CASE HISTORY Shane,a 21-Year-Old College Student,was Spending Spring Break at the Beach

CASE HISTORY
Shane,a 21-year-old college student,was spending spring break at the beach in Cancun,where he frequently ordered drinks from the beachfront stands.Upon his return to classes,Shane fell ill with a fever of 38.9°C (102°F) ,severe abdominal cramps,and watery diarrhea.
By the second day,Shane had blood in his stools,a condition known as dysentery.Upon admission to the hospital,Shane showed dehydration,and his rectal exam was very painful with bleeding.
A fecal culture was performed on Hektoen agar,a selective medium for Gram-negative bacteria;bile salts exclude Gram-positives.Hektoen agar also differentiates among the enteric Gram-negative pathogens Salmonella,Shigella,and Escherichia.Most Salmonella species show black colonies due to H₂S formation from thiosulfate,whereas Escherichia species form colonies that are orange from lactose fermentation.Shane's culture produced translucent colonies,indicating Shigella species,which neither ferment lactose nor reduce thiosulfate to H₂S.
Serotyping (determining which antibodies react with the pathogen) confirmed the species Shigella flexneri,a common cause of "traveler's diarrhea" from drinking contaminated water.Shane received intravenous rehydration and was given the antibiotic quinolone,chosen based on laboratory tests of the pathogen's antibiotic sensitivity.Afterward,Shane made a full recovery.
On Hektoen agar,Salmonella colonies appear black,whereas Escherichia species form colonies that are orange.This helps microbiologists identify the organisms,and is actually representative of

A) differences in the enzymes produced by different species, which normally allow them to utilize nutrients in their environments.
B) differences in the color of the cells themselves.
C) how quickly the organisms are able to grow on this medium and how quickly they take up dyes.
D) the virulence or pathogenicity of the organisms (whether they can infect humans) .