Last week, in the aftermath of an E. coli outbreak that has caused major problems for Chipotle, the chain's co-chief executive Steve Ells made a promise on national television.

"We're going to layer on this culture of food safety and make sure that we're the safest place to eat," Ells said. "That's priority number one."

"We can assure you today that there is no E. coli in Chipotle," Ells added."We have thoroughly tested our food, we have thoroughly tested our surfaces and we are confident that Chipotle is a safe place to eat."

But on Friday, the Centers for Disease Control and Prevention announced that it was investigating a new outbreak that could be linked to the popular Mexican-inspired chain, identifying five people in three states who had caught the E. coli bug. Then on Monday, the center added an update that indicated another report had surfaced in Pennsylvania.

The new outbreaks occurred before Ells' pledge — the cases are from a month ago, and the CDC has yet to confirm a source. But they underscore a credibility problem for Chipotle as it tries to convince consumers that it has finally squashed any possibility of disease in its supply chain. On Friday, shares of Chipotle fell by 3 percent, indicating a sense among investors that the news would hurt business. On Monday, the drop worsened — shares were down 3 1/2 percent when the market closed. Overall, the firm's shares are down nearly a third from their high.

Chipotle, which has reported robust growth for years, said recently that it projects big declines at its flagship stores as a result of the outbreak. On social media, some customers were vowing never to return. Chipotle insists that the backlash, which could extend well beyond this week, is misguided.

"We have indicated before that we expected that we may see additional cases stemming from this, and CDC is now reporting some additional cases," Chris Arnold, the company's director of communications, said in a statement. "Since this issue began, we have completed a comprehensive reassessment of our food safety programs with an eye to finding best practices for each of the ingredients we use."

The latest crisis began in October when Chipotle closed 43 restaurants in Washington state and Oregon after health authorities linked an E. coli outbreak to six restaurants in the region. Illnesses contracted at Chipotles were then reported in seven more states.

Earlier this month, at least 80 students at Boston College got sick after eating at a Chipotle. Boston health officials said the cause was norovirus, a common virus, while citing the restaurant for two health violations: improper handling of poultry and the presence of a sick employee.

In response to the outbreaks, Chipotle pledged to sanitize its operations, hired food safety consultants and announced that it would introduce stricter scrutiny of its ingredients. Chipotle has about 2,000 locations.

A a pedestrian walks past a closed Chipotle restaurant in Seattle, Washington. (Elaine Thompson/AP Photo)

But the more food safety problems and Chipotle are uttered in the same breath, the harder it will be to shed the association. Chipotle's reputation is particularly   threatened because it has made an emphasis out of the fact that it adheres to rigorous standards.

"The real red flags are repetitive failures," Arun Lakshmanan, a marketing professor at the University at Buffalo's School of Management, said earlier this month. "When there is repetition, that's what really damages credibility. It's a risky position for Chipotle to be in now."

Other firms from Costco to Blue have also confronted serious questions when disease outbreaks were linked to food sold by them. About a decade ago, Taco Bell's business contracted for more than a year after an E. coli outbreak, even though it rapidly fixed the problem.

In each case, the company recovered, but the pressures on companies continue to get tougher, as customers demand better-quality food without being willing to pay much more for it.