Safeguarding Against False Test Results

Beyond human error, equipment problems can cause wrong incorrect results. Instrument failure can skew test results. Also, the reliability of test kits varies from manufacturer to manufacturer, and faulty kits produce incorrect results. Garbage in, garbage out.

Conventional Testing Methods

There are three main detection paradigms used to test for Salmonella and Campylobacter: Culture, ELISA, and PCR.

Culture. A pathogen test in which a sample of the food in question is placed on traditional selective culture media to allow the target microorganism to grow while simultaneously preventing the growth of other organisms. Culturing is the oldest method of pathogen detection as well as the gold standard because of its clear, visible endpoint. However, cultures are time consuming, taking several days to produce results depending on the target organism, which can be prohibitive when dealing with perishable food with a short shelf life. It also requires trained laboratory staff following Good Laboratory Procedures and, even when technicians take the utmost care, cultures are vulnerable to the interference of background flora.

ELISA. Enzyme-Linked ImmunoSorbant Assay determines if a pathogen is present by detecting the presence of an antibody that has linked to it. An enriched sample solution is placed in a 96-well plate coated with a protein that will bind to an antibody to the target microbe. The solution is removed and a second antibody, linked to an enzyme, is added that will bind to the first antibody, making an antibody-antigen-antibody sandwich that will cling to the side of the well. The solution is washed away and a substrate is added to the well that will cause the colorless antibodies to become colored products, thus signaling the presence of the target bacteria. A detraction from ELISA is its cost. A different test kit is needed for each unique strain of bacteria, and this can impact the reliability of results. Companies will do a cost-benefit analysis and test for usually two or three strains that are the most likely to appear in the products, potentially overlooking a harmful microorganism.

PCR. Polymerase chain reaction is arguably the most sensitive of the three methods because it looks for the actual DNA of Salmonella or Campylobacter strains. A sample is heated in a thermocycling instrument, splitting the double-helix DNA into a single strand. An enzyme called “Taq polymerase” is added which builds two new strands of DNA using the originals as templates, and then proceeds to amplify the DNA exponentially, creating a large enough sample of DNA for pathogen detection.

What’s New?

One example of a platform that claims to lower the risk of technician mistakes is the Molecular Detection Assay (MDA). Launched by 3M in 2011, MDA uses BART (Bioluminescent Assay in Real Time) technology to recognize distinct sections of a bacteria’s genome. In an email, 3M’s food safety division explained that MDA involves two processes: Isothermal DNA amplification, meaning it is done without a thermocycling instrument, and bioluminescence which uses luciferase, the enzyme that causes fireflies’ abdomens to light up. An enzymatic process in the enriched sample produces ATP which reacts to luciferase, causing the target pathogen DNA to glow. MDA reduces the risk of human error by requiring only a single instrument and preparation protocol across most assays. The technician does not have to match the protocol to the pathogen, thereby lowering the opportunity for confusion. Additionally, MDA uses color-coded assay tubes to differentiate pathogen assays to help shrink the margin of error.

Safeguarding against instrument failure is another way to improve testing accuracy. MDA does not require calibration and features an automatic diagnostic program that runs on startup.