In this article we will discuss about the growth curve of bacteriophage.
One-step growth curve, developed by Max Delbriick and Emory Ellis (1939), marks the starting of modern bacteriophage research.
In this experiment, susceptible bacterium (E. coli) is mixed with bacteriophage (T2) and is allowed for a short period to get attached with the host cells. The culture is then diluted much, thereby any virus particle – if released by lysis of host cell —will not be able to infect new cells. The number of phage particle released from bacteria is subsequently determined at different time intervals by plaque count.
The result is plotted as bacteriophages released from bacteria versus time, indicating several distinct phages.
Growth curve of this type consists of two periods:
1. Latent Period:
This period immediately follows phage addition and thus there is no release of virions. This is the shortest period required for virus reproduction and release.
In the first part of this phage, the bacteria do not contain any complete infective virions. This can be shown by lysing them with chloroform. The initial stage of the latent period is called eclipse period.
It is so named because the virions are not detectable even by disrupting the bacteria by treating with chloroform. At the end of the eclipse period, the phage virions increase in number within the host bacterium and the host bacterium is prepared for lysis.
2. Rise Period:
After the latent period, the host cells rapidly lyse and release all infective phages, called rise period or burst. At the end, no more viruses are liberated. The term burst size is the number of viruses released from a single infected bacterium. The total number bacteriophages released can be used to calculate the burst size.
The experiment is carried out in enriched medium at 37°C and growth plateau is reached in approximately 30 minutes. Thus a plot of plaque forming unit (PFU) per ml against time in minutes produces a sigmoid curve (Fig. 2.49).
