(Reuters) -Abbott Laboratories, the biggest U.S. supplier of powder infant formula including Similac, has agreed with regulators on steps needed to resume production at its Sturgis, Michigan, manufacturing plant. Abbott initiated a recall of its infant formula products and closed its Michigan plant in February after reports of serious bacterial infections in four infants, worsening a shortage among multiple manufacturers that began with pandemic supply chain issues. Here are some details about what has happened: WHY DID ABBOTT INITIATE A RECALL AND PLANT SHUTDOWN? Consumers reported four infan…