Two E.coli outbreaks linked to Chipotle Mexican Grill Inc that affected more than 50 people across 14 states appear to be over, according to the US Centers for Disease Control and Prevention (CDC).
The company's shares were up 4.7 per cent at $US474.45 ($A669.80) in early afternoon trading.
The regulator said on Monday investigators were not able to identify the ingredient that was responsible for the contamination.
Chipotle has been grappling to overcome a series of foodborne illnesses linked to its chain since October that have driven away diners, hammered its high-flying stock and spawned both a federal criminal probe and a shareholder lawsuit.
The most recent illness reported to the CDC was on December 1, the regulator said on Monday.
Testing of multiple food items collected from Chipotle restaurant locations did not identify the bacteria, the CDC said.
Chipotle said last month it was confident that the steps it had undertaken to tighten food safety will stop future food poisoning outbreaks.
The Wall Street Journal first reported the CDC's findings on Sunday.
Chipotle's stock has fallen nearly 30 per cent since the end of October when the first case of E.coli was reported.
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