New Rapid Listeria Test Provides Peace-of-Mind in Food Testing Laboratories

Thermo Scientific Limited has extended the O.B.I.S. (Thermo Scientific Biochemical Identification Systems) range of rapid colourimetric tests with the launch of O.B.I.S. mono - a simple new biochemical test that differentiates Listeria monocytogenes from other Listeria species on standard diagnostic culture media.

Due to their similar appearance on Listeria selective media, Gram-positive, catalase positive, oxidase negative colonies could be one of a number of Listeria species or even a different species altogether, such as Bacillus. If the presence of Listeria monocytogenes (one of the most significant food-borne pathogens) is suspected, further lengthy identification procedures are required. In the meantime, product release is delayed and plant-cleaning procedures may be initiated at considerable cost to the manufacturer.

O.B.I.S. mono is able to offer valuable peace-of-mind at this critical point by indicating rapidly whether suspect colonies are Listeria monocytogenes or not - providing an answer in just 10 minutes.

All Listeria species, with the notable exception of Listeria monocytogenes, possess D-alanyl aminopeptidase (DALAase) activity. O.B.I.S. mono was developed to detect this enzyme using a non-carcinogenic substrate, D-alanyl-7-amido-4- methylcoumarin (DALA), in response to health concerns associated with b-naphthylamines (potent carcinogens traditionally used to detect aminopeptidase activity).

Simply smear colonies (taken from a purity plate*) onto the disposable reaction card and add the DALA substrate. In the presence of DALAase, DALA is hydrolysed to form a chemical that produces a vivid purple colour when mixed with the O.B.I.S. mono Developing Solution. This colour reaction, which appears in 20 seconds, reassures food manufacturers that the suspect colonies are NOT Listeria monocytogenes.

If DALAase is not present, there is no colour reaction. In this case, the suspect colonies are presumptive Listeria monocytogenes and appropriate measures can be taken with confidence and at the earliest opportunity.

In trials, O.B.I.S. mono demonstrated 100% sensitivity and 99% specificity with naturally contaminated samples, and 100% sensitivity and 98% specificity with pure cultures compared to the Gold Standard method.

O.B.I.S. is a range of rapid biochemical tests that build on the information obtained from the culture plate to differentiate between organisms with similar colonial appearance. They are quick, simple and safe to use, with vivid colour reactions that are extremely easy to interpret.


* The use of multiple colonies from primary isolation is not recommended for identification testing as this may lead to a mixed culture and an incorrect result. International standards recommend sub-culturing presumptive Listeria species on to purity plates TSA (CM131), TSA-YE, or a recognised chromogenic Listeria medium.