The oxidase test assays for the presence of cytochrome c in a cell’s respiratory pathway. Cells are smeared onto a sheet coated with tetrametyl-p-phenylenediamine (TMP), a colorless electron donor. A functional cytochrome c removes electrons from TMP; this oxidized TMP has an intense purple-blue color. A positive control for the oxidase test is Pseudomonas fluorescens, and a negative control for the oxidase test is S. epidermidis.