Plaque-forming unit

Plaque-forming unit

A plaque-forming unit (PFU) is a measure used in virology to quantify the number of virus particles capable of forming plaques per unit volume. Plaques are clear zones formed on a layer of host cells, such as bacteria or cultured cells, where the virus has infected and lysed the cells. The PFU is an important metric in virology for determining the concentration of infectious virus particles in a sample.

Methodology

The process of determining the PFU involves several steps:

1. Preparation of Host Cells: A monolayer of host cells is prepared in a petri dish or multi-well plate. These cells are susceptible to infection by the virus being studied.

2. Serial Dilution: The virus sample is serially diluted to ensure that the number of plaques formed is countable. Typically, tenfold dilutions are used.

3. Inoculation: A small volume of each dilution is added to the host cell monolayer and allowed to adsorb for a period, usually around one hour.

4. Overlay: After adsorption, the cells are overlaid with a semi-solid medium, such as agar or agarose, which restricts the spread of the virus to neighboring cells.

5. Incubation: The plates are incubated for a period, often several days, to allow plaques to form.

6. Counting Plaques: After incubation, the plaques are counted. Each plaque corresponds to a single infectious virus particle in the original sample.

Calculation of PFU

The PFU per milliliter is calculated using the formula:

\[ \text{PFU/mL} = \frac{\text{Number of plaques} \times \text{Dilution factor}}{\text{Volume of diluted virus added to the plate (mL)}} \]

This calculation provides an estimate of the concentration of infectious virus particles in the original sample.

Applications

Plaque-forming units are used in various applications, including:

• Vaccine Development: Determining the potency of viral vaccines by measuring the concentration of live virus.
• Antiviral Testing: Evaluating the efficacy of antiviral compounds by observing their ability to reduce plaque formation.
• Virus Purification: Assessing the purity and concentration of virus stocks.

Limitations

While PFU is a widely used measure, it has limitations:

• Sensitivity: The method may not detect low concentrations of virus.
• Plaque Morphology: Some viruses do not form distinct plaques, making counting difficult.
• Host Cell Specificity: The choice of host cells can affect the ability of the virus to form plaques.

See Also

• Virus quantification
• Plaque assay
• Virology