special edition: Rapid Pathogen Detection

DNA Diagnostic’s Salmonella test backed by NordVal

By Joseph James Whitworth

- Last updated on GMT

Picture: © iStock/Wiktory
Picture: © iStock/Wiktory

Related tags Polymerase chain reaction

DNA Diagnostic A/S’s test to detect Salmonella spp has been certified by NordVal International.

Salmonella Velox is certified for testing three matrix categories: raw meat (25 to 125g samples), raw poultry (25g samples) and raw fish/seafood (25g samples).

Detection limit is 1 CFU per 25-125g sample and Salmonella Velox runs in 96 well format.

The Denmark-based firm said the test, which takes 5.5 hours from sample to result, enables a more flexible production line and a faster release of the daily production.

AnalyTech Miljølaboratorium of Denmark carried out validation of the system.

Method protocol

The method includes an enrichment step, a DNA extraction step and a quantitative polymerase chain reaction (qPCR) step. The enrichment is carried out with Salmonella Velox Broth for 4.5 h at 41°C or 42.5°C ± 0.5°C.

The DNA extraction step uses the Salmonella Velox DNA Extraction Kit for DNA extraction from 1.5 -1.8 mL of the enrichment.

The qPCR step uses the Salmonella Velox qPCR Kit and 5 μL DNA extract. The DNA sample is added to a 96 well qPCR plate containing qPCR reaction mix. The qPCR reaction runs for 40 minutes.

For testing the selectivity, i.e. the method’s ability to detect the target microorganism from a range of strains (the inclusivity), and the lack of interference from a relevant range of non-target microorganisms (the exclusivity), 110 Salmonella strains and 30 non-Salmonella bacteria were tested with Salmonella Velox and ISO 6579.

Two samples with Salmonella strains were excluded due to failure of spiking. The remaining 108, representing 101 different Salmonella serovars, all tested positive with Salmonella Velox and the reference method.

All non-Salmonella strains were negative with both methods.

For the sensitivity study, i.e. the ability to detect the analyte, 186 samples were tested with both methods.

The matrix categories were raw meat (including poultry), ready-to-cook meat products and raw and ready-to-cook fish and seafood.

NordVal International said the only deviation in the results between the methods, was that five of the results were positive, and confirmed positive, with the alternative method, but were negative with the reference method.

So sensitivity is better with the alternative method than the reference method, it added.

LOD and inter-laboratory study

The level of detection (LOD) for the Salmonella Velox method was compared against the LOD of ISO 6579, by analysing 92 samples at low levels and results were satisfactory with both.

AnalyTech Miljølaboratorium organised an inter-laboratory study (ILS) of the Salmonella Velox method earlier this year.

Eight laboratories analysed pre-enriched samples in duplicates. The matrix was raw unprocessed pork samples, inoculated with cold stressed Salmonella typhimurium at three different contamination levels.

The results showed no statistical difference in the performances between the Salmonella Velox and the reference method ISO 6579:2002.

Certificate renewal

NordVal certificates renewed included the Transia Plate Listeria from Biocontrol. An ELISA method tested for detection of Listeria in foods and environmental samples.

Certificates for RAPID’Salmonella method short protocol and double enrichment protocol (24 hours) were also renewed.

RAPID’Salmonella from Bio-Rad is a chromogenic agar medium and the principle of the method relies on demonstration of two enzymatic activities.

The RAPID’Salmonella method with short protocol and RAPID’Salmonella method with double enrichment protocol are applicable to foods, animal feeds and environmental samples.

NordVal said the agreement between the RAPID’Salmonella method with double enrichment protocol and the reference method were not satisfactory due to a high number of false positives with the alternative method.

It said according to the study, the double enrichment protocol will cause false positives in 20% of cases, and confirmation is necessary.

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