Use Violet Red Bile Lactose Agar for examination of food, animal feed, and environmental samples for the presence of indicator groups such as coliforms. Besides safety, indicator groups provide a measure of food quality and spoilage potential. One of the most common groups of bacteria employed as indicator organisms by the food industry are the coliforms, which can be regarded as a subgroup within the Enterobacteriaceae
- Easy-to-read: organisms rapidly fermenting the lactose (coliforms) form red-purple colonies with red-purple halos; while late lactose fermenters form staw or colorless colonies with greenish halos.
- Selective: crystal violet and bile salts suppress the growth of non target Gram-positive bacteria.
- Adheres to ISO 4832:2006 and ISO/TS 11133-2:2003 standards.
- Improved selectivity by incubation at >42°C or by anaerobic incubation.
Coliforms are commonly defined by their ability to ferment lactose rapidly, producing acid and gas, typically within 24 hours. The genera that would be commonly expected to belong to this group include Enterobacter
, and Escherichia
, particularly Escherichia coli
. However, species belonging to other genera, for example Erwinia
, can also ferment lactose, albeit slowly, and some strains of Citrobacter
, as well as Salmonella arizonae
and Hafnia alvei
, show delayed or variable fermentation ability.
Coliforms rapidly ferment the lactose in VRBL Agar (ISO) and so reduce the pH of the medium, producing red-purple colonies, due to the inclusion of neutral red and crystal violet. These colonies are usually surrounded by red-purple halos of precipitated bile salts.
Non-lactose or late lactose fermenters produce straw or colorless colonies with greenish halos. Crystal violet and bile salts inhibit the growth of Gram-positive flora.
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