According to Tom Weschler, president of Strategic Consulting Inc.
( www.strategic-consult.com), “2010 testing for E. coli O157 in the U.S. totaled
4. 9 million tests, with red meat accounting for 80 percent or 3. 9 million tests.
In 2012, we project that STEC screening test volumes will eventually equal or
exceed 2010 E. coli O157 test volumes, which represents a major shift in U.S.
Types of Tests: Today, our choices of STEC test kits include rapid methods
based upon PCR detection or a combination of immunomagnetic separation
with PCR. Soon, additional methods will be introduced to the market, increasing
options available to the food safety professional. When selecting a test kit, it is
important to understand how the method works and also how the method has
been validated and certified. The following information is a review of test kit
selection fundamentals.
Molecular methods are based on DNA detection by PCR. Following incubation
of the sample in enrichment media, an aliquot is transferred for “amplification”
of the target DNA using a thermocycler, which measures the change in
concentration of DNA over time to determine the status of the sample—positive
or negative. One method uses immunomagnetic separation to preferentially
select E. coli having the target STEC “O” factors, which are then subjected to PCR
analysis. Other methods perform multiple sequential PCR analyses to determine
the status of the sample—positive or negative.
Validated & Certified Methods: Choosing a method that has been validated
and certified by an appropriate independent authority increases your assurance
that your investment in testing will withstand scientific scrutiny—and meet
regulatory requirements. It is important to review the details of the methods you
are considering to verify that the method has been validated for the food matrix
you are testing and that you clearly understand other important factors such
as sample size, media volume/ratio and enrichment time. You should use the
online resources of the appropriate scientific authority in addition to requesting
information from the test kit supplier. AOAC International is the primary U.S.
authority for certifying test methods.
Inclusivity & Exclusivity: These data tell you what organisms the test will and
will not detect. In addition to providing information about “true positives” that
the test will detect, interpretation of the data helps you understand the potential
for false-positive and false-negative results.
Inclusivity testing is performed by the kit manufacturer to determine from a
broad range of organisms those that will produce a positive result by the test.
In the case of STEC, a review of inclusivity data will indicate which E. coli are
detected by the kit. If there are organisms that produce a false positive result,
these may be indicated in the inclusivity data. By contrast, if there are target STEC
that are not detected by the kit, then the absence of these will be your indication
that the method may not detect all of the target STEC. It is important to review
the inclusivity data when choosing a kit.
Exclusivity testing is performed by the kit manufacturer to test a broad range
of non-target organisms that produce an accurate negative test result. These data
provide additional information about the test accuracy—in this case, the ability of
the test to accurately produce negative results for nontarget bacteria.
Sensitivity & Specificity: These measures capture the ultimate test accuracy
compared with a reliable benchmark. Sensitivity and specificity are reported as a
percent.
Sensitivity of 100 percent
indicates that during the validation,
there were zero false-negative
results—in other words, all of the
intended STEC were detected by the
method.
Specificity of 100 percent
indicates that during the validation,
there were zero false-positive
results—only target STEC intended to
be detected were detected.
Sample Enrichment Media:
Sample enrichment using nutritive
media is necessary when testing
food and environmental samples
because of the very low prevalence
of STEC and other pathogens in
food products. Enrichment times
vary based on the performance of
the media in resuscitating weak
or injured cells and the detection
capabilities of the test assay. As you
consider different test systems, you
will have the opportunity to discuss
enrichment media with your test
supplier and evaluate the potential
benefits of using conventional
media or proprietary media. Here
are highlights of each option:
Conventional Enrichment uses
less expensive conventional media
that may be acceptable when
performance of the test assay is not
affected. Cost savings are typically
offset by the need for additional
enrichment time.
Selective Enrichment for STEC
takes advantage of optimization of
the media to grow target STEC to
the exclusion of other organisms.
Additives included in the media
formulation will prevent the growth
of undesirable organisms while
nourishing target STEC to grow
faster in an environment where
competition has been minimized.
When deciding to use selective
media, it is important to confirm that
the media have been validated to
work with your test assay.
