Public Health England (PHE) said tests had shown it was the same strain of E. coli O55 that had affected 26 other people in the county.
In May, two children were also admitted to hospital with hemolytic uremic syndrome (HUS), the most common cause of acute kidney injury in children .
No common source has been identified.
PHE said close family contacts and pets were being tested and follow-up investigations were taking place.
It urged Dorset residents and visitors to be “extra vigilant” with hand hygiene before preparing food and after contact with animals.
In June, the O55 strain was found in animal droppings outside the home of an affected family.
It is not yet clear whether the fecal sample, which tested positive, came from a wild or domestic animal.
Clusters of this particular strain had not been identified in England since records began in 1994.