This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: general microbiology, Subject name: Microbiology
[{'content': "All of the following pathogenic bacteria fulfill Koch's postulates, except? \nSelect and explain. \nA. Treponema pallidum, B. Yersinia pestis, C. Bacillus anthracis, D. Helicobacter pylori", 'role': 'user'}, {'content': "Answer: A \nExplanation: According to Koch's postulate: it must be possible to isolate a bacterium from the site of lesion and grow it in a cell-free culture medium. Pathogenic Treponemes do not grow in a cell-free aificial culture medium. Ref: Textbook of Microbiology; Ananthanarayan and Paniker; 9th edition; Pg no: 370", 'role': 'assistant'}]