Hektoen Enteric Agar, Isolation Techniques and Selective Media, A Photographic Atlas for the Microbiology Laboratory,
Purpose
Hektoen Enteric (HE) Agar is used to isolate and differentiate Salmonella and Shigella species from other Gram-negative enteric organisms.
Principle
Hektoen Enteric Agar is an undefined medium designed to isolate Salmonella and Shigella species from other enterics based on the ability to ferment lactose, sucrose, or salicin, and to reduce sulfur to
hydrogen sulfide gas (H2S). In addition to the three sugars, sodium thiosulfate is included as a source of sulfur. Ferric ammonium citrate is added to react with H2S and form a black precipitate. Bile salts are included to inhibit most Gram-positive cocci. Bromthymol blue and acid fuchsin dyes are added as color indicators. Differentiation is possible as a result of the various colors produced in the colonies and in the agar. Enterics that produce acid from fermentation will produce yellow to salmon-pink colonies. Organisms like Salmonella, Shigella, and Proteus that do not ferment any of the sugars produce blue-green colonies. Proteus and Salmonella species that reduce sulfur to H2S form colonies containing a black precipitate. Refer to Figures 2-17 and 2-18.
2-17 HEKTOEN ENTERIC AGAR HE Agar inoculated
with (clockwise from top), Escherichia coli, Proteus
mirabilis, Shigella flexneri, and Enterococcus faecalis.
E. coli produces yellow color because acid is an endproduct
of its fermentation of lactose. P. mirabilis does
not ferment lactose but does produce a black precipitate
from the reaction between ferric ammonium citrate in
the medium and H2S from sulfur reduction. Shigella is
also a lactose nonfermenter and is blue-green; it is not
a sulfur reducer. E. faecalis is inhibited by the bile salts
2-18 HEKTOEN ENTERIC AGAR STREAKED
FOR ISOLATION HE agar streaked with Salmonella
typhimurium and Escherichia coli. Note the black
Salmonella colonies due to sulfur reduction and
the yellow E. coli colonies due to lactose fermentation with acid end-products.
Suggested Reading
- Michael J. Leboffe & Burton E. Pierce. A Photographic Atlas for the Microbiology Laboratory 4th edition 2011
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