Department of Microbiology, University of Hong Kong, Pokfulam, Hong Kong, China. jamescai@hkusua.hku.hk|Department of Microbiology, University of Hong Kong, Pokfulam, Hong Kong, China. jamescai@hkusua.hku.hk|Department of Microbiology, University of Hong Kong, Pokfulam, Hong Kong, China. jamescai@hkusua.hku.hk|Department of Microbiology, University of Hong Kong, Pokfulam, Hong Kong, China. jamescai@hkusua.hku.hk