Ecology of Listeria spp. in a fish farm and molecular typing of Listeria monocytogenes from fish farming and processing companies Hanna Miettinen, Gun Wirtanen ⁎ VTT, P.O. Box 1000, FI-02044 VTT, Finland Received 27 June 2005; received in revised form 28 April 2006; accepted 2 June 2006 Abstract This study focused on the ecology of Listeria monocytogenes in a fish farm by following the changes in its occurrence in different types of samples for a three year period. In addition, L. monocytogenes isolates from different seafood industry areas were compared with pulsed field gel electrophoresis (PFGE) typing to discover possible associations between primary production, further processing and final products. Weather conditions were found to have a strong influence on the probability of finding Listeria spp. in a fish farm environment. The number of samples contaminated with Listeria spp. was typically bigger after rainy periods. Brook and river waters as well as other runoff waters seemed to be the main contamination source at the farm studied. The farmed fish originally found to carry L. monocytogenes become gradually Listeria free. The time needed for the purification of the fish was several months. The sea bottom soil samples were the ones that preserved the L. monocytogenes contamination the longest time. It can be stated that the fish and fish farm equipment studied did not spread listeria contamination. On the contrary, they were found to suffer from listeria contamination coming from outside sources like the brook water. There was a wide range of different L. monocytogenes PFGE-pulsotypes (30) found at 15 Finnish fish farms and fish processing factories. L. monocytogenes isolates from the final products often belonged to the same pulsotypes as did the isolates from the processing environment as well as from the raw fish. This suggests that, in addition to the fish processing factory environment, the fish raw materials are important sources of L. monocytogenes contamination in final products. © 2006 Elsevier B.V. All rights reserved. Keywords: Listeria monocytogenes; Ecology; Contamination; Fish 1. Introduction The Gram-positive intracellular bacterium Listeria monocyto- genes has been recognized as a food-borne pathogen since the early 1980s. A diverse range of foods has been linked to outbreaks of listeriosis. Seafood-related outbreaks have been linked to at least shrimps (Riedo et al., 1994), smoked mussels (Brett et al., 1998), twice to rainbow trout (Ericsson et al., 1997; Miettinen et al., 1999) and tuna and sweetcorn salad (Aureli et al., 2000). Pregnant women, and newborn infants as well as children, elderly people and adults whose immune systems are weakened are more susceptible to the infection than healthy individuals (Rocourt et al., 2000). L. monocytogenes is ubiquitously distributed throughout the environment. Forest soil, cultivated and uncultivated fields, mud, feed, feeding grounds, wildlife faeces and birds (Weis and Seeliger, 1975) have been found to be extensively contaminated with listeria. In addition, a large proportion of faecal samples collected from healthy animals with no clinical symptoms of listeriosis may contain L. monocytogenes (Wesley, 1999). Moreover, the bacterium has also been detected in different kinds of fish, squid and crustaceans (Jeyasekaran et al., 1996; Laciar and de Centorbi, 2002; Miettinen and Wirtanen, 2005). Hence it is no surprise that contamination with Listeria spp. has been found in water environments such as coastal sea waters and rivers containing organic load (Colburn et al., 1990). In addition, even springs and groundwater wells have been discovered to harbour the bacterium (Korhonen et al., 1996; Schaffter and Parriaux, 2002). Process contamination in particular has proved to be an important source of listeria contamination in food production and numerous studies show that in-house L. monocytogenes flora contaminates seafood during processing (Autio et al., 1999; Fonnesbech Vogel et al., 2001; Medrala et al., 2003). Neverthe- less, there are some indications that L. monocytogenes contam- inates final products from seafood raw materials (Eklund et al., International Journal of Food Microbiology 112 (2006) 138 – 146 www.elsevier.com/locate/ijfoodmicro ⁎ Corresponding author. Tel.: +358 20 722 5222; fax: +358 20 722 7071. E-mail address: gun.wirtanen@vtt.fi (G. Wirtanen). 0168-1605/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ijfoodmicro.2006.06.016