A rare strain of E.coli O157 was behind the outbreak of the bug linked to a nursery in Fife, according to experts.
The rare strain is more difficult to detect in tests
Health Protection Scotland (HPS) said the strain had been seen in Europe and Australia but was rare in the UK.
A spokesman said the unusual bacterium was hard to identify and clinicians were being urged to be vigilant when testing samples.
The strain has been linked to 13 cases in Dunfermline, four other single cases in Scotland and four cases in England.
HPS said it was working with its sister agency in England to determine the possible origin of the bug, known as Sorbitol-fermenting Vero cytotoxin.
HPS consultant Dr John Cowden said clinicians had been urged to ensure the unusual strain was not missed when dealing with samples from patients who were suspected of having any kind of E.coli O157.
He said: "This unusual strain reacts differently in the laboratory from other types of E.coli O157 and this is why we are interested in finding out more about it.
"It produces the same symptoms as other strains and our advice to reduce the possibility of becoming infected with any type of E. coli remains the same."
Dr Cowden said this included effective hand washing, storing food correctly and making sure food was thoroughly cooked.
The bug is most commonly found in the intestines of healthy cattle.
Humans can become infected by eating contaminated foods or through direct contact with animals and also by person-to-person spread.
It is thought the outbreak in Fife was the result of bacterium spreading from person-to-person.
Two children infected in Dunfermline are still being treated in hospital.