West, P.A., Daniel, R.M., Knowles, C.J. & Lee, J.V. (1978). Tetramethyl-p-phenylenediamine (TMPD) oxidase activity and cytochrome distribution in the genus. FEMS Microbiology Letters, 4(6), 339-342.
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Kovacs'  qualitative test for N,N,N',N',-tetramethyl- p-phenylenediamine (TMPD) oxidase activity is a fundamental test in the identification of bacteria [2,3]. TMPD (E~ = +0.26 V at pH 7.0) acts as an artificial electron donor to the terminal portion of aerobic respiratory systems , and the ability to oxidise TMPD, reduced by ascorbate, has been widely used as a quantitative assay of terminal oxidase activity . In a wide range of heterotrophic bacteria there is a close correlation between their response to the Kovacs test and their ascorbate-TMPD oxidase activity measured manometrically . The inability to oxidise ascorbate-TMPD or a negative response to the Kovacs test, however, does not necessarily indicate that an organism lacks an aerobic respiratory system and a terminal oxidase. For example, species belonging to the family Enterobacteriaceae are oxidase- negative yet form aerobic respiratory systems[5,7].