Selective Lysine Agar

Principle And Interpretation

Selective Lysine Agar is recommended by AOAC (1), АРНА (2) and FDA (3) for the isolation and identification of Salmonellae in foods. As per AOAC (The hydrophobic grid membrane filter method), membrane filters are used containing hydrophobic lines, printed in a grid pattern, that serve as barrier to the spread of colonies from one area of the filter to another.

Peptic digest of animal tissue, yeast extract provide nitrogenous compounds, sulphur, vitamin B complex and other essential growth nutrients. Dextrose serves as an energy source. Bromo cresol purple is the pH indicator that changes from purple to yellow at acidic pH. Organisms such as Salmonella, which are able to decarboxylate lysine reverse this acid reaction and form bluegreen, blue or purple colonies. Bile salts mixture and Sulphapyridine inhibit gram-positive organisms. Food sample is processed and suspended in a general enrichment broth and incubated for 18 to 24 hours at 35-37°C and then subjected to selective enrichment procedures in broth media. Following a 6-8 hours incubation period at 35-37°C, aliquots of the broth are filtered through a selective hydrophobic grid membrane filter. The filter is then placed on the surface of a plate of Selective Lysine Agar, which has been pre-dried to eliminate excess surface moisture. The trapping of air bubbles between the filter and agar surfaces must be avoided. A second filter is similarly placed onto a plate of Hektoen Enteric Agar (M467). The plates of Selective Lysine Agar are incubated for 24 ± 2 hours at 43°±0.5°C.