Gassner Lactose Agar, 500 g

Gram-negative bacilli belonging to Enterobacteriaceae are widely dispersed in nature and have been frequently isolated from clinical specimens. The genera in Enterobacteriaceae include pathogens such as Salmonella, Shigella, Yersinia, diarrheogenic E.coli and others. Definitive identification of members of Enterobacteriaceae may require a battery of biochemical tests. Differentiation of the Enterobacteriaceae however is based primarily on the presence or absence of different enzymes coded by the genetic material possessed. These enzymes direct the metabolism of bacteria along one of several pathways that can be detected in vitro. Substrates on which these enzymes can react are incorporated into the culture medium together with an indicator that can detect either utilization of the substrate or the formation of specific metabolic products. Gassner Lactose Agar was originally developed by Gassner for the detection and isolation of pathogenic Enterobacteriaceae from food and other materials (1). This medium has been prescribed in the regulations for the execution of the German Meat Inspection Law (Deutsches Fleischbeschaugesetz) (2). This medium is also known as Water-blue Metachrome-Yellow Lactose Agar. Metachrome-yellow primarily inhibits grampositive microorganisms present in the food materials. Lactose fermenters produce acid, indicated by the water blue indicator, which turns blue in acidic range and colourless in the alkaline range. Original colour of the prepared medium is green, but in the acidic pH it becomes blue-green to blue while in the alkaline conditions the yellow colour of metachrome yellow becomes increasingly apparent. Medium ingredients like meat peptone provide essential nutrients and sodium chloride maintains osmotic balance respectively. Storage and Shelf-life: Store below 30°C in tightly closed container and prepared medium at 2-8°C. Use before expiry period on the label. Reference: 1.Gassner G., 1918, Centralbl. F. Bakt. I. Orig., 80:219-222. 2.Deutsches Fleischbeschaugesetz: Anlage 1 zu § 20 Abs. 4: Vorschriften über die bakteriologische Fleischuntersuchung.