A plaque is a (small) clearing in a lawn of bacteria grown on a plate. This plaque usually indicates the presence of a virus that infects the bacteria and uses it to reproduce (such viruses are called phages or bacteriophages). Although some may look similar, each phage forms a unique plaque. The way a plaque look is called its morphology.
Plaque morphologies tell us a lot about the type of phages that made them. For example if the plaques the phage makes are very clear (one could see clearly through them) the phage is highly lytic, whereas if the plaques are hazy or turbid (one could not see clearly when looking through them) the phage is probably lysogenic. In some cases plaques have clear centers and hazy rings, or halos.
No one is absolutely certain what causes these halos but one hypothesis is that the halos are rings of bacteria that are resistant to the phage. Larger plaques often mean more phage and smaller plaques, less phage.
Looking at plaque morphologies is a preliminary way (before electron microscopy) to tell if one has more than one phage on a plate. It is still unknown weather a phage’s plaque morphology is at all related to the phage’s morphology.
