Automated colorimetric assay for the detection of Pasteurella haemolytica leucotoxin

A rapid and easily performed colorimetric method has been adapted for the detection of the Pasteurella haemolytica leucotoxin using the lysis of bovine leucocyte BL-3 cells (a bovine lymphoid cell line obtained from Dr. G. Theilen, University of California, Davis). The method involves incubating BL-3 cells in the presence of P. haemolytica culture supernatant and assessing cell survival at the end of the assay by staining remaining BL-3 cells with the dye neutral red. Solubilization of the cells with concomitant dye release allows the assay to be read by measuring optical density (540 nm) with an automated spectrophotometer. This assay can also be used for the determination of neutralizing antiserum titres.