Food Safety Magazine, February/March 2012

TESTINGsites for Listeria growth, the test methodyou choose should be broad based andcapable of detecting all six strains of Listeria.A method that detects all six Listeriastrains will provide the mostcomplete information about yourprocesses and facilities.L. monocytogenes testing for productsamples and direct product contact surfaces.Because of ourinterest in protectinghealth, we test productfor L. monocytogenes.This is your final opportunityfor assurancethat systems are workingproperly and theproduct manufacturedis indeed safe to eat.This part of your overalltesting program ensuresthat product iswithheld from commerceuntil test resultsfor product or productcontact surfaces have been received andresults are negativeNew Listeria Test MethodsAs food scientists, we can expect 2012to bring new, significant developmentsin sophistication of Listeria testing andmicrobiological testing generally. Somenotable and interesting new technologiesare being developed for both Listeriaspp. and L. monocytogenes testing.One innovative technology is basedupon highly accurate multiplex polymerasechain reaction (PCR) for simultaneousanalysis of 10 to 30 targets,building upon conventional PCR capabilityand combining DNA tagging withliquid chromatography-mass spectrometry.“Mass tags” are coupled to PCRprimers that, following amplification,provide two unique identifiers per genetictarget and appear in the mass spectrometricanalysis only if the target issuccessfully and specifically amplified.Results can be displayed and shared electronicallyutilizing easy-to-use applicationsoftware with no need forconventional PCR gel analysis.““Successful food”manufacturers makefinding Listeria apriority to be able toeliminate it fromtheir facilities.”In a departure from the amplificationand detection of DNA, another technologyinvolves an rRNA method combinedwith several sample preparationinnovations that offers unique advantages.Exploiting the relative abundanceof RNA over DNA in bacterial cells(100–1,000 times more), the methoduses enzymes and isothermal transcription-mediatedamplificationto produce anRNA amplicon underrapid kinetics, resultingin up to a billionfoldamplification in 15–60minutes. Target captureinvolving oligonucleotidesenhances themethod’s ability toovercome potential interferencesand inhibitoryeffectsassociated with the matrixor sample.Another advance involvesnew tests for Listeria spp. and L.monocytogenes that combine proven molecularscience with innovation in bioluminescence.The result of this innovativethinking is a user-friendly method thatincorporates isothermal amplification ofDNA coupled with simultaneous analysisof bioluminescence to produce ahighly accurate and flexible test platform.Choosing a Listeria TestIn determining which is your best testkit option, consider several factors inmaking your decision.Types of TestsPathogen testing methods may be categorizedinto two major types, culturalmethods and rapid methods. While culturalmethods are accurate and the costof materials is relatively low, they can belabor intensive and may require specializedskills and training to perform. Rapidmethods offer good value, even thoughthey may carry a higher materials cost,because they are accurate, faster thancultural methods and usually require lessspecialized training.Rapid MethodsRapid methods in the market todaythat are AOAC certified can be groupedgenerally into one of two broad categories:either protein-based detection orDNA-based detection. Protein methodsutilize immunoassay antibody technologyfor detection. Following incubation,an aliquot of enrichment media is transferredto the assay and a positive or negativeresult is indicated. The format ofimmunoassays can vary widely fromuser-friendly lateral flow devices to moresophisticated automated ELISA (enzyme-linkedimmunosorbent assay) systems.Molecular methods are based onDNA detection by PCR. Following incubationof the sample in enrichmentmedia, an aliquot is transferred for “amplification”of the target DNA using athermocycler, which measures thechange in concentration of DNA overtime to determine the status of the sample—positiveor negative. Some PCRmethods involve three steps: enrichmentfollowed by thermocycling and finallydetection using pulsed-field gel electrophoresisor another method.Validated & Certified MethodsChoosing a method that has beenvalidated and certified by an appropriateindependent authority increases your assurancethat your investment in testingwill withstand scientific scrutiny—andmeet regulatory requirements. It is importantto review the details of the methodsyou are considering to verify that themethod has been validated for the foodmatrix that you are testing and that youclearly understand other important factorssuch as sample size and enrichmenttime. You should use the online resourcesof the appropriate scientific authorityin addition to requestinginformation from the test kit supplier.AOAC International is the primary U.S.authority for certifying test methods. Dependingon your objectives, you maybenefit by visiting several other authoritiesonline, including the U.S. Food andDrug Administration, the U.S. Departmentof Agriculture Food Safety and In-14 F O O D S A F E T Y M A G A Z I N E