A green colony with a black center is isolated on HE agar from a stool specimen. It is most likely which organism?
a. E. coli
b. Salmonella spp.
c. Shigella spp.
d. K. pneumoniae
B
The black center indicates H2S production; therefore, Salmonella spp. is the only H2S producer in the group. A green colony represents a lactose nonfermenter. E. coli and K. pneumonia are both lactose-positive and would appear yellow on HE. Shigella are lactose nonfermenters and H2S-negative.
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