Food Safety

Detection and Isolation of foodborne pathogens

Sunday, July 11, 2010 @ 4:42 PM

Iummnoassays(ELISA method)

The purpose of ELISA is to determine if a particular protein is present in a sample and if so, how much. This method produces quantitative and automatable results which thus, takes longer time than qualitative test. The test works by using anti-bodies specific for proteins encoded by rDNA sequences. It relies on the reaction between antibodies and a “foreign substances” called antigen. Antibodies are made in response to infection. Thus, an antibody ELISA can detect whether the sample had been contaminated with viruses. For an antibody ELISA, antigens are stuck onto a plastic surface, a sample is added and any antibodies for the disease we are testing for will bind to the antigens. Following, a second antibody with a marker will be added. Positive reaction will be noted if appropriate substrate is added. If there are no antibodies in the sample, the second antibody will not be able to stick and there will be no colour change.These reactions can be detected by a color change or a flourometric reaction.

Figure 1. Detection of antigen-antibody binding (IFA and ELISA tests)

i:Test serum is incubated with antigen immobilised on a 96-well plate or microscope slide
ii:Secondary antibodies labelled with an enzyme are added
iii:After washing, any bound secondary antibodies can be detected using the marker. The label used is an enzyme which induces a colour change when the substrate is added

Figure 2. A typical ELISA at Biobest

Imunoassays methods are less sensitive than PCR methods.One problem with immunoassays is that the technique does not distinguish between different sources of biotechnology-derived rDNA that may exhibit similar characteristics. One good example would be, it will not detect whether BT protein is derived from soy or corn. To add on, proteins are denatured during food processing thus ELISA is more appropriate for raw food and food that undergone minimal processing.

An ELISA is a five-step procedure: 1) coat the microtiter plate wells with antigen; 2) block all unbound sites to prevent false positive results; 3) add antibody to the wells; 4) add anti-mouse IgG conjugated to an enzyme; 5) reaction of a substrate with the enzyme to produce a colored product, thus indicating a positive reaction. There are many different types of ELISAs. One of the most common types of ELISA is "sandwich ELISA."

Asia food information centre(2004) Detecting Genetically Modified Foods - Easier said than done. Retrieved on 30th June 2010 from http://www.afic.org/Detecting%20Genetically%20Modified%20Foods%20-%20Easier%20said%20than%20done.htm

Farid E Ahmed(2004) Testing of Genetically Modified Organism in food. Binghamton, NY: Haworth press

Biobest( nd) ELISA- How does the test work?. Retrieved on 30june from http://www.biobest.co.uk/diagnostics/elisa.html

Youtube(2007). Elisa. Retrieved on 1st july from http://www.youtube.com/watch?v=BBQWWi6cFXU&feature=player_embedded

Posted By: Alicia Chee