How to Perform an ONPG Test: Principle and Protocol for Beta-Galactosidase Detection

The ONPG (ortho-nitrophenyl-β-D-galactopyranoside) test is a biochemical method used to detect the presence of the enzyme β-galactosidase in bacterial isolates. This test is particularly valuable for identifying bacteria that are late lactose fermenters—organisms that possess the genetic capacity to produce β-galactosidase but may not produce sufficient enzyme activity to yield a visible color change in traditional lactose fermentation tests within 24–48 hours. The ONPG test uses a chromogenic substrate that, when cleaved by β-galactosidase, releases a yellow compound (ortho-nitrophenol) that is readily detectable, providing a clear positive result even when enzyme activity is low. This method is useful for differentiating members of the Enterobacteriaceae family and for confirming β-galactosidase activity in slow or non-lactose-fermenting gram-negative rods.

At a Glance

Scientific Principle

The ONPG test relies on the enzymatic activity of β-galactosidase, an enzyme encoded by the lacZ gene in many bacteria. This enzyme normally catalyzes the hydrolysis of lactose into glucose and galactose. The ONPG substrate is a synthetic analog of lactose in which the galactose moiety is linked to ortho-nitrophenol instead of glucose. When β-galactosidase cleaves the β-galactosidic bond in ONPG, it releases free ortho-nitrophenol, which is yellow in alkaline conditions. The reaction proceeds as follows:

ONPG (colorless) + H₂O → β-galactosidase → galactose + ortho-nitrophenol (yellow)

The intensity of the yellow color is proportional to the amount of enzyme activity present. Because ONPG is a smaller molecule than lactose, it can enter bacterial cells more readily and is cleaved more efficiently, making the test more sensitive than traditional lactose fermentation tests for detecting low levels of β-galactosidase activity [1]. This sensitivity is why the ONPG test is particularly useful for identifying bacteria that are late or weak lactose fermenters—organisms that may take 48 hours or longer to produce visible acid and gas in lactose broth but still possess functional β-galactosidase.

The test does not measure lactose fermentation directly; rather, it measures the presence of the enzyme required for the first step of lactose catabolism. Bacteria that are true non-lactose fermenters (e.g., Salmonella spp., Shigella spp. except S. sonnei) lack β-galactosidase and will remain colorless in the ONPG test. However, some bacteria that lack β-galactosidase may still ferment lactose through alternative pathways, so the ONPG test should be interpreted in conjunction with other biochemical tests.

Materials and Instrumentation

Bacterial Culture Requirements

The test requires a pure bacterial culture grown on a non-selective, non-differential medium such as tryptic soy agar (TSA) or nutrient agar. The culture should be 18–24 hours old to ensure active metabolism. Avoid using cultures grown on lactose-containing media (e.g., MacConkey agar) because residual lactose or its metabolites may interfere with the test. Similarly, avoid using cultures from selective media that contain inhibitors that could affect enzyme activity.

ONPG Substrate Preparation

ONPG substrate can be prepared in-house or purchased as commercial disks or tablets. For in-house preparation, dissolve 0.06 g of ONPG in 10 mL of distilled water, then add 10 mL of 0.01 M sodium phosphate buffer (pH 7.5). Sterilize by filtration through a 0.22-μm membrane filter. Store the solution at 4°C in a light-protected container; it is stable for approximately one month. Commercial ONPG disks are available from several manufacturers and are convenient for routine use. Follow the manufacturer's instructions for storage and expiration dates.

Additional Materials

• Sterile test tubes or microcentrifuge tubes
• Sterile saline (0.85% NaCl) or phosphate-buffered saline (PBS)
• Sterile inoculating loops or swabs
• Incubator set to 35–37°C
• pH indicator strips (optional, for buffer verification)
• Timer or clock
• Positive and negative control organisms

Instrumentation

No specialized instrumentation is required beyond a standard microbiological incubator. A spectrophotometer can be used for quantitative measurement of ortho-nitrophenol production at 420 nm, but this is not necessary for routine qualitative testing. For research applications, a microplate reader can be used to process multiple samples simultaneously.

Controls

Positive Control

Use a known β-galactosidase-positive organism such as Escherichia coli ATCC 25922 or a laboratory strain of E. coli K-12. This control should produce a visible yellow color within 4–6 hours. If the positive control fails to develop color, the substrate may be degraded, the buffer pH may be incorrect, or the incubation temperature may be inappropriate.

