Between July and October 2010, 136 people in London and the east of England were sickened by the same strain of Salmonella – Java phage type 3b variant 9.
According to a report from the Health Protection Agency (HPA) the results of the investigation were compatible with salad vegetables.
However, no salad suppliers were identified as the source and no organisms were isolated from environmental or food samples.
The HPA report, which was published on the Eurosurveillance website said: “We cannot exclude the possibility that the study may have missed the right vehicle of the outbreak such as sprouted seeds which have been implicated in two recent outbreaks in Europe.”
An E.coli outbreak in Germany earlier this year, which was later linked to sprouted seeds from Egypt, killed 50 and infected over 4000.
“It is likely that the consumption of smaller food items (seeds, sprouted seeds and herbs) in salads prepared by commercial caterers was not remembered or was not noticed by cases,” the report added.