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IBEX Sciences Oxidase Reagent Solution is a ready-to-use liquid biochemical reagent widely used in microbiology laboratories. It functions as a rapid indicator test to detect the presence of the intracellular enzyme cytochrome c oxidase in bacterial strains. [1, 2, 3]
This diagnostic solution plays a vital role in differentiating between major groups of Gram-negative bacteria. [1, 2]
Core Principle and Mechanism
The solution typically contains a redox indicator dye, most commonly 1% Tetramethyl-p-phenylenediamine dihydrochloride (Kovács reagent) or N,N-dimethyl-p-phenylenediamine hydrochloride (Gordon-McLeod reagent). [1, 2, 3, 4]
- Positive Reaction: If the target bacteria produce cytochrome c oxidase, the enzyme catalyzes the oxidation of the reagent. The reduced, colorless reagent acts as an electron donor and converts into a highly visible, oxidized, deep blue or dark purple compound within 10 to 30 seconds.
- Negative Reaction: If the bacteria lack the enzyme, the solution remains colorless or light pink/yellow. [1, 2, 3, 4]
Primary Diagnostic Applications
The oxidase test is a critical baseline step for identifying unknown bacterial isolates: [1]
- Identifying Pathogens: It differentiates oxidase-positive genera like Pseudomonas, Neisseria, Vibrio, and Campylobacter from oxidase-negative families. [1, 2, 3]
- Screening Enterobacteriaceae: It isolates members of the Enterobacteriaceae family (such as E. coli and Salmonella), which are characteristically oxidase-negative, from other non-fermenting Gram-negative rods.
Common Laboratory Testing Methods
- Filter Paper Method (Indirect): Place a strip of clean filter paper in a Petri dish and moisten it with 1–2 drops of the Oxidase Reagent. Using a sterile plastic loop or a wooden applicator stick, smear a fresh colony onto the moistened paper. Observe for a purple color shift within 30 seconds. [1]
- Direct Plate Method: Drop 1–2 drops of the reagent solution directly onto isolated colonies growing on a nutrient agar plate. Avoid flooding the whole plate, and monitor the treated colonies for an immediate purple color transition. [1, 2]
Crucial Practical Limitations
- Auto-Oxidation: Oxidase reagents are highly sensitive to air and light, meaning they gradually auto-oxidize over time. If the solution turns blue or dark brown while sitting in the bottle, it has deteriorated and must be discarded.
- Loop Material Interference: Never use nichrome or iron-containing inoculation loops. These metals can catalyze a false-positive oxidation reaction on their own. Always use sterile plastic loops, wooden sticks, or platinum wire.
- Timing Constraints: Results must be read strictly within 10 to 30 seconds. Any color change appearing after 60 seconds is non-specific and should be interpreted as a negative result. [, 2, 3, 4, 5]
Storage Conditions
To maximize shelf life, store the tightly closed container under refrigeration at 2°C to 8°C (unless otherwise stated by the manufacturer’s specific instructions) and protect it from prolonged light exposure. [1]
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