Oxidase test adapted from Dhar & Johnson . Smears (arrows) of organisms were applied to two Whatman 3 M filterpapers folded, vertically, into three. The filter papers were placed vertically in Petri dishes so that the solution within soaked vertically across the smears of organisms. Top panel: filterpaper stood in 10 ml distilled water in a Petri dish, bottom panel: filterpaper stood in 10 ml 1% aqueous solution of N, N, N', N'-tetramethyl-1, 4-phenylene-diamine dihydrochloride (Fluka 87890, Czech Republic) in a Petri dish. The organisms smeared were left E. coli ATCC 25922 (negative control), centre C. violaceum isolate from patient and right P. aeruginosa ATCC 27853 (positive control). Water (top panel) failed to produce any streaks whilst N,N,N',N'-tetramethyl-1, 4-phenylene-diamine dihydrochloride (bottom panel) led to pronounced vertical flame-like streaks across smears of C. violaceum and P. aeruginosa within 5 minutes, indicating oxidase test positivity. The flame-like streaks had faded by the next day. The profound violet color of the C. violaceum smear is clearly visible.