The municipal government of Takasaki, Gunma Prefecture, said Saturday it did not find toxic bacteria in food samples taken from a local factory after potato salad produced there caused E. coli food poisoning earlier in the month.

Twelve people who ate the salad sold in Gunma and neighboring Saitama prefectures earlier this month were found to have been infected with the O-157 strain of the bacteria, and one became gravely ill.

Those infected ate salad that was made from Aug. 5 to 7 at a factory in Takasaki. Tests on potato salad samples made between Aug. 1 and 18 and preserved at the factory found no toxic bacteria, the Takasaki Municipal Government said.

So far, no evidence of toxic bacteria has been found at delicatessens that sold the salad, and employees there have tested negative for O-157 infection. Bagged potato salad shipped from the plant was mixed with ham and apple pieces at delicatessens where customers used tongs in the self-service areas.

The Gunma-based operator of the delicatessens, which also runs supermarket chains in eastern Japan, has suspended operations at all 17 delicatessens.