New rapid biochemical test differentiates listeria monocytogenes from other listeria species on standard diagnostic culture media to provide peace-of-mind in food testing laboratories
Oxoid has extended the Obis (Oxoid biochemical identification systems) range of rapid colourimetric tests with the launch of Obis 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.
Obis 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 ten minutes.
All listeria species, with the notable exception of listeria monocytogenes, possess D-alanyl aminopeptidase (Dalaase) activity.
Obis 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 Obis 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, Obis 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.
Obis 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.
