updated 1/12/2007 2:13:40 PM ET 2007-01-12T19:13:40

Contaminated California-grown lettuce was the possible source of the E. coli outbreak that sickened 81 people late last year at Taco John’s restaurants in two states, health officials said Friday.

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State and federal investigators said they have matched the strain of the bacteria associated with the outbreak to two samples taken from dairy farms in California’s Central Valley. The farms are near lettuce fields, the Food and Drug Administration said.

Investigators continue to study whether bacteria-laden manure from the dairy farms could have contaminated the nearby lettuce-growing areas, the FDA said. The FDA said other sources of contamination were possible.

The outbreak sickened 81 people who had eaten at Taco John’s restaurants in Minnesota and Iowa in November and December. Among those sickened, 26 were hospitalized. There were no deaths.

The Taco John’s outbreak occurred at roughly the same time as a separate and more widely publicized outbreak of food poisoning that sickened 70 patrons of Taco Bell restaurants in the Northeast. Though E. coli-contaminated iceberg lettuce was the likely culprit behind both outbreaks, the two are not thought to be linked.

Friday’s announcement also marks the second time in recent months that investigators have suggested Californian cattle or dairy operations as the source of contamination of produce grown on close-by fields. Bacteria tracked by wild pigs from a cattle ranch in California’s Salinas Valley onto nearby spinach fields likely sparked an E. coli outbreak last year that sickened more than 200 people and killed three.

Taco John’s spokesman Brian Dixon said the Cheyenne, Wyo., company had suspended purchases from the produce company that had supplied the three restaurants linked to the outbreak. The company also is in discussions with its other suppliers to do more testing of irrigation water and possibly to hold shipments of produce until samples can be tested for contamination, Dixon added.

The recent spate of food-poisoning cases includes two salmonella outbreaks blamed on fresh tomatoes that made about 400 people sick in October and November.

The FDA said Friday it would step up its produce-safety efforts, including consideration of new regulations. The regulatory agency also plans to hold public meetings to discuss ways of improving the safety of fresh produce.

And in Congress, Democrats in both the Senate and House plan to hold food-safety hearings in the coming months.

© 2013 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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