The use of serotyping and PFGE-typing of Listeria monocytogenes in food processing contamination studies and human foodborne infections

Maria Sjöman

Tutkimustuotos: OpinnäyteVäitöskirjaArtikkelikokoelma


Two different typing methods, serotyping and pulsed-field gel electrophoresis (PFGE) typing, were used to study Listeria monocytogenes contamination in food processing plants and human foodborne L. monocytogenes infections.

L. monocytogenes contamination was studied in two different types of food processing plants: one ice cream plant and two broiler abattoirs. A dominant L. monocytogenes PFGE type of serotype 1/2b was found to have persisted in this ice cream plant for at least seven years. This strain was found in the production environment, ice cream, and equipment, especially in the packaging machine. Two broiler abattoirs were evaluated for L. monocytogenes contamination of the processing environment, broiler meat, and broiler meat products. Contaminated sites in the broiler processing environment included the air chiller, the conveyor belt leading to the meat packaging area, and the skin removing machine, possibly suggesting important contamination points of broiler meat. The two broiler abattoirs harboured different L. monocytogenes PFGE types. Retail broiler meat samples were also analyzed in order to compare the results to those from the broiler abattoirs. Of the raw broiler meat pieces bought from retail stores, 62% tested positive for L. monocytogenes. PFGE types found in the retail raw broiler meat pieces were identical to the PFGE types found at the broiler abattoirs where they had been processed. PFGE characterized these industrial strains and enabled targeted cleaning and disinfection practices at these food processing plants.

L. monocytogenes isolates from human invasive infections from 1990-2001 in Finland were studied in order to detect a possible clustering of cases. The most common L. monocytogenes serotype was 1/2a. Altogether 81 different PFGE types were found using one restriction enzyme (AscI). A strain of an identical PFGE type of serotype 1/2a had appeared in 1994, accounting for 12% of the invasive infections.

In addition to the human invasive infections, we studied two foodborne L. monocytogenes outbreaks in Finland. In the first outbreak in 1997, five healthy persons fell ill with febrile gastroenteritis. The outbreak was associated with the consumption of vacuum-packed cold-smoked rainbow trout containing L. monocytogenes. Indistinguishable L. monocytogenes PFGE types of serotype 1/2a were isolated from the incriminated fish product lot and human stool samples. This same L. monocytogenes PFGE type was identical to the most common PFGE type in human invasive L. monocytogenes infections in Finland during 1994-2001.

The second outbreak studied occured in 1999, when 25 patients acquired listeriosis from butter, 6 of whom died. This outbreak was initially identified with the help of timely serotyping and PFGE typing, and resulted from a globally uncommon strain of L. monocytogenes of serotype 3a and unique PFGE type.

For regular epidemiological surveillance of human L. monocytogenes infections with PFGE typing, the use of one restriction enzyme is sufficient, but in the case of contamination studies or suspected outbreaks, the use of at least two restriction enzymes is recommended in order to increase the discrimination power of PFGE typing.
Painoksen ISBN978-952-92-6945-7
Sähköinen ISBN978-952-10-6112-7
TilaJulkaistu - 2010
OKM-julkaisutyyppiG5 Tohtorinväitöskirja (artikkeli)


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