SUMMARYA total of 79 Australian isolates of beta-haemolytic Escherichia coli from cases of porcine postweaning diarrhoea (PWD), and 18 isolates of serotype O 149:K91 :K88 (F4) from unweaned pigs from Australia, Indonesia and Denmark, were examined by multilocus enzyme electrophoresis. These were divided into 57 electrophoretic types (ETs), with an overall mean genetic diversity per enzyme locus of 0-466. This value closely resembled that previously recorded for the whole species. Not only was the collection diverse, but there was considerable genetic heterogeneity amongst PWD isolates of the same serogroup.Isolates from serogroups 0 8 and 0 138 were most varied, whilst many from serogroups 0 141 and 0 149 were more closely related. In contrast, the isolates from the unweaned pigs all belonged to only one ET.

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The genetic diversity of 87 isolates ofEscherichia coli recovered from Australian pigs with neonatal diarrhea was examined by multilocus enzyme electrophoresis. The isolates were of serogroups 09, 020, and 0101, and although most isolates lacked K88(F4), K99(F5), 987P(F6), and F41 fimbriae, they were considered to be involved in the etiology of the diarrhea. The isolates were extremely diverse, considering their origin from a single pathological condition in one country. There were estimated to be 18, 16, and 12 clones of the three respective serogroups in the collection, with serogroup diversities of 0.387, 0.448, and 0.275, respectively. Comparison with the results previously obtained for isolates from piglets with postweaning diarrhea suggested that bacteria from piglets with these two conditions did not come from any particular common genetic background. The overall genetic diversity for the combined collection was the same as that reported by others for representative isolates selected from throughout the species (0.47). The current results indicate that if isolates of these 0 groups are involved in porcine diarrhea, their pathogenicity is directly linked to their 0 somatic antigen type and is not simply due to the wide distribution of a small number of virulent clones.