Oxidase Test Procedure for Bacterial Identification

The oxidase test is a fundamental diagnostic tool in clinical microbiology, primarily used to detect the presence of the enzyme cytochrome c oxidase in bacterial species. It is a biochemical test that differentiates among Gram-negative bacteria, particularly in distinguishing members of the family Enterobacteriaceae (oxidase-negative) from non-enteric oxidase-positive organisms like Pseudomonas, Neisseria, and Aeromonas.

Let us explore the oxidase test principle and procedure, variations in testing methods, and how results are interpreted in clinical practice. We aim to provide a clear, evidence-based explanation of the oxidase test procedure for pathologists, lab technicians and microbiologists.

Historical Development

The oxidase test was first introduced by Gordon and McLeod in 1928 to distinguish Neisseria gonorrhoeae from Staphylococcus spp. and Streptococcus spp. Later, Kovacs modified the test using tetramethyl-p-phenylenediamine dihydrochloride (now known as Kovacs' oxidase reagent) for improved identification of the cytochrome oxidase enzyme.

Gaby and Hadley further modified the test, using p-amino dimethylaniline oxalate with α-naphthol as a reagent to detect the cytochrome oxidase enzyme in tube culture.

Oxidase Test Principle

The oxidase test is based on the ability of bacterial cytochrome c oxidase to catalyse the transfer of electrons from an electron donor (usually reduced cytochrome c) to molecular oxygen, resulting in the formation of water.

When artificial electron donors such as tetramethyl-p-phenylenediamine dihydrochloride (the primary oxidase test reagent) are present, bacteria possessing cytochrome c oxidase promote oxidation of the reagent, producing a visible colour change.

Cytochrome c oxidase is a large transmembrane protein that functions as the terminal enzyme of the electron transport chain in aerobic bacterial and mitochondrial respiration. Some bacteria possess this enzyme and transfer terminal electrons to molecular oxygen, whereas others lack it or use alternative cytochromes.

The oxidase reagent is colourless in its reduced form and turns deep blue or purple when oxidised, indicating a positive reaction.

Clinical Applications of the Oxidase Test

• Initial screening in diagnostic bacteriology for Gram-negative rods.
• Differentiating Enterobacteriaceae from oxidase-positive organisms such as Pseudomonas aeruginosa.
• Identification of fastidious organisms, including Neisseria spp. and Campylobacter spp.
• Differentiating Neisseria spp. from Staphylococcus spp. and Streptococcus spp.

Oxidase Test Procedure in Microbiology

Here is a step-by-step explanation of the various oxidase test procedures followed in clinical labs.

Materials Required

• Fresh bacterial culture (18–24 hours old)
• Oxidase test reagent: 1% tetramethyl-p-phenylenediamine dihydrochloride (Kovacs' reagent) or 1% dimethyl-p-phenylenediamine dihydrochloride (Gordon and McLeod reagent)
• Filter paper or oxidase discs
• Inoculating loop (plastic) or sterile wooden stick
• Glass slides (optional)
• Timer or stopwatch
• Petri plates (for certain methods)
• Cotton swabs (for swab method)
• Nutrient broth (for tube method)

Procedure Types

There are more than five methods to conduct the test. Some of them are explained below.

Filter Paper Method

1. Place a strip/disc of Whatman no. 1 filter paper in a sterile petri plate.
2. Soak the filter paper with 1% Kovacs' oxidase reagent and allow it to dry.
3. Using a sterile inoculating loop, pick up a well-isolated colony from a fresh culture and make a smear on the reagent-soaked filter paper.
4. Observe for colour change and note the time required for up to 60 seconds.

Alternative approach:

1. Make a bacterial smear on filter paper first.
2. Add 1-2 drops of oxidase reagent over the smear.
3. Observe colour development for up to 60 seconds.

Oxidase Disc Test Procedure

1. Place a commercially prepared oxidase disc on a clean, dry slide.
2. Moisten the disc with sterile distilled water (unless the manufacturer states otherwise).
3. Using a sterile wooden stick or plastic loop, rub a colony onto the disc.
4. Observe for a colour change to deep purple within 10–30 seconds.

Oxidase Test Strip Procedure

1. Remove one oxidase strip from its container and allow it to come to room temperature.
2. Apply a fresh colony directly onto the test pad.
3. Wait for 30 seconds to 1 minute.
4. A blue or purple colour change indicates a positive result.

Swab Method

1. Moisten a sterile swab with 1% Kovacs' oxidase reagent.
2. Touch a well-isolated colony from a fresh culture with the swab.
3. Observe the development of colour in the swab for up to 60 seconds.

