What causes a metallic taste with food poisoning?

Not sure: What kind of "food poisoning" are you referring to? This term is very confusing and needs further definition. The basic answer, however, is that if you are not being treated with medication which may be responsible for the taste, any inflammation of the upper GI tract has the potential for altering your taste perception.

Spoiled fish: Eating spoiled fish---especially dark meat fish such as tuna, mackerel, bonito and mahi mahi---may cause a temporary metallic taste in the mouth. This type of food poisoning is called scombroid or histamine fish poisoning.