available from a
number of vendors and has proven successful for rapid screen-
ing of samples for target pathogens. Of concern, however, is the
specificity of the antibodies used in the strip that may not be able
to detect all strain types present in a sample. Fakhr et al. (2006)
found that the a lateral flow kit can fail to detectSalmonellain
some poultry samples that were positive forSalmonellaby other
methods including real-time PCR and selective culture, suggest-
ing that specificity of the antibodies used in the assay may not
always detect the target.Automated Methods of IdentificationAlthough not strictly part of this review, it is important to dis-
cuss the differences between detection of a pathogen and its
identification. While there are a range of technologies avail-
able for the detection of pathogens, there are also a significant
number of technologies available for the rapid identification of
pathogens once detected. These include a range of automated
systems that are based on biochemical tests or metabolic pro-
files Vitek – (BioM ́erieux) and Microstation (Biolog), or cellular
fatty acid analysis systems (gas chromatograph based, Sherlock
Microbial Identification system). In addition, there are multiple
smaller rapid identification kits that can successfully identify
a pathogen in a period of a few minutes to a few hours, these
include systems such as microID, Trek’s GNID, API, and so on;
the majority of these kits are visual and based on biochemical
reactions leading to changes in color (or fluorescence if read
automatically, TREK GNID). In addition, DNA-based methods
can successfully identify a potential pathogen by amplification
of single or multiple genes associated with a particular species.Immunologic-Based Methods of
Pathogen DetectionImmunological-based methods for pathogen detection rely
on the binding of a bacterial antigen to a monoclonal or