Listeria monocytogenes : detection and behaviour in food and in the environment

In this thesis, Listeria monocytogenes, a bacterial pathogen was studied, with emphasis on the detection and behaviour in food and environment.Epidemics of foodborne listeriosis have raised concern about the incidence of L. monocytogenes in foods. In the past 10-15 years listeriosis has emerged as a foodborne illness in a series of large outbreaks from contaminated milk, coleslaw, soft cheese and paté. The organism is ubiquitous in the environment and has been isolated from a variety of raw and ready-to-eat food products.As there is a lack of reliable enrichment procedures, factors influencing the isolation, confirmation and identification of L. monocytogenes were investigated. It was shown that the other (faster growing) listerias can mask the presence of this pathogen in enrichment media. The use of lithium chloride did not overcome this effect. It was also observed that the use of acriflavine in enrichment media affected the isolation of L. monocytogenes both directly and indirectly. Because these effects will lead to inferior detection of L. monocytogenes, it is worthwhile to introduce an isolation medium on which the pathogen can be differentiated from non-pathogenic listerias. On enhanced hemolysis agar (EHA), L. monocytogenes can be distinguished from other listerias on the basis of hemolysis.The traditional methods for the detection of L. monocytogenes are both time consuming and labour intensive. Therefore, rapid test kits for Listeria and L. monocytogenes have been developed. Differences among the various test kits may be contributed to the enrichment protocols, the detection limits of the tests and the concentration of Listeria cells in the samples.In the second part of this thesis, the behaviour of L. monocytogenes in food and environment was investigated.In the literature contradictory results on the growth of the organism on meat surfaces have been reported. In this study it was shown that growth on raw meat was strain dependent and mainly determined by initial pH and storage temperature. On cooked meat products the growth was not or only slightly affected by the storage atmosphere and the presence of lactic acid bacteria. At the end of shelf life, levels of 10 7cfu per g could be reached.Under osmotic stress conditions exogenously added proline, betaine and carntine significantly stimulate growth in laboratory media. Because these compounds are also present in foods, they can contribute to growth of L. monocytogenes in various foods at high osmolarities.In sporadic cases of listeriosis it is often difficult to link food products with this disease, unknown sources may also be responsible for illness. From an investigation in the domestic environment it became clear that L. monocytogenes may be present in high numbers.To be able to control the problems caused by L. monocytogenes, international microbiological criteria, based on < 10 2L. monocytogenes cells per gram on sell-by-date should be established. Persons at risk (immuno-compromized, pregnant) should be strongly advised not to eat food products likely to contain the pathogen. Meanwhile, more attention should be given to challenge studies, factory ecology, cleaning and disinfection and both personal and domestic hygiene.