A 21 year old male has diarrhea for 4 days and enters the ER complaining of bloo

Question

A 21 year old male has diarrhea for 4 days and enters the ER complaining of bloody stools. A stool culture is collected. The blood agar plate grows pungent smelling, large, gray, non-hemolytic colonies after 24 hours. The MacConkey plate grew small colorless colonies. The Hectoen Enteric (HE) plate grew blue colonies with black spots. The Campy plate grew nothing. 1. The fact that this organisms grows on MacConkey and HE tells us what? 2. The fact that the organisms did not grown on Campy tells us what? 3. Colorless colonies on MacConkey and Blue colonies on HE tells us what about this organism? 4. What do the black spots growing on HE tell us about the organism? 5. Give one possible organism that this could be. (Just name a genus at this point.)

Explanation / Answer

1.Macconkey agar is selective for Gram negative organisms and helps to differentiate lactose fermenting gram negative rods from non-lactose fermenting gram negative rods. It is primarily used for detection and isolation of members of family Enterobacteriaceae and Pseudomonas spp and Hektoen Enteric Agar (HE) is a selective and differential medium designed to isolate and differentiate members of the species Salmonella and Shigella from other Enterobacteriaceae.

2. Organisms do not grow on campy plate indicates absences of Campylobacter species from stool specimens.

3.Colorless colonies on MacConkey indicates Non-lactose fermenting gram negative bacteria and Blue colonies on HE indicates, Salmonella colonies.

4.Black spots growing on HE shows salmonella producing Hydrogen sulphide gas in plate.

5. The possible organism is Salmonella.

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