Negative Control

Use a known β-galactosidase-negative organism such as Proteus mirabilis ATCC 29906 or Salmonella enterica serovar Typhimurium. This control should remain colorless throughout the incubation period. If the negative control develops any yellow color, contamination may have occurred, or the substrate may have been contaminated with β-galactosidase-producing organisms.

Reagent Control

Include a tube containing ONPG substrate and buffer but no bacterial inoculum. This control verifies that the substrate does not spontaneously hydrolyze during incubation. Any yellow color in this tube indicates substrate degradation and invalidates all test results.

pH Control

The reaction requires an alkaline pH for ortho-nitrophenol to appear yellow. If the test medium becomes acidic due to bacterial metabolism, the yellow color may be masked. Some protocols include a drop of 1 M sodium carbonate after incubation to raise the pH and reveal any ortho-nitrophenol that may have been produced. This step is particularly important when testing organisms that produce acid during metabolism.

Conceptual Workflow

Step 1: Prepare Bacterial Suspension

Using a sterile loop, transfer a small amount of the 18–24 hour pure culture to a tube containing 0.5 mL of sterile saline or PBS. Emulsify the bacteria thoroughly to create a uniform suspension with a turbidity equivalent to approximately 0.5 McFarland standard (approximately 1.5 × 10⁸ CFU/mL). This standardized inoculum ensures consistent enzyme-to-substrate ratios across tests.

Step 2: Add ONPG Substrate

Add 0.25 mL of ONPG substrate solution to the bacterial suspension. If using commercial ONPG disks, add one disk to the suspension and ensure it is fully submerged. Mix gently by vortexing or tapping the tube.

Step 3: Incubate

Place the tube in a 35–37°C incubator. Incubate for up to 24 hours, checking for color development at 1, 2, 4, 6, and 24 hours. Most positive reactions appear within 4–6 hours. Record the time of first visible color change.

Step 4: Read and Interpret Results

Examine the tube against a white background under good lighting. A positive result is indicated by the development of a yellow color in the suspension. The intensity can range from pale yellow to deep yellow. A negative result is indicated by no color change (the suspension remains colorless or appears slightly turbid white). If no color develops after 24 hours, the test is considered negative.

Step 5: Confirm with pH Adjustment (Optional)

If no yellow color is observed after 24 hours, add 1–2 drops of 1 M sodium carbonate solution to the tube. Mix gently and observe immediately. The addition of base raises the pH, which may reveal ortho-nitrophenol that was produced but remained colorless due to acidic conditions. A yellow color after pH adjustment indicates a weak positive result.

Quality Checks

Pre-Test Quality Assurance

Verify that the ONPG substrate is within its expiration date and has been stored properly (refrigerated, protected from light). Check that the buffer pH is 7.5 ± 0.2 using pH indicator strips or a pH meter. Confirm that control organisms are viable and pure by streaking them on non-selective agar and checking for typical colony morphology.

During-Test Quality Checks

Include positive and negative controls with each batch of tests. Record the time of inoculation and the time of each observation. Note any unusual turbidity, color changes in the reagent control, or contamination in control tubes. If the positive control fails to develop color within 6 hours, do not interpret any test results from that batch.

Post-Test Quality Checks

Document all results, including the time to color development for positive tests. Compare the intensity of the test reaction to the positive control. If the test reaction is very weak (pale yellow only after 24 hours), consider repeating the test with a fresh culture and fresh substrate. Confirm negative results by performing a lactose fermentation test in phenol red lactose broth to rule out alternative lactose utilization pathways.

