An interesting mode of infection and transmission of viruses was found in bacterial system. Normally a bacterium infected with bacteriophage would lyse after some time and release a progeny of bacteriophages. In many experiments it was seen that the bacterium does not lyse and happily undergoes division. Analysis of such bacteria showed that the phage DNA becomes incorporated in the bacterial chromosome. In this case the phage DNA is replicated along with the bacterial DNA, and is passed on to the daughter bacterial cells in the course of the cell division. The phage DNA retains its integrity and remains latent throughout many bacterial generations. Then, for some unknown reason, the phage DNA would detach from the bacterial chromosome and act as a lytic phage. The point of attachment for the lysogenic phage DNA to the bacterial chromosome is highly specific. It attaches in such a way that only one point of the phage DNA is attached and the rest forms two DNA arms on either side of the point of attachment. There are many questions that remain unanswered, such as what determines whether a phage will act in a lytic or lysogenic manner?
The phage infection in bacterial cultures can be demonstrated by simple experiments. The colonies of host bacterium may be allowed to grow on semi-solid agar-nutrient medium. A dilute suspension of phage particles is spread over the bacterial culture. After some time hole or plaques will appear on the bacterial colonies, indicating the process of lysis.