APIListeria, a New andPromising One-DaySystem ToIdentify Listeria Isolates

APIListeria isa new 10-test strip for24-hbiochemical identification ofListeria isolates. Withthiscommercial system, 85%of646Listeria strains, including atypical isolates selected forthisstudy, were recognized atthe species andsubspecies level without a complementary test. A new testdifferentiates Listeria monocytogenes fromL.innocua on thebasis oftheabsence ofarylamidase fromtheformer. Withthis system,97.7%(252of 258)oftheL.monocytogenes strains tested were correctly identified anddifferentiated from99.4%(175of176) oftheL.innocua strains alsotested. Gram-positive bacteria otherthanListeria spp. gave quitedifferent biochemical patterns. Thissystemconsiderably reduced thetimeneededforconventional identification, since results were available within 18to24h. ThegenusListeria comprises sixrecognized species: L. monocytogenes,L.ivanovii subspp. ivanovii andlondonien- sis(3), L.innocua, L.welshimeri, L.seeligeri, andL.grayi (2,28).Allofthese species arepsychrotrophic andwidely spread intheenvironment, butonlyL.monocytogenesisa significant humanandanimalpathogen. Duringthelast decade, L.monocytogenesemerged as a foodborne patho- gen, andvarious contaminated foodstuffs (milkanddairy products, meatandmeatproducts, vegetables, andseafood) were implicated inmajoroutbreaks andsporadic cases in NorthAmericaandEurope(10). Thetotal costofhuman listeriosis was estimated to$255million peryear,andfood recalls were evaluated at$15million in1985through 1987 (27). Thissituation ledtoregular controls ofbothprocessed and nonprocessed foodand theirenvironment-related sources.Sinceall Listeria species arepotential foodcontam- inants, rapid andreliable detection andidentification ofL. monocytogenesappeartobeofutmostimportance. Rapidmethodssuchasflowcytometry (9), enzyme-linked immunosorbent assay techniques (21), DNA hybridization (6,16,24),andthepolymerase chainreaction (1,33)have beenrecently developed tospecifically detect Listeria spp. and,insome cases,more precisely L. monocytogenes. Nevertheless, inmany laboratories, isolation followed by identification ofthemicroorganism remains themethodused todetect Listeria spp.(19, 22). Atthepresent time, Listeria isolates aremainly recognized on thebasis ofmorphological andbiochemical characteristics (2). Amongthesecharacter- istics, hemolysis istheonlymarkerthatdistinguishes L. monocytogenes (hemolytic andpathogenic) fromL.innocua (nonhemolytic andnonpathogenic), thetwo species most frequently isolated fromfoodstuffs (4,25). Thepurposeofthisstudy was toevaluate a new API system,specifically designed toidentify Listeria isolates at thegenus,species, andsubspecies levels. Thestrip consists of10testsandallows complete identification in24hwithout additional testssuchashemolysis.