Isolation Techniques and Selective Media, A Photographic Atlas for the Microbiology Laboratory, Eosin Methylene Blue Agar
Purpose
Eosin Methylene Blue (EMB) Agar is used for isolation of fecal coliforms. EMB Agar can be streaked for isolation or used in the Membrane Filter Technique as discussed on page 228.
Principle
Eosin Methylene Blue agar contains peptone, lactose, sucrose, and the dyes eosin Y and methylene blue. The sugars provide fermentable substrates to encourage growth of fecal coliforms. The dyes inhibit growth of Gram-positive organisms and, under acidic conditions, also produce a dark purple complex usually accompanied by a green metallic sheen.
This green metallic sheen serves as an indicator of the vigorous lactose and/or sucrose fermentation typical of fecal coliforms. Smaller amounts of acid production (typical of Enterobacter aerogenes and slow lactose fermenters) result in a pink coloration of the growth. Nonfermenters remain their normal color or take on the coloration of the medium (Figures 2-15 and 2-16).
2-15 EOSIN METHYLENE BLUE AGAR EMB agar
inoculated with (clockwise from top) Escherichia coli,
Enterobacter aerogenes, Salmonella typhimurium, and
Enterococcus faecalis. Note the characteristic green
metallic sheen of E. coli and the pink coloration with
slight darkening of E. aerogenes. Both organisms are
coliforms; the difference in color is due to the degree
of acid production. Some E. coli strains appear black
without the green sheen or produce colonies with dark
centers. See Figure 2-16.
2-16 EOSIN METHYLENE BLUE AGAR STREAKED
FOR ISOLATION EMB agar inoculated with Escherichia
coli and Salmonella typhimurium. Note the dark centers
in the E. coli colonies. S. typhimurium is a noncoliform
and remains its natural color.
Suggested Reading
- Michael J. Leboffe & Burton E. Pierce. A Photographic Atlas for the Microbiology Laboratory 4th edition 2011
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