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Method, TCID50 and pfu

The virus suspension is diluted out in ten-fold dilutions steps (log10) because a suspension may contain 108 infectious virions per 0.1ml. Remember to change
your pipette at each step to avoid any carry-over of virus! Replicates of each dilution are inoculated into cells in microwells or hens eggs. The infectivity is the dilution at which 50% of the cultures have become infected within a week. This is called the tissue culture infectious dose 50 (TCID50 per 0.1ml.) similar to the LD50 in pharmacology.

In a plaque assay virus growth is confined to the area of infection by a semi-solid overlay eg agar in the medium. If there were 50 plaques at a dilution of 1:10,000 using 0.1ml of inoculum the number of plaque forming units, (pfu) is 5 x 105 plaques per 0.1ml of inoculum and the virus could be diluted 1/5000 for a virus neutralisation test..