VIOLET RED BILE GLUCOSE AGAR 500g

Description
Results from tests that may be applied to water to detect coli-aerogenes organisms as possible indicators of faecal contamination possess far less significance when applied to raw foods. In the examination of foodstuffs, detection of a more defined group of organisms, the Enterobacteriaceae, that ferment glucose to produce acid and/or gas has been recommended^1,2. In addition to coliforms this group includes salmonellae and shigellae, which do not ferment lactose, and enterotoxigenic Escherichia coli. It also contains organisms, such as Klebsiella and Citrobacter, which are more resistant to heat than coliforms and are therefore better indicators of failure of processes that use minimal heat.

The difficulties of measuring the total Enterobacteriaceae content of foodstuffs have been studied by Mossel et al.³, who showed that the addition of glucose to an existing medium for the detection of coliforms improves the performance. They added 10g per litre of glucose to crystal violet neutral red bile lactose agar (Violet Red Bile Agar CM0107), and named the modified formulation MacConkey Glucose Agar.

Further work by Mossel et al.^4,5 showed that the lactose could be omitted resulting in the formulation of Violet Red Bile Glucose Agar. The continued inclusion of lactose would not provide test results leading to more accurate identification. Exclusion of lactose renders the medium more economical to make as less weight is required per litre.

Media that contain bile salts have an intrinsic toxicity for Enterobacteriaceae, even for cells that have not been under stress^{6,7,8,9,10,11}.

Considerable differences have been observed among six commercial preparations of Violet Red Bile Agar^4,5 with regard to productivity for Enterobacteriaceae¹², and the intensity of their metabolism. In conjunction with Oxoid the components of the medium were examined and Mossel drafted a specification as follows:

1. Approved media have to be clear and yield colonies of satisfactory size. They have to give reproducible counts of typical colonies of Enterobacteriaceae.
2. When challenged for intrinsic toxicity by an anaerobic metabolic test ¹³ using a strain of Yersinia enterocolitica (Serotype 03) as a sensitive indicator, media must promote adequate growth, acid formation and, where required, adequate gas formation.
3. Media have to satisfy the confirmation rate of typical colonies, i.e. the number of colonies confirmed as Enterobacteriaceae divided by the number of colonies tested.

Violet Red Bile Glucose Agar has been developed to satisfy all of these criteria.

VRBGA CM0485 is tested in accordance with ISO11133:2014¹⁵