1. Key Laboratory of Biomedical Polymers of Ministry of Education, Department of Chemistry, Wuhan University, Wuhan, 430072, P. R. China, State Key Laboratory of Chemical Resource Engineering and College of Material Science and Engineering, Beijing University of Chemical Technology, Beijing, 100029, P. R. China, and Department of Microbiology, Peking University Health Science Center, Beijing, 100083, P. R. China