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  • Archive for February 2011

    Product Testing, Part 6: “Current testing methods: the positive-negative challenge (continued)

    Tuesday, February 1st, 2011

    Julia Stewart:
    Hello, this is PMA PR Director Julia Stewart, and welcome back to PMA’s audio blog, “Ask Dr. Bob” with PMA’s Chief Science & Technology Officer Dr. Bob Whitaker. We’re in the middle of a continuing series on the challenges of pathogen testing in produce. In the last post we talked about how a positive is not always a positive and the importance of looking at the selectivity and sensitivity of tests.

    Bob, in the last post you also said that a negative test result might not actually be a negative. That seems even more worrisome to me. I mean if you test a product and the result is negative and you ship it out only to find later that it was positive, that can be a big problem.  Can you elaborate on that?

    Bob:
    Sure, Julia.  There are basically three issues to look at here: enrichment, interference and sampling issues.  Let’s take enrichment first.  Enrichment is simply the method used to permit any bacteria that might be in a sample to grow and multiply so you can get enough DNA to be able to isolate it and start the process of determining if any of it comes from a pathogen.

    Since pathogens are typically present only in very low levels, when present at all, this process of enrichment can be critical.  You also have to appreciate that these pathogens thrive in warm, humid, high nutrient environments — in other words our intestinal tracts and the surface of a fruit or vegetable is not necessarily the best environment for them.  This enrichment method permits these bacteria to recover, if you will, and begin growing again.  So, it’s important that conditions and times are maximized to permit pathogen growth.  This typically takes 12 to 18 hours.  Failure to use proper enrichment methods can result in pathogens going undetected even though they may have been present in the original sample.

    If that wasn’t enough of a challenge, we’ve got the issue of interference. Fruits and vegetables are extremely complex in terms of their chemistry. They’re made up of thousands of chemicals, and that composition can sometimes change dramatically based on variety or growing conditions.  Some of these chemical constituents can actually interfere with pathogen detection by interfering with DNA isolation, purification or detection.  It seems like it is almost necessary to develop specific isolation protocols for every commodity to be sure the tests are selective and sensitive enough to find pathogens if they are present.

    Lastly, we have the sampling issues. If your sampling program is not sufficient, it is quite possible that if there is a low level or non-uniform contamination event, like we typically see in produce, that you could simply miss including contaminated fruits or leaves in your sample.  We’ll talk about this more in a future post.  In fact, I think this is actually a more important consideration in product testing than even the tests themselves so it warrants a more extensive discussion.   But in the end, if you don’t have a dynamic sampling program, you can easily miss contamination and get a “negative” result for a field or batch of finished product that might indeed be contaminated.

    Julia:
    Thanks Bob.  You’ve raised some good points here.  After hearing this, I can see why you always say that food safety programs should first focus on preventing contamination from ever happening.  Detecting it after the fact is not a simple issue.  I can also see why you repeatedly emphasize how important it is for those doing testing to ask questions and fully understand what their labs are doing, and what the data may or may not mean. 

    Next time, let’s dive into this issue of confirmation testing and how that can be used to verify these initial or PCR-based screening test results.  Listeners, thanks for tuning us in today and we look forward to speaking with you further on this issue of product testing.

     
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