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. (AP Photo/J. David Ake, File)

Longtime head of FDA stepping down

After nearly six years on the job, Margaret Hamburg announced her resignation as FDA director Thursday in an email to staff. She said the ag…