Dose units for pathogens
It is often difficult to measure the precise dose of a pathogen. Individual pathogens may clump together, and clumps are indistinguishable from single pathogens to many assays. If clumping is uncommon (or if the investigator has ignored this possibility), the unit may be given as 'pathogens', 'virions', 'cells', 'spores', etc.
Commonly encountered units for measuring pathogen doses are listed below.
Colony-forming units (CFU)
Obtained by counting discrete (usually bacterial) colonies on solid media.
Focus-forming units (FFU)
Similar to plaque-forming units (PFU). Areas (foci) of cytopathic effect that indicate replication of the virus on a lawn of cells may be counted, instead of actual plaques.
Plaque-forming units (PFU)
Obtained by counting discrete plaques (clear circular areas) on a lawn of cells, which are formed by the pathogen infecting and lysing the cell. The size of the plaque increases with time as the pathogen consumes more cells.
Tissue culture infectious dose 50 (TCID50)
The approximate dose at which 50% of tissue culture specimens show a cytopathic effect indicating replication of the virus being quantified. This is similar to the 'most probable number' (MPN) method of quantifying bacteria.