This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Microbiology
Stalactite growth in ghee broth is due to the following organism? 
Select and explain. 
A. Y.pestis, B. T.palladium, C. H.influenzae, D. C.diphtheriae
A: Y.pestis
Answer: A: Y.pestis 
Explanation: Cultural characteristics of Yersinia pestis: 1. When grown in a flask of broth with oil / ghee floated on top (ghee broth) a characteristic growth occurs which hangs down into the broth from the surface, resembling stalactite. 2. On nutrient agar, colonies are small, delicate, transparent discs, becoming opaque on continued incubation. 3. On blood agar colonies are dark brown due to absorption of the hemin pigment. 4. On MacConkey agar colourless colonies are formed. 5. In broth a flocculent growth occurs at the bottom and along the sides of the tube. Ref: Textbook of Microbiology Ananthanarayanan, 8th edition