Isolation Techniques and Selective Media, A Photographic Atlas for the Microbiology Laboratory, Desoxycholate Agar
Purpose
Desoxycholate (DOC) Agar is used for isolation and differentiation among the Enterobacteriaceae. It is also used to detect coliform bacteria in dairy products.
Principle
Desoxycholate Agar is a selective and differential medium containing lactose, sodium desoxycholate, and neutral red dye. Lactose is a fermentable carbohydrate, desoxycholate isa Gram-positive inhibitor, and neutral red, which is colorless above pH 6.8 and red below, is added as a pH indicator. Neutral red will reveal lactose fermentation (Figure 2-10) by turning the bacterial growth red where acid products have lowered the pH (Figures 2-11 and 2-12). Lactose nonfermenters will remain their natural color or the color of the medium. Thus, the characteristics to look for on DOC are the quality of growth and color production.
2-10 LACTOSE FERMENTATION WITH ACID END PRODUCTS
2-11 DESOXYCHOLATE AGAR DOC medium inoculated
with (clockwise from top): Escherichia coli, Enterobacter
aerogenes, Shigella flexneri, and Enterococcus faecalis. Note
the inhibition of the Gram-positive E. faecalis. Note also the
red coloring of the lactose fermenting coliforms E. coli and E.
aerogenes.
2-12 DESOXYCHOLATE AGAR STREAKED FOR ISOLATION
DOC medium inoculated with the coliform Escherichia coli
(pink) and Shigella flexneri (buff). The color difference is because
Shigella is a noncoliform and doesn’t ferment lactose.
Suggested Reading
- Michael J. Leboffe & Burton E. Pierce. A Photographic Atlas for the Microbiology Laboratory 4th edition 2011
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