A microbiologist observes colonies on Hektoen enteric agar (HE)that appear green.Which of the following organisms should the microbiologist suspect?

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HE contains bile salts,two indicators,bromthymol blue,acid fuchsin,and lactose.Gram-positive organisms are inhibited because of the addition of bile salts and dyes.Most enteric nonpathogens will ferment lactose and appear yellow.Therefore this agar is selective and differential for the growth of enteric pathogens,Salmonella and Shigella spp.,because they do not ferment lactose.There are additional components that aid in the detection of Salmonella spp.They are sodium thiosulfate and ferric ammonium citrate; when an organism that produces hydrogen sulfide is present,the organism will form a black precipitate.Therefore,colonies of Salmonella (hydrogen sulfide producer)will appear green or transparent with black centers.Shigella spp.do not produce hydrogen sulfide and will appear green (the color of the media)or transparent.

A Microbiologist Observes Colonies on Hektoen Enteric Agar (HE)that Appear