Characterisation of Persistent and Sporadic Listeria monocyto- genes Strains by Pulsed-Field Gel Electrophoresis (PFGE) and Amplified Fragment Length Polymorphism (AFLP) Tiina Autio, Riikka Keto-Timonen, Janne Lundén, Johanna Björkroth, and Hannu Korkeala Department of Food and Environmental Hygiene, Faculty of Veterinary Medicine, University of Helsinki, Finland Received: May 27, 2003 Summary This study was set up to evaluate the genetic similarity or dissimilarity of persistent and sporadic Listeria monocytogenes strains existing in eleven food processing facilities, including fish, dairy, meat and poul- try processing plants. In each plant persistent and sporadic strains were selected on the basis of PFGE typing results. A total of 17 strains representing persistent strains and 38 sporadic strains originating from eleven food processing plants were included in the study. PFGE macrorestriction patterns of persis- tent and sporadic strains from different processing plants were compared and the strains were further studied by amplified fragment length polymorphism (AFLP), being a characterisation method giving more whole genome based information. The 17 persistent and 38 sporadic strains showed 14 and 35 pul- sotypes, 14 and 28 AFLP types, respectively. The combination of PFGE and AFLP typing results yielded a total of 48 genotypes. Thirteen of 15 genotypes presented by persistent strains were only associated with persistent strains and similarly 94% (33/35) of genotypes showed by sporadic strains were recovered among sporadic strains only. Our results showed that L. monocytogenes strains causing persistent con- tamination differ from sporadic strains. In AFLP analysis persistent strains did not, however, form any specific clusters and neither was there any difference between the known two genomic groups. These re- sults indicate that even though persistent strains differ from sporadic strains there seems not to be any specific evolutional lineage of persistent strains. Key words: Listeria monocytogenes contamination – pulsed-field gel electrophoresis typing – amplified fragment length polymorphism Introduction L. monocytogenes is a troublesome bacterium for the food processing industry, since it grows at low tempera- tures, adheres to various food contact surfaces, and may adapt to disinfectants [7, 15]. Molecular typing tech- niques, such as ribotyping [19], randomly amplified polymorphic DNA (RAPD) [6] and pulsed-field gel elec- trophoresis (PFGE) typing [4, 14], have been used for characterisation of isolates associated with foods and food processing in order to analyse the L. monocyto- genes contamination routes of the products. Several au- thors [3, 8, 16, 20] have pointed out a typical feature of L. monocytogenes contamination: prolonged processing plant contamination. These studies have illustrated that some L. monocytogenes strains may cause persistent contamination of a processing plant over several months or years, whereas other strains are recovered only spo- radically. However, overall genetic similarity/dissimilarity be- tween persistent and sporadic strains recovered from dif- ferent food processing plants has remained obscure and it is not known whether the persisting strains are delineated in specific genetic groups. This study was set up to evalu- ate the genetic similarity of persistent and sporadic L. monocytogenes strains existing in various food processing facilities. The persistent and sporadic strains, determined by PFGE typing, of eleven food processing plants (fish, dairy, meat and poultry processing plants) were estab- lished. The strains were further characterised by amplified fragment length polymorphism (AFLP). AFLP was chosen for further characterisation as it has been shown not only 0723-2020/03/26/04-539 $ 15.00/0 System. Appl. Microbiol. 26, 539–545 (2003) © Urban & Fischer Verlag http://www.urbanfischer.de/journals/sam