Chipotle, the Mexican fast food chain in a long slog out of last year's E. coli outbreak, has engaged Joele Frank, Wilkinson Brimmer Katcher as the company eyes a new potential threat: activist investor Bill Ackman.

chipotleAckman's Pershing Square Capital Management took a 9.9% stake in Chipotle in September, making the hedge fund the company's No. 2 shareholder as its share price remains depressed.

Bloomberg reported Chipotle last week posted its tenth straight month of more than 20-percent year-over-year sales declines.

Reuters said the company has tapped investment banking and legal help, as well, noting Chipotle and Pershing Square have met at least once amid other talks.

The Joele Frank firm has years of experience in public tussles with Ackman, including his ongoing, high-profile showdown with Herbalife.

Burson-Marsteller picked up consumer PR duties for Chipotle in March.