Zoonotic aspects of Listeria monocytogenes : with special reference to bacteriology

Listeria monocytogenes is a non acid-fast, Gram-positive facultative anaerobic pathogen, which is considered as food- and feed-borne. Whereas poor quality silage is the main cause of animal listeriosis, contaminated food of animal origin is the main cause of human listeriosis. That the raw material for food is of animal origin does not necessarily mean that the L. monocytogenes bacteria also spring from animals. The bacteria may have contaminated the food product while processed. Knowledge of the direct or indirect transmission of L. monocytogenes between animals and humans, via e.g. foods, is limited. To highlight the zoonotic aspects of L. monocytogenes we need more comparative data concerning isolates of animal and human origin. The aim of the present study was to characterize clinical L. monocytogenes isolates from different animal’s species and to compare the patterns with those obtained from previously characterized clinical human strains. Animal isolates were characterized by use of restriction enzymes Asc I and Apa I followed by PFGE. Out of 104 animal strains 47 belonged to clonal types identical or closely related to clonal types seen among clinical human strains. The clonal types shared by animals and humans may indicate that there is an exchange of L. monocytogenes strains between these two groups or there may be a common environmental pool of strains. On the other hand, 42 animal strains belonged to clonal types that were unfamiliar to our collection of human strains. Finally, 15 animal isolates distributed into eight clonal types yielded Asc I profiles familiar to our human clonal types yet unfamiliar Apa I profiles. Human and animal isolates of L. monocytogenes have rarely been compared by use of PFGE. Further studies is needed to highlight routes of transmissions between animals and humans, e.g., via food