Enzyme linked immunosorbent assay (ELISA)
Enzyme linked immunosorbent assay was performed using a commercially available sandwich ELISA kit for E. coli HCP testing from Cygnus Technologies (Southport, NC). Both the standard included in the kit (kit standard) and an E. coli cell lysate supernatant were used to calibrate the amount of residual E. coli HCPs in the final API samples. In addition, the immunoreactivity of several other enzymes used in the reaction was also tested by the commercial kit to determine if the kit antibody recognizes those enzymes.