Production was continuing at the PM Beef plant linked to seven E. coli illnesses, the company said on Thursday.  PM Beef spokeswoman Lisa Hernandez said the plant is working with federal and state authorities on the matter, but in the meantime, the plant has not shut down.  State health officials said seven people were sickened by E. coli O157:H7 bacteria. All involved ground beef purchased since April 12 from one of four Lunds or Byerly’s stores in the western Twin Cities and supplied by PM Beef. Three of those who got sick were hospitalized, but all have since been discharged.  The U.S. Department of Agriculture is leading the investigation, but it has not said whether other sellers received meat that may have been contaminated.