Result Interpretation

Positive ONPG Test

A yellow color indicates the presence of β-galactosidase activity. The organism is capable of cleaving lactose into glucose and galactose, even if it does not produce visible acid and gas in traditional lactose fermentation tests within 24–48 hours. Organisms that are typically ONPG-positive include:

• Escherichia coli (usually positive within 1–4 hours)
• Enterobacter spp. (positive within 4–6 hours)
• Klebsiella spp. (positive within 4–6 hours)
• Citrobacter spp. (variable, some strains positive)
• Shigella sonnei (positive, while other Shigella species are negative)
• Vibrio spp. (positive)

Negative ONPG Test

No color development after 24 hours (or after pH adjustment) indicates the absence of β-galactosidase activity. The organism cannot cleave lactose and is considered a non-lactose fermenter. Organisms that are typically ONPG-negative include:

• Salmonella spp. (negative)
• Shigella spp. except S. sonnei (negative)
• Proteus spp. (negative)
• Providencia spp. (negative)
• Morganella morganii (negative)
• Yersinia spp. (negative)

Interpretation Caveats

Some bacteria possess β-galactosidase but lack lactose permease, the transport protein needed to bring lactose into the cell. These organisms will be ONPG-positive (because ONPG enters the cell more readily than lactose) but may appear as non-lactose fermenters on MacConkey agar. Conversely, some bacteria may ferment lactose through pathways that do not involve β-galactosidase (e.g., through β-galactoside permease and alternative enzymes), so a negative ONPG test does not absolutely rule out lactose utilization. Always interpret ONPG results in the context of other biochemical tests, including lactose fermentation in broth, oxidase test, and IMViC (indole, methyl red, Voges-Proskauer, citrate) reactions.

Troubleshooting

Limitations

The ONPG test detects β-galactosidase activity but does not measure lactose fermentation directly. Some bacteria may produce β-galactosidase but lack the transport proteins or downstream metabolic enzymes needed for complete lactose fermentation. Conversely, some bacteria may ferment lactose through alternative pathways that do not involve β-galactosidase, yielding a false-negative ONPG result.

The test is not quantitative in its standard form. The intensity of the yellow color provides a rough estimate of enzyme activity but cannot be used to compare enzyme levels across different organisms without spectrophotometric measurement and standardization.

The test requires a pure culture. Mixed cultures can produce ambiguous results, as β-galactosidase-positive organisms in the mixture will produce a positive signal even if the target organism is negative.

The test is not suitable for all bacterial groups. Some gram-positive bacteria produce β-galactosidase but may not grow well in the saline suspension used for the test. For these organisms, alternative methods such as the MUG (4-methylumbelliferyl-β-D-galactopyranoside) test may be more appropriate.

The test does not differentiate between constitutive and inducible β-galactosidase production. Some bacteria produce the enzyme only when lactose is present in the growth medium, while others produce it constitutively. The test measures total enzyme activity at the time of testing, regardless of induction state.

Documentation

Record Keeping

Document the following information for each ONPG test performed:

• Date and time of test initiation
• Organism identification and source
• Culture age and growth medium used
• ONPG substrate lot number and expiration date
• Buffer pH (if prepared in-house)
• Incubation temperature
• Observation times and results (color intensity, time to color development)
• Control results (positive, negative, reagent)
• Any deviations from standard protocol
• Final interpretation

Reporting Results

Report ONPG test results as "Positive" or "Negative" based on the presence or absence of yellow color after 24 hours. If pH adjustment was performed, note this in the report (e.g., "Positive after pH adjustment"). Include the time to color development when relevant (e.g., "Positive at 4 hours").

Quality Records

Maintain records of control organism performance, including the expected and observed results for each batch of tests. Document any lot-to-lot variations in substrate performance. If commercial ONPG disks are used, retain the manufacturer's quality certificates.

Biosafety Considerations

The ONPG test is typically performed with BSL-1 organisms in teaching laboratories and research settings. However, clinical isolates may contain pathogens that require BSL-2 containment. Always perform a risk assessment before testing unknown isolates [2]. Use standard microbiological practices, including hand washing, use of personal protective equipment (lab coat, gloves, safety glasses), and decontamination of work surfaces before and after procedures.

All bacterial cultures and test materials should be decontaminated by autoclaving at 121°C for 30 minutes before disposal. Liquid waste containing ONPG substrate can be treated with 10% bleach (final concentration 0.5% sodium hypochlorite) for 30 minutes before disposal, but autoclaving is preferred for all microbiological waste [2].

The ONPG substrate itself is not considered hazardous at the concentrations used in this test, but standard chemical hygiene practices should be followed. Avoid skin contact and inhalation of powder when preparing substrate solutions. If using commercial disks, follow the manufacturer's safety recommendations.

