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': 'Which is a gram negative anaerobic bacteria: September 2007 \nSelect and explain. \nA. Streptococci, B. Bacteroides, C. Actinomyces, D. Clostridium welchii', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Ans. B: Bacteroides', 'role': 'assistant'}]