Submitted to: Journal of Food Protection
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/16/2003
Publication Date: 5/3/2003
Citation: PORTO, A.C., WONDERLING, L.D., CALL, J.E., LUCHANSKY, J.B. USE OF PULSED-FIELD GEL ELECTROPHORESIS TO DETERMINE THE SUCCESSION OF A FIVE-STRAIN MIXTURE OF LISTERIA MONOCYTOGENES IN VACUUM-SEALED PACKAGES OF FRANKFURTERS DURING EXTENDED STORAGE AT 4 DEG C. JOURNAL OF FOOD PROTECTION. 2003. V. 66. p. 1465-1468. Interpretive Summary: Contamination of frankfurters by Listeria monocytogenes has been responsible for several outbreaks of listeriosis in the past decade. Although there has been considerable research conducted on L. monocytogenes in frankfurters, few studies have assessed how individual strains within a mixture of strains may fare in packages of frankfurters and/or if certain strains are more successful in this environment. In a previous study, a combination of five different L. monocytogenes strains were added to vacuum-sealed packages of frankfurters formulated with and without potassium lactate, a food-grade antimicrobial. The viability of the five-strain mixture was subsequently monitored during extended refrigerated storage of the frankfurter packages. In the present study, those isolates recoverd from the frankfurter packages after 28 and 90 days of storage were analyzed by a technique called pulsed-field gel electrophoresis. This DNA fingerprinting technique allowed us to calculate the relative prevalence of each strain and, therefore, to determine if one or more of the five-strains were better able to survive the frankfurter environment. The results indicated that strain MFS-2, a strain originally recovered from a pork processing plant, predominated after 90 days in the packages of frankfurters formulated with and without added potassium lactate. These results suggest that strain diversity is an important aspect of competition in foods. Further DNA fingerprinting of L. monocytogenes strains will investigate genetic determinants that influence the ability of this pathogen to survive in foods and/or cause disease.
Technical Abstract: In a previous study, the viability of Listeria monocytogenes strains Scott A (serotype 4b, clinical isolate), 101M (serotype 4b, beef/pork sausage isolate), F6854 (serotype 1/2a, turkey frankfurter isolate), H7776 (serotype 4b, frankfurter isolate), and MFS-2 (serotype 1/2a, pork plant isolate) was monitored during refrigerated storage of frankfurters prepared with and without 3.0% added potassium lactate. Throughout 90 days, the initial inoculum remained relatively constant in frankfurters containing added potassium lactate, but there was a 4.6-log10 increase of the pathogen in the absence of added potassium lactate. To determine which strains persisted under these conditions, randomly-selected colonies obtained after 28 and 90 days of refrigerated storage of frankfurters were distinguished by pulsed-field gel electrophoresis (PFGE). Then, using PFGE as an identification tool, the succession of each strain was compared between days 28 and 90 of the growth study. In frankfurters without added potassium lactate, all five strains were present at a frequency of 10 to 43% among the 58 isolates tested on day 28; however, by day 90, a significant majority (83%) of the 60 isolates analyzed were identified as strain MFS-2. In frankfurters formulated with added potassium lactate, all five strains were present at a frequency of 5 to 36% among the 19 isolates tested on day 28; however, by day 90, strain MFS-2 comprised the significant majority (63%) of the 27 isolates tested. These results indicated that strain MFS-2, an isolate recovered from a pork processing plant, was the most persistent strain tested during extended refrigeration storage of frankfurters.