In: Biology
Plaques is a clear area formed on agar plate due to lysis or destruction of host bacteria cells.
Plaques are characterstics of lytic life cycle, because cells are destroyed in this life cycle and virus are released. While lysogenic life cycle moves with reproduction proliferating inside the cells. Progeny viruses are free and infect other cells. The plaque is formed because cells are destroyed.
2. Pfu or plaque forming unit measures number of infectious viral particles. Since measurement is done by counting number of viral particles capable of forming plaques. So the number of plaques formed by one ml would be pfu per ml. We are not counting virus cells individually hence can not call it phage per ml.