PULSED-FIELD GEL-ELECTROPHORESIS APPLIED FOR COMPARING LISTERIA-MONOCYTOGENES STRAINS INVOLVED IN OUTBREAKS

Recent food-borne outbreaks of human listeriosis as well as numerous s poradic cases have been mainly caused by Listeria monocytogenes serova r 4b strains. Thus, it was of interest to find out whether a certain c lone or a certain few clones were responsible for these cases and espe cially for outbreaks., We used pulsed-field gel electrophoresis of lar ge chromosomal DNA restriction fragments generated by ApaI, SmaI, or N otI to analyse 75 L. monocytogenes strains isolated during six major a nd eight smaller recent listeriosis outbreaks. These strains could be divided into 20 different genomic varieties. Thirteen of 14 strains is olated during major epidemics in Switzerland (1983-1987), the United S tates (California, 1985) and Denmark (1985-1987) demonstrated indistin guishable DNA restriction patterns. In contrast, strains responsible f or the outbreaks in Canada (Nova Scotia, 1981), the United States (Mas sachusetts, 1983), France (Anjou, 1975-1976), New Zealand (1969), and Austria (1986) and some smaller outbreaks in France (1987, 1988, 1989) were each characterized by particular combinations of DNA restriction patterns. Seventy-seven percent of the tested strains could be classi fied into the previously described ApaI group A (Brosch et al. 1991), demonstrating a very close genomic relatedness. Because 49% of the epi demic strains selected for this study belonged to phagovar 2389/2425/3 274/2671/47/108/340 or 2389/47/108/340, fifty-six additional strains o f these phagovars, isolated from various origins, were also typed to d etermine whether differences in DNA restriction profiles between epide mic and randomly selected strains of the same phagovars could be point ed out. Variations in DNA patterns appeared more frequently within ran domly selected strains than within epidemic strains.