Correlation between geographic distance and genetic similarity in an international collection of bovine faecal Escherichia coli O157:H7 isolates

Evidence from epidemiological and molecular studies of bovine Escherichia coli O157:H7 suggests that strains are frequently transmitted across wide geographic distances. To test this hypothesis, we compared the geographic and genetic distance of a set of international bovine Escherichia coli O157:H7 isolates using the Mantel correlation. For a measure of genetic relatedness, pulsed-field gel electrophoresis of six different restriction enzyme digests was used to generate an average Dice similarity coefficient for each isolate pair. Geographic distance was calculated using latitude and longitude data for isolate source locations. The Mantel correlation between genetic similarity and the logarithm of geographic distance in kilometers was -0.21 (P<0.001). The low magnitude of the Mantel correlation indicates that transmission over long distances is common. The occurrence of isolates from different continents on the same cluster of the dendrogram also supports the idea that Escherichia coli O157:H7 strains can be transferred with considerable frequency over global distances.