Food Biochemistry and Food Processing (2 edition)

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842 Part 8: Food Safety and Food Allergens

production 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 Pathogens

Commercial 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 Identification

Although 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 Detection

Immunological-based methods for pathogen detection rely

on the binding of a bacterial antigen to a monoclonal or