title: Real-time PCR detection of Listeria monocytogenes in infant formula and lettuce following macrophage-based isolation and enrichment.
abstract: OBJECTIVE
  To develop a rapid detection procedure for Listeria monocytogenes in infant formula and lettuce using a macrophage-based enrichment protocol and real-time PCR.
  
  
  RESULTS
  A macrophage cell culture system was employed for the isolation and enrichment of L. monocytogenes from infant formula and lettuce for subsequent identification using real-time PCR. Macrophage monolayers were exposed to infant formula and lettuce contaminated with a serial dilution series of L. monocytogenes. As few as approx. 10 CFU ml(-1) or g(-1) of L. monocytogenes were detected in infant formula and lettuce after 16 h postinfection by real-time PCR. Internal positive PCR controls were utilized to eliminate the possibility of false-negative results. Co-inoculation with Listeria innocua did not reduce the L. monocytogenes detection sensitivity. Intracellular L. monocytogenes could also be isolated on Listeria selective media from infected macrophage lysates for subsequent confirmation.
  
  
  CONCLUSIONS
  The detection method is highly sensitive and specific for L. monocytogenes in infant formula and lettuce and establishes a rapid identification time of 20 and 48 h for presumptive and confirmatory identification, respectively.
  
  
  CONCLUSIONS
  The method is a promising alternative to many currently used q-PCR detection methods which employ traditional selective media for enrichment of contaminated food samples. Macrophage enrichment of L. monocytogenes eliminates PCR inhibitory food elements and contaminating food microflora which produce cleaner samples that increase the rapidity and sensitivity of detection.
authors.affiliation: U.S. Food and Drug Administration, Center for Food Safety and Applied Nutrition, Colleg Park, MD, USA.
authors.forename: J B
authors.initials: JB
authors.lastname: Day
authors.forename: U
authors.initials: U
authors.lastname: Basavanna
mesh_terms: D002469:Cell Separation; D005516:Food Microbiology; D006801:Humans; D007223:Infant; D041943:Infant Formula; D018545:Lactuca / Q000382:microbiology*; D008087:Listeria; D008089:Listeria monocytogenes / Q000235:genetics / Q000302:isolation & purification*; D008264:Macrophages / Q000382:microbiology*; D060888:Real-Time Polymerase Chain Reaction / Q000379:methods*
keywords: Listeria; detection; food; identification; polymerase chain reaction