Question

If you exposed an unknown bacteria to four different bacteriophages. Susceptibility of the bacteria will be determined by observing for the production of plaques. Describe how these plaques are formed?

Answer 1

1. Most phage infections of bacteria after certain incubation time can result in formation of plaques of certain size and definitive boundary. The boundary may be fuzzy or clear cut edge.

Each plaque is initiated by the infection of a single virus particle. The circular plaque morphology is typical of most phages. It is the result of cycles of infection of the host cells by the numerous viral progeny. These progeny phages disseminate in all directions from the original focus of infection. Phage morphology is consistent in standard condition. Hence, if there is change in morphology, then the strain may have a mutation.

Phages are clear zones seen in the soft agar containing bacteria. When a single bacterium is infected by a single phage, the virus will enter the cell. It will integrate its nucleic acid in host chromosome and will utilize the host cell machinery for replication and synthesis of viral proteins. The viral proteins will assembly along with the nucleic acid and for virions. The number of virions released depends on the phage. Upon lysis of the host cell, the phage particles are released and diffuse in the surrounding soft agar. Each of these phages will infect other bacterium and start the whole process again. The release of more phages will result in killing of more bacteria. As a result, the lawn of bacteria surrounding the area where the first phage had infected will clear and form a plaque. Thus 1 plaque= 1 phage.

Morphology of the plaque depends on bacterial host, the phage and growth conditions. The size of the plaque is proportional to the adsorption efficiency, latent period length, and the burst size of the phage. If the plaque formed is turbid, then some of the forms have initiated a lysogenic cycle where the virus nucleic acid is integrate in host chromosome. Hence, the phage will not lyse the host cells.

Most phages form clear plaques. These are lytic phages. Some phages that exhibit both lysogeny and lytic cycles will form turbid plaques. Bulls eye morphology is the morphology where in there is greater turbidity at peripheries. These are caused by the lysis inhibition phenomenon seen in T even phages or by decrease in lytic cycle due to aging. Some phages show plaques that have semi-transparent haloes due to diffusion of soluble phage products that destroy cell envelopes. Phages with larger heads form smaller plaques while small-headed phages produce larger plaques.