Genetic characterisation of isolates of Listeria monocytogenes from man, animals and food.

Multilocus enzyme electrophoresis was used to assess genetic relationships between 95 isolates of Listeria monocytogenes, most of which were isolated in Australia and New Zealand from man, animals and food. The isolates were separable into two major genetic divisions; the majority of those from human patients and animals were in division I, and the majority from those foods that were not specifically associated with human listeriosis were in division II. Isolates in division I were virulent, whereas many isolates from food were probably less virulent and did not pose a large threat to human health. However, isolates from certain foods, particularly paté, were indistinguishable from those causing disease in man, and the consumption of these products represented a clear risk factor for infection. Isolates from infected human patients in Australia and New Zealand belonged to the same clone of serotype 4b that has been responsible for major epidemics in the northern hemisphere. However, a separate clone of serotype 1/2b strains, present in both Australia and New Zealand, was responsible for two major outbreaks that occurred in Western Australia in 1978-80 and 1990-91.