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Neogen improves colony colours in chromogenic media

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By Joe Whitworth+

Last updated on 17-Feb-2017 at 09:46 GMT2017-02-17T09:46:10Z

Chromogenic Agar for Salmonella Esterase (CASE)
Chromogenic Agar for Salmonella Esterase (CASE)

Lab M has developed a chromogenic media that can simultaneously detect Salmonella strains while inhibiting or minimizing other organisms.

Chromogenic Agar for Salmonella Esterase (CASE) uses a proprietary dual chromogenic system to differentiate between Salmonella and non-target organisms that grow on the agar.

It detects all Salmonella spp. that have the presence of esterase and absence of ß-glucosidase activity.

The dual chromogenic system pairs an esterase target chromogen with a secondary chromogen for ß-glucosidase activity. 

Neogen, which bought Lab M in late 2015, said this allows a colour differentiation between target and non-target organisms on the agar.

Unequal serovar performance and background flora

The company told us that Salmonella spp. is a complex pathogen with over 2,500 different serovars, a lot of which are of interest to the food safety industry.

“The current ISO standard allows the user to test for multiple strains with one method and ensures that different serovar variations are detected. This is achieved by allowing customers to use a second choice of media, often chromogenic plates, which can detect the different characteristics.

“An issue we found with the current generation of Salmonella chromogenic agars, which are often chosen as a second media, was that each serovar did not perform equally, heightening the risk of reading false negative results.

“With CASE, there is superior detection of atypical and weaker serovars including S. Dublin to eliminate this risk.”

The formulation means growth from background flora is reduced and closely related Enterobacteriaceae, such as Citrobacter and Enterobacter, are defined.

These are able to grow on CASE but they do so very poorly - typically with less than 50% recovery. 

Those which do grow are either colourless or show as a distinct negative black colony which can be differentiated from the positive turquoise Salmonella spp.

This makes matrices easier to read and can reduce unnecessary confirmations which can be costly and time-consuming, added the firm.

“Currently available Salmonella chromogenic media typically produce pink purple colonies that are sometimes hard to distinguish - whilst this new agar gives turquoise-blue and black colonies that are very easy to identify particularly with low C8-esterase activity Salmonella strains,” said Steve Chambers, Neogen’s European sales and marketing director.

Chambers added the media was developed at Neogen’s Lab M facilities for food and animal feed manufacturers, as well as poultry and pork producers and processors.

Pre-poured plate or DCM

Neogen said following industry feedback that current Salmonella chromogenic agars were sometimes difficult to interpret, the R&D team developed the CASE media to give improved coloured colonies by using different chromogens.

The media, in pre-poured plate or dehydrated culture media (DCM), is for use within the ISO 6579 protocol - the international standard for testing food and animal feed for Salmonella species.

The protocol has initial primary and secondary sample enrichment steps before plating for detection.

Neogen said CASE fits into the protocol as the secondary selective detection agar and gives results in 24 ±3 hours after incubation.

Results are obtained in 72 hours for negatives or five days for presumptive positives followed by full confirmation testing.

Neogen can supply the media ready to use as part of Lab M’s Pinnacle pre-poured media range, which includes all necessary supplements with no need to weigh, cool or autoclave media.

It can also be pre-supplemented in DCM format as part of Lab M’s Harlequin chromogenic media range.

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