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J. David Ake, File, Associated Press
In this May 30, 2014 file photo, Food and Drug Administration (FDA) Commissioner Dr. Margaret Hamburg answers questions during an interview at The Associated Press in Washington. Hamburg has told employees she is resigning her post in March. Hamburg told employees of the Food and Drug Administration in an email Thursday that the agency's chief scientist, Stephen Ostroff, will serve as acting commissioner.

WASHINGTON — Food and Drug Administration Commissioner Margaret Hamburg is resigning in March after six years in the position.

Hamburg told employees of the FDA in an email Thursday that the agency's chief scientist, Stephen Ostroff, will serve as acting commissioner.

She is among the longest-serving commissioners to head the agency and helped oversee the creation of a new food safety system, reforms in how drugs are reviewed and new tobacco regulations.

President Barack Obama named Hamburg to the post in 2009, following a series of high-profile safety problems at the agency ranging from contaminated blood thinners to salmonella-tainted peanut butter that required one of the largest food recalls in U.S. history.

Under Hamburg's tenure, the FDA was more active on food policy than it had been since nutrition labeling rules were first written in the early 1990s. The agency has worked to put new food safety rules in place, phased out artery-clogging trans fats from the food supply, proposed updates to nutrition facts on the backs of food packaging and required restaurants and retailers to label calories on menus.

In her goodbye note to staffers, she emphasized the importance of science in these decisions and other reforms aimed at speeding up drug reviews and overseeing tobacco products.

"At the heart of all of these accomplishments is a strong commitment to science as the foundation of our regulatory decision-making and of our integrity as an agency," Hamburg said.

She took control of the agency at a time when its reputation had been tarnished by accusations that agency officials were allowing politics to influence their decisions. A federal judge ruled that in 2006 the agency deliberately delayed making a decision on the Plan B morning-after pill at the behest of the Bush administration.

Before joining the FDA, Hamburg, 59, was primarily known as a bioterrorism expert who served as New York City health commissioner

News of Hamburg's departure comes just a week after the agency announced that Robert Califf, a prominent cardiologist from Duke University, would take on the agency's No. 2 leadership position. Califf was considered for the agency's top job under the administrations of President George H.W. Bush and George W. Bush. Many FDA observers expect him to eventually move into the commissioner position.