320,000,000
PFU/mL
320,000,000
PFU/mL
The Phage Titer Calculator determines the concentration of bacteriophages (bacterial viruses) in a sample using plaque assay results. The plaque assay is the gold standard for quantifying infectious phage particles. Each plaque (clear zone in a bacterial lawn) represents a single phage particle that infected and lysed surrounding bacteria.
Phage titer is expressed as plaque-forming units per milliliter (PFU/mL) and is used in phage biology research, phage therapy development, and molecular biology applications involving phage vectors.
The phage titer formula is:
PFU/mL = Number of Plaques / (Dilution Factor × Volume Plated in mL)
Count plates with 20-200 plaques for statistical accuracy. Too few plaques give high sampling error; too many cause overlapping plaques.
Inputs
Results
32 plaques at 10⁻⁶ dilution with 0.1 mL plated gives 3.2 × 10⁸ PFU/mL, a typical titer for a well-grown phage lysate.
Inputs
Results
5 plaques at 10⁻² dilution gives 1000 PFU/mL. Low plaque counts have higher uncertainty; consider concentrating the sample.
PFU stands for plaque-forming unit, which represents a single infectious phage particle capable of infecting a bacterial cell and producing a visible plaque. One PFU corresponds to one plaque on the plate. PFU/mL is the standard unit for phage concentration, analogous to CFU/mL for bacteria.
Prepare serial 10-fold dilutions (10⁻¹ to 10⁻¹⁰) and plate several. Choose the plate with 20-200 countable plaques for your titer calculation. If you expect a titer around 10⁸ PFU/mL, focus on the 10⁻⁵ to 10⁻⁷ dilutions. Always plate at least two dilutions in duplicate for reliability.
Low titers can result from degradation during storage (phages are sensitive to temperature, UV, and pH extremes), using bacteria in the wrong growth phase (log phase is optimal), poor adsorption conditions (wrong buffer or temperature), or interference from chemicals in the sample. Ensure proper storage at 4°C with chloroform (if phage is chloroform-resistant) and use fresh host bacteria.
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