Some aspects of the ecology of Listeria monocytogenes in salmonid aquaculture

In this thesis, several related aspects of the ecology of Listeria spp. especially L. monocytogenes in aquatic environments and foods were studied. The ecology of the organisms in aquatic habitats was investigated in North West Bay, southern Tasmania, over a 12 month period. Listeria spp. most frequently occurred in effluent and river water but less often in receiving estuarine waters. Sediments and shellfish served as a better reservoir for the organisms. Recent rainfall and the level of faecal coliforms and E. coli were the most significant variables (P ‚Äöv¢¬ß 0.01) related to the presence/absence of Listeria species and L. monocytogenes in estuarine water. Secondly, the relationship between the occurrence of the human pathogenic species, L. monocytogenes, in aquatic environments and in a nearby salmon processing plant and its products was studied. Molecular subtyping methods (multilocus enzyme electrophoresis and repetitive sequence element-PCR) were employed to help trace the distribution of L. monocytogenes strains. A high diversity of L. monocytogenes was found in the aquatic environment but only a small group was detected in fish and the fish processing environment. Thirdly, to be able to understand the physiology and growth response of L. monocytogenes to temperature, water activity, pH, and lactic acid and which in tum may be used to minimise the consequences of contamination by the pathogen of foods, quantitative microbiology (predictive microbiology) studies were conducted. The results were incorporated into 2 different types of mathematical model. The first type of model, a kinetic model, was developed using a \square root type model\" which is useful for predicting the shelf-life of foods. The second type of model a probability model (a so called \"growth/ no growth interface\" model) which is a novel model for L. monocytogenes growth limits was developed using a new approach viz \"generalised nonlinear regression method\". This type of model is useful for predicting the condition when micro-organisms especially pathogenic bacteria might grow or might not grow. Finally model predictions were evaluated by comparing them to novel and literature data broadly relevant to the range of conditions in foods for which the models were developed. Limited tests involving direct addition of different levels of lactic acid onto traditional cold-smoked salmon products were performed as an approach to non-thermal inhibition or inactivation of L. monocytogenes and also to test the performance of the models."