For research involving recombinant organisms or synthetic nucleic acids, consult the NIH Guidelines for Research Involving Recombinant or Synthetic Nucleic Acid Molecules to determine appropriate containment levels [3]. The BACTH system described in the literature uses E. coli strains that are typically BSL-1, but any genetic modifications should be reviewed by the institutional biosafety committee [1].

Frequently Asked Questions

1. Why would a bacterium be ONPG-positive but lactose-negative on MacConkey agar?

This discrepancy occurs because ONPG is a smaller molecule than lactose and can enter bacterial cells even when lactose permease (the transport protein for lactose) is absent or nonfunctional. Once inside the cell, β-galactosidase cleaves ONPG to produce the yellow color. On MacConkey agar, lactose must be transported into the cell before it can be fermented to produce acid, which then causes the pH indicator to change color. Bacteria that lack lactose permease will appear as non-lactose fermenters on MacConkey agar (colorless colonies) but will test positive in the ONPG test. This is commonly observed with Shigella sonnei and some strains of Escherichia coli.

2. Can the ONPG test be used for gram-positive bacteria?

Yes, but with limitations. Many gram-positive bacteria, including enterococci and some staphylococci, produce β-galactosidase and will test positive. However, the standard protocol using a saline suspension may not provide optimal conditions for gram-positive organisms, which may require longer incubation times or different buffer systems. For gram-positive bacteria, the MUG test (using 4-methylumbelliferyl-β-D-galactopyranoside) is often preferred because it is more sensitive and can be detected using a UV lamp. If using the ONPG test for gram-positive organisms, include appropriate gram-positive controls and extend the incubation time to 48 hours if necessary.

3. How long should I incubate the ONPG test before calling it negative?

The standard incubation time is 24 hours at 35–37°C. Most positive reactions appear within 4–6 hours, but some weak or late lactose fermenters may require the full 24 hours to produce detectable color. If no color develops after 24 hours, the test is considered negative. However, if you suspect a very weak positive (e.g., from an organism known to be a late lactose fermenter), you can add 1–2 drops of 1 M sodium carbonate to raise the pH and reveal any ortho-nitrophenol that may have been produced but remained colorless due to acidic conditions. If still no color appears after pH adjustment, the result is definitively negative.

4. What is the difference between the ONPG test and the lactose fermentation test?

The ONPG test detects the presence of β-galactosidase, the enzyme that cleaves lactose into glucose and galactose. The lactose fermentation test detects the complete process of lactose catabolism, including transport into the cell, cleavage by β-galactosidase, and subsequent acid production from glucose fermentation. A bacterium can be ONPG-positive (has β-galactosidase) but lactose fermentation-negative (cannot transport lactose or cannot produce detectable acid). Conversely, a bacterium can be ONPG-negative (lacks β-galactosidase) but still ferment lactose through alternative pathways, though this is rare. The two tests provide complementary information and are often used together for bacterial identification.

References and Further Reading

1. Olson MG, Goldammer M, Gauliard E, Ladant D, Ouellette SP. A Bacterial Adenylate Cyclase-Based Two-Hybrid System Compatible with Gateway® Cloning. Methods in Molecular Biology. 2018;1794:87-100. PubMed – This reference describes the BACTH system, which uses β-galactosidase activity (detected via ONPG or similar substrates) as a reporter for protein-protein interactions in E. coli. The principles of β-galactosidase detection described here are directly applicable to the ONPG test methodology.

2. CDC and NIH. Biosafety in Microbiological and Biomedical Laboratories (BMBL), 6th Edition. U.S. Department of Health and Human Services, 2020. CDC – This authoritative reference provides biosafety guidelines for handling microorganisms in laboratory settings, including the risk assessment and containment practices relevant to performing biochemical tests like the ONPG test.

3. National Institutes of Health. NIH Guidelines for Research Involving Recombinant or Synthetic Nucleic Acid Molecules. NIH Office of Science Policy. NIH OSP – This document provides the regulatory framework for research involving recombinant organisms, which may be relevant when using genetically modified bacterial strains in ONPG testing.

4. National Center for Biotechnology Information. NCBI Bookshelf: Molecular Biology and Laboratory Methods. NCBI Bookshelf – This searchable collection includes authoritative references on biochemical testing methods, bacterial identification protocols, and laboratory quality control procedures that support the ONPG test methodology.

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