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
Organisms that has not been cultured successfully so far is- 
Select and explain. 
A. Leptospira, B. Treponema pallidum, C. Bordetella, D. Staphylococcus
B: Treponema pallidum
Answer: B: Treponema pallidum 
Explanation: None