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Doctoral thesis, 1998

Listeria monocytogenes with special reference to food products and human listeriosis

Loncarevic, Semir


Listeria monocytogenes has been the cause of numerous sporadic cases and outbreaks of human listeriosis worldwide. In most of these cases the source and route of infection were unknown. Foodbome transmission of listeriosis, however, is a common mode of infection. In Papers I and II we show that L. monocytogenes often could be isolated from soft cheeses, especially those made of raw milk, as well as from vacuum-packed smoked and gravad salmon and rainbow trout. Serotyping has shown that the majority of the isolates belong to serogroup 1/2. A change in serogroup distribution among Swedish human isolates has been observed. Serogroup 4 was predominant until 1987, and after that it was counterbalanced with serogroup 1/2. Changes in eating habits are a possible explanation of this alteration. Serotyping of isolates is not sufficient to estimate a suspected source of human listeriosis. Application of more discriminating methods such as molecular typing methods must be used. In Paper III, using pulsed-field gel electrophoresis (PFGE), we show that the same food product may harbour more than one L. monocytogenes clonal type. It is recommended to isolate several colonies from both the direct plating and enrichment procedures. Practical application of the approach mentioned has been applied in tracing a source of human listeriosis in Paper IV. Among 23 food items collected from a patient’s refrigerator, two harboured L. monocytogenes. The isolated strains belonged to the same serovar. By use of PFGE, medwurst was found to be the source of illness. This is the first reported foodbome case oflisteriosis in Sweden. To find out if two groups of frequently contaminated ready-to-eat food products, soft cheeses and gravad and cold smoked salmon and rainbow trout, are potential risk foods in Sweden, PFGE was used in Paper V. An association between human cases of listeriosis and consumption of salmon and rainbow trout is presented. However, a link to soft cheese was not found.
​​​​​​​In conclusion, raw milk soft cheeses, gravad and cold smoked salmon and rainbow trout were often contaminated with L. monocytogenes and thus potential risk foods, especially for immuno-compromised persons, including pregnant women. In the case of routine investigation of food products, an enrichment method and characterization of one isolate could be sufficient. In investigating outbreaks and sporadic cases it might be necessary to use two different isolation methods and to characterize several isolates with more than one typing method, where at least one should be a molecular typing method. PFGE was shown to be a highly discriminative method for this use.


Listeria monocytogenes; food; human listeriosis; serotyping; phage-typing; PFGE

Published in

Acta Universitatis Agriculturae Sueciae. Veterinaria
1998, number: 31
ISBN: 91-576-5432-8
Publisher: Swedish University of Agricultural Sciences

Authors' information

Loncarevic, Semir
Swedish University of Agricultural Sciences, Department of Food Hygiene

UKÄ Subject classification

Food Science

URI (permanent link to this page)