Direct Plate Method

1. Add a few drops of oxidase reagent directly over well-isolated colonies.
2. Tilt the plate and shake it gently to expose the colonies to oxygen.
3. Observe for colour development over the reagent-moistened colonies for up to 60 seconds.

Tube Method (Gaby-Hadley Oxidase Test)

1. Inoculate the nutrient broth with the sample bacteria and incubate aerobically at 35±2°C for 18-24 hours.
2. Add 0.2 ml of Gaby-Hadley Reagent A (1% α-naphthol).
3. Add 0.3 ml of Gaby-Hadley Reagent B (1% p-amino dimethylaniline oxalate) and mix well.
4. Observe for colour change for up to 3 minutesm

Note: The tube method is especially useful for microaerophilic or fastidious organisms.

Interpretation of Results

Reagent Used	Positive Result	Negative Result
Kovacs' Reagent	- Blue or purple colour within 10–30 seconds (strong positive) - Colour change within 30–60 seconds (weak/delayed positive)	- No colour change or colour change after 60 seconds (considered negative due to reagent auto-oxidation)
Gaby-Hadley Reagents	- Purple to deep blue colour within 15–30 seconds (positive) - Colour change within 2–3 minutes (weak/delayed positive)	- No colour change within 3 minutes - Colour change after 3 minutes is negative

Organisms Tested

Organism	Oxidase Test Result
Pseudomonas aeruginosa	Positive
Neisseria gonorrhoeae	Positive
Vibrio cholerae	Positive
Campylobacter jejuni	Positive
Aeromonas spp.	Positive
Brucella spp.	Positive
Moraxella spp.	Positive
Micrococcus spp.	Positive
Bordetella pertussis	Positive
Escherichia coli	Negative
Klebsiella pneumoniae	Negative
Salmonella spp.	Negative
Staphylococcus spp.	Negative
Streptococcus spp.	Negative
Pseudomonas maltophilia	Negative
Mycoplasma spp.	Negative
Bordetella parapertussis	Negative
Listeria spp.	Negative
Haemophilus spp.	Variable
Pasteurella spp.	Variable

Quality Control

For reliable results, quality control should be performed with known positive and negative controls.

Positive control: Pseudomonas aeruginosa ATCC 2783 (rapidly produces deep blue/purple colour within 10-30 seconds)

Negative control: E. coli ATCC 25922 (no colour change within 60 seconds)

Limitations and Precautions

While the oxidase test is simple and rapid, accuracy depends on proper technique and awareness of its limitations.

• Metal loops (nichrome or iron) can cause false positives due to oxidation of the reagent; use plastic or wooden applicators instead.
• Old cultures may lack sufficient enzyme activity, leading to false negatives.
• Bacteria grown on media containing dyes (like EMB, MAC medium) may give misleading results.
• Bacteria grown on glucose-rich media often show false-negative results.
• The test is not suitable for strict anaerobes.
• Some organisms produce delayed colour changes unrelated to cytochrome oxidase, leading to false positives.
• Contaminated reagents or over-exposure to air can yield misleading results.
• The reagent-soaked colonies in the direct plate method become nonviable quickly and require immediate subculture if needed.

To minimise variability:

• Use freshly prepared oxidase reagents or store as per the manufacturer's guidelines.
• When preparing the oxidase reagent, store it in a dark place at -20°C.
• Record the exact time for color development to differentiate between rapid positive, delayed positive, and negative reactions.
• Test only well-isolated colonies from fresh cultures (18-24 hours old).
• Don't flood plates with excessive oxidase reagent.

Glucose Oxidase Test Procedure vs. Cytochrome Oxidase Test

Though similar in name, the glucose oxidase test procedure is unrelated to the oxidase test for bacterial ID. Glucose oxidase tests are used in biochemistry to measure glucose levels in biological fluids, relying on the enzyme glucose oxidase, which catalyses glucose oxidation.

The oxidase test discussed here is strictly a microbiological tool centred on cytochrome c oxidase, not glucose metabolism.

Storage and Stability of Oxidase Reagent

• Store oxidase discs and strips in airtight containers at 2–8°C.
• Avoid exposure to light and humidity.
• Discard reagents past their expiration date or if they show discolouration before use.
• It is recommended to prepare reagents daily, as they tend to auto-oxidise and are photosensitive.

Final Thoughts

A knowledge of the oxidase test opens the door to smarter laboratory practices, but equally important is developing the skill to interpret borderline results with consistency. What goes unspoken is the value of technician experience—subtle differences in timing, reagent freshness and organism handling can influence outcomes.

Continuous hands-on training and observation of test patterns across different bacterial isolates help refine accuracy over time. As laboratories move toward automation, retaining expertise in foundational methods like the oxidase test promise deeper insight into microbial behaviour.

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