This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Systemic Bacteriology (Gram Positive Bacilli, Gram Negative Bacilli), Subject name: Microbiology
[{'content': 'The first pathogenic bacterium to be observed under microscope was- \nSelect and explain. \nA. Vibrio cholera, B. Bacillus anthracis, C. Staphylococcus aureus, D. Streptococcus pyogenes', 'role': 'user'}, {'content': "Answer: B \nExplanation: The first pathogenic bacterium to be observed under microscope was Bacillus anthracis. Bacillus anthracis: First pathogenic bacterium to be observed under microscope. First bacterium isolated in pure culture. First communicable disease shown to transmit by inoculation of infected blood. First used for preparation of attenuated vaccine- Pasteur's vaccine-nontoxigenic strain. Recently used for biological warfare as spores enclosed in envelopes.", 'role': 'assistant'}]