Although I agree with 123 to a certain extent, a lump that appears post administration does not always attribute to an immune response, in some cases, sure, in all, absolutely not. I've had this welt appear from time to time with various peptides over the last several years. It will appear, on say my fifth pin out of the same vial, not having happened before with the same vial or after with the same vial. My body would not randomly attack this substance if its already been exposed to it 5 times already.The reason my lump appeared was as already mentioned, I injured/ruptured a capillary. The resulting lump and BRUISE is due only to this. A mosquito bite or any other immune induced inflammation will not present with a bruise, but merely a lump.
And unless your analyzing a sample, how can you presume the reaction is due to a very specific strain of bacteria, as you said, E. Coli? There are billions of bacterium that could violate the integrity of an unsanitary product.