This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Bacteriology, Subject name: Microbiology
[{'content': 'This test is positive in \nSelect and explain. \nA. Streptococcus pyogenes, B. Streptococcus pneumoniae, C. Streptococcus mitis, D. Streptococcus agalactiae', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Test is quellung reaction quellung reaction: a small amount of growth is mixed with saline, antisera against the capsule polysaccharide, and methylene blue stain. After incubation at room temperature for 1 hour, the reaction is observed under the microscope.The organisms are outlined in light blue. A positive reaction shows clumping because of cross-linking of the antibodies and pneumococci. The halo effect around the pneumococci is apparent capsular swelling.Jawetz 7e pg 224', 'role': 'assistant'}]