Chipotle restaurants in Washington and Oregon could re-open after E.coli tests come up negative
A sign is seen at a Chipotle Mexican Grill restaurant in San Francisco, California on July 21, 2015. Photo by Robert Galbraith for Reuters.

Chipotle Mexican Grill Inc could reopen locations in Washington state and Oregon as soon as Wednesday after a batch of tests found no E. coli bacteria in food samples, health officials said on Monday.

Public health officials are still working to identify the cause of the E. coli food poisoning that has sickened 43 people, most of whom dined at eight Chipotle restaurants in the greater Seattle and Portland areas. All of Chipotle's 43 outlets in those areas have been closed since Oct. 31.

Scott Lindquist, Washington state's epidemiologist for communicable disease, said Chipotle could start shipping produce to restaurants on Tuesday and Wednesday. The outlets could reopen on Wednesday and Thursday, after meeting certain requirements.

Chipotle needs to sanitize each restaurant and throw out all food items and pre-test certain high-risk foods such as lettuce and peppers as it brings in new food, Lindquist said.

Locations also need to have an on-site inspection by a county health official, he said.

Lindquist said Oregon locations are expected to open as soon as Wednesday as well, though public health officials there did not immediately respond to a request for comment.

A first round of test results did not find E. coli bacteria in food samples taken from several Chipotle restaurants, the Washington health department said, citing officials at the Food and Drug Administration.

Food outbreak investigations do not always identify a specific food source as the culprit, because contaminated food is at times consumed before the samples are collected, the health department added.

"In addition to the testing done by health departments and the FDA, we have received results from nearly 900 test samples that include food, surfaces in our restaurants, and restaurant equipment. None of them have shown E. coli. All have been negative. We are, of course, sharing those results with investigators," said Chipotle representative Chris Arnold.

Arnold added that Chipotle would release more information on Tuesday.

This is the third outbreak of food contamination at Chipotle restaurants since August. Those earlier cases involved salmonella and the highly infectious norovirus.

The tally of those sickened varies as people are tested. On Monday, officials in Washington state said 28 cases had been recorded, while officials in Oregon said they had 15 cases.

(Reporting by Eric M. Johnson in Seattle and Subrat Patnaik in Bengaluru; Editing by Gopakumar Warrier and Richard Pullin)