Oxoid has extended its O.B.I.S. (Oxoid Biochemical Identification Systems) range of rapid colourimetric tests with the launch of O.B.I.S. mono - a biochemical test that differentiates Listeria monocytogenes from other Listeria species on standard diagnostic culture media.
Due to their similar appearance on Listeria selective medium, 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, Oxoid claims.
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).
The procedure involves smearing colonies onto a disposable reaction card and adding a 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 per cent sensitivity and 99 per cent specificity with naturally contaminated samples, and 100 per cent sensitivity and 98 per cent 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 said to be quick, simple and safe to use, with vivid colour reactions that are extremely easy to interpret.