Attention:

Certain features of Sigma-Aldrich.com will be down for maintenance the evening of Friday August 18th starting at 8:00 pm CDT until Saturday August 19th at 12:01 pm CDT.   Please note that you still have telephone and email access to our local offices. We apologize for any inconvenience.

Innovative Detection of Listeria

By: Jvo Siegrist, Microbiology Focus Edition 2.2

By Jvo Siegrist, Product Manager Microbiology…. ivo.siegrist@sial.com

Today, Real Time PCR is the standard for modern microbiology, but there are also other easier methods.

Our HybriScan® test system is able to detect Listeria ssp. and specifically Listeria monocytogenes. PCR is not needed but nevertheless the result is based on the genetic information, the target molecule being ribosomal RNA. Each bacterial cell posseses several hundred copies of ribosomal RNA and therefore no PCR is needed. The test method is designed on simple 96-well microplate with 12 strips of 8 wells and is sandwich hybridization. The test is not sensitive to sample matrix and detects only living cells. No special expensive equipment is needed and the test is done in approximately 2 to 2.5 hours. A positive result is visible to the naked eye, but can also be read by a standard microplate reader to quantify the number of cells at 450 nm. With a pre-enrichment step (24-30 hours in Half Fraser and Fraser broth) a sensitivity of 1 CFU/25 g can be reached.

The system was validated according to the German § 64 LFGB method. 25 g sample was taken and enriched in 225 mL ONE-Broth for 24 h at 30 °C. Then the broth was streaked out on PALCAM agar incubated for 48 h at 37 °C and all Listeria appears as grayish-black, caved in, colonies. The colonies are then confirmed as Gram-positive rods using biochemical differentiation with BD BBL CRYSTAL. As the HybriScan system missed three L. monocytogenes positive samples, the single colonies grown on PALCAM agar, sequenced by 16S rDNA and the three undetected species turned out to be L. innocua, L. seeligeri and L. welshimeri. So the cultivation based method (§ 64 LFGB method) lead to false positive results due to a wrong biochemical identification (see more in table 1).

Table 1 HybriScanD Listeria monocytogenes test validation (*subsequently identified as false positive result with cultivation based; § 64 LFGB method )

HybriScan®D Listeria monocytogenes has a relative accuracy of 99.0%, a relative specificity of 98.2% and relative sensitivity of 100%. For more information about the HybriScan-System and the range of yeast and bacteria detection kits, visit www.sigma-aldrich.com/hybriscan.

back to top Back to Top

Related Links