Plaques are clear zones formed in a lawn of cells due to lysis by phage.
The morphology of the plaque depends upon the phage, the host, and the growth conditions. The size of the plaque is proportional to the efficiency of adsorption, the length of the latent period, and the burst size of the phage. A clear plaque will be formed if the host is completely susceptible to the phage. If a phage lysogenized a host cell immediately upon infection, it would never form a plaque. This characteristic plaque morphology is due to the role of the MOI and cell physiology on the lysis-lysogeny decision.