UA-38165594-3
  • The one-step growth curve, developed by Max Delbriick and Emory Ellis (1939) using the Escherichia coli-T4 bacteriophage system, marks the start of modern bacteriophage research.
  • Viral replication is often analyzed by growth curves, in which viral multiplication in the presence of host cells is measured as a function of time.
  • The virus growth curve is a representation of the overall change, with time, in the amount of infectious virus in a single cell that has been infected by a single virus particle
  • In one step growth experiment, only a single or one cycle of virus growth is observed, that’s why it’s termed as One-Step Growth Curve.
  • Comparing growth curves is one of the most sensitive ways of comparing viral growth under different conditions or for comparing the replication of different viral mutants.

 

Phases of One-Step Growth Curve

The One Step Growth Curve consists of three periods known  as Latent Period, Rise Period and the plateau period

1. Latent Period

  • The latent period is described as the time period that follows immediately from the infection of the viruses  into the host cell until prior to the release of infection particles or the appearance of extracellular phages or viruses (cell lysis).
  • In the latent period, attachment, entry, replication, transcription, translation and assembly of progeny phages or viruses occur.
  • During this period there is no release of new phage or virus particles therefore plaque/virus count remains constant
  • The latent period can be divided into two phases
    a. Eclipse: This period is the initial portion of the latent period that follow immediately after the penetration of viral particles into the host cell. Eclipse is characterized by the incapacity to detect free virions since viruses are actively transcribing and replicating inside the host. The eclipse usually lasts from minutes (bacteriophages) to hours or days (animal or plant viruses).
    b. Intracellular accumulation: During the eclipse period all structural proteins and viral genomes have been produced and massively accumulated in the cytoplasm of the host cell. Both components self-assembly to form new viral particles that accumulate intracytoplasmatically

2. Exponential growth or Burst or Rise period

  • The Rise Period or Burst Period begins after the latent period when the host cells rapidly lyse and release all infective phages.
  • In this period the new viral particles are released from the infected cells and therefore, plaque count increases rapidly or exponentially over a period of time.
  • This period is slightly extended due to the asynchrony of infection.
  • At the end, no more viruses are liberated.
  • The number of bacteriophages released from the infected cells can be used to calculate the burst size.

3. Plateau period

  • This period represents the end of all infected host cell lysis.
  • The newly liberated phage or virus particles fail to meet uninfected host cells due to high dilution.
  • Therefore during this phase, the plaque count remains constant.
  • Burst Size: Average yield of infectious virus per cell is called burst size.
    Burst Size = pfu/ml at plateau/ pfu/ml at latent period
    There is much variation in burst size between different kinds of cells (range between 20-3000 pfu/ml)
    Burst size for T4 = 100
  • Maximum yield/cell = characteristic of each virus-cell system
  • Reflects balance between

(a) the rate at which virus components continue to be synthesized and assembled into virions, and

(b) the  rate at which cells lose synthetic capacity & structural integrity ~ 8-72+ hours, yielding 100-10,000 virions/infected cell