Delvotest® is a diagnostic tool that detects inhibitory factors like antibiotics in milk by monitoring bacterial growth and pH changes. The test requires incubation at 64°C, and a color shift from purple to yellow indicates a negative result, while the presence of antibiotics halts bacterial growth, keeping the medium predominantly purple.
Working principle
Delvotest® products such as ampoules and plates, are designed to detect inhibition in milk. The tests contain a culture medium seeded with a standardised number of spores of thermophilic bacteria (Geobacillus stearothermophilus). The medium is coloured purple by the pH indicator bromocresol purple. This indicator will change color based on the metabolic activity of the bacteria when the conditions allow germination and growth of the bacteria. When bacteria grow normally, they produce acid, which lowers the pH, leading to a color change of the test from purple to yellow. Let's see an example below:
- Negative Result: If the milk is free from inhibitors or contains antibiotics below the detection limit of the test, the bacteria grow normally, the pH drops, and the indicator changes from purple to yellow, indicating the milk is free of antibiotic residues.
- Positive Result: If antibiotics are present above the detection limit of the test, bacterial growth is inhibited, and the pH remains unchanged. As a result, the color of the medium stays purple, indicating the presence of antibiotic residues in the milk sample.
Incubation conditions
Delvotest® must be kept at a specific temperature, and the presence of milk plays a crucial role for the bacterial activity.
Temperature: Delvotest® needs to be incubated at 64 °C +/- 2 °C as this type of bacteria is thermophilic and likes the heat.
Nutrients: Delvotest® kits are designed for testing milk samples. The milk also adds ‘nutrients’ to the Delvotest®, such as lactose that the bacteria will utilize to grow.
In the absence of a milk sample, the metabolism process is slower and different, resulting in a color change from purple to brownish or even yellowish. The yellowish coloration only occurs with extended incubation time. Despite this color change, incubating Delvotest® without a milk sample does not introduce any inhibitory substances, unlike those found in positive milk samples.
Do you incubate you plates using the DAS? It is recommended not to leave wells empty during testing. Instead, it is advisable to add negative raw milk to these wells. This will help the algorithm in accurately calculating the control time. [ link to the article with how many samples of NEG needed]
see also this article: Science Uncovered // How does Delvotest® work?
Product range
- Delvotest® T has the broadest detection spectrum for antibiotics than Delvotest® SPNT including the EU MRL for Tetracyclines.
- Delvotest® T is most often used as payment analysis in the National laboratories. We advised to use the same test as the National laboratories (Milk Payments Laboratories)
SAQs:
- Why do the color changes occur occasionally when the test is incubated without the addition of milk?