Elucidation of Listeria monocytogenes contamination routes in cold-smoked salmon processing plants detected by DNA-based typing methods

The contamination routes of Listeria monocytogenes in cold-smoked salmon pr ocessing plants were investigated by analyzing 3,585 samples from products (produced in 1995, 1996, 1998, and 1999) and processing environments (sampl es obtained in 1998 and 1999) of two Danish smokehouses. The level of produ ct contamination in plant I varied from 31 to 85%, and no L. monocytogenes was found on raw fish (30 fish were sampled). In plant II, the levels of bo th raw fish and product contamination varied from 0 to 25% (16 of 185 raw f ish samples and 59 of 1,000 product samples were positive for L, monocytoge nes). A total of 429 strains of L. monocytogenes were subsequently compared by random amplified polymorphic DNA (RAPD) profiling, and 55 different RAP D types were found. The RAPD types detected on the products were identical to types found on the processing equipment and in the processing environmen t, suggesting that contamination of the final product (cold-smoked salmon) in both plants (but primarily in plant I) was due to contamination during p rocessing rather than to contamination from raw fish. However, the possibil ity that raw fish was an important source of contamination of the processin g equipment and environment could not be excluded. Contamination of the pro duct occurred in specific areas (the brining and slicing areas). In plant I , the same RAPD type (RAPD type 12) was found over a 4-year period, indicat ing that an established in-house flora persisted and was not eliminated by routine hygienic procedures. In plant II, where the prevalence of L, monocy togenes was much tower, no RAPD type persisted over long periods of time, a nd several different L, monocytogenes RAPD types were isolated. This indica tes that persistent strains may be avoided by rigorous cleaning and sanitat ion; however, due to the ubiquitous nature of the organism, sporadic contam ination occurred. A subset of strains was also typed by using pulsed-field gel electrophoresis and amplified fragment length polymorphism profiling, a nd these methods confirmed the type division obtained by RAPD profiling.