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
[{'content': "All 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: Ans. is 'a' i.e., Treponema pallidum", 'role': 'assistant'}]