Principles of Virology, Volume 1. Jane Flint

2.9 Transformation assay. Chicken cells transformed by two different strains of Rous sarcoma virus are shown. Loss of contact inhibition causes cells to pile up rather than grow as a monolayer. One focus is seen in panel A and three foci are seen in panel B at the same magnification. Courtesy of H. Hanafusa, Osaka Bioscience Institute.

BOX 2.6

      METHODS

       End-point dilution assays

      End-point dilution assays are usually carried out in multiwell plastic plates (see the figure above). In the example shown in the adjacent table above, 10 monolayer cell cultures were infected with each virus dilution. After the incubation period, plates that displayed cytopathic effect were scored +. At high dilutions, none of the cell cultures are infected because no infectious particles are delivered to the cells; at low dilutions, every culture is infected. The end point is the dilution of virus that affects 50% of the test units. This number can be calculated from the data and expressed as 50% infectious dose (ID₅₀) per milliliter. Fifty percent of the cell cultures displayed cytopathic effect at the 10^–5 dilution, and therefore, the virus stock contains 10⁵ TCID₅₀ (tissue culture infectious dose) units.

Virus dilution	Cytopathic effect
10–2	+	+	+	+	+	+	+	+	+	+
10–3	+	+	+	+	+	+	+	+	+	+
10–4	+	+	–	+	+	+	+	+	+	+
10–5	–	+	+	–	+	–	–	+	–	+
10–6	–	–	–	–	–	–	+	–	–	–
10–7	–	–	–	–	–	–	–	–	–	–

      In most cases, the 50% end point does not fall on a dilution tested as shown in the example; for this reason, various statistical procedures have been developed to calculate the end point of the titration. In one popular method, the dilution containing the ID₅₀ is identified by interpolation between the dilutions on either side of this value. The assumption is made that the location of the 50% end point varies linearly with the log of the dilution. Because the number of test units used at each dilution is usually small, the accuracy of this method is relatively low. For example, if six test units are used at each 10-fold dilution, differences in virus titer of only 50-fold or more can be detected reliably. The method is illustrated in the second example below, in which the lethality of poliovirus in mice is the end point. Eight mice were inoculated per dilution. In the method of Reed and Muench, the results are pooled, as shown in the table below, which equalizes chance variations (another way to achieve the same result would be to utilize greater numbers of animals at each dilution). The interpolated value of the 50% end point, which in this case falls between the 5th and 6th dilutions, is calculated to be 10^–6.5. The virus sample therefore contains 10^6.5 LD₅₀ (50% lethal dose). The LD₅₀ may also be calculated as the concentration of the stock virus in PFU per milliliter (1 × 10⁹) times the 50% end-point titer. In the example shown, the LD₅₀ is 3 × 10² PFU.

       Reed LJ, Muench H. 1938. A simple method of estimating fifty percent endpoints. Am J Hyg 27:493–497.

Dilution	Alive	Dead	Total alive	Total dead	Mortality ratio	Mortality (%)
10–2	0	8	0	40	0/40	100
10–3	0	8	0	32	0/32	100
10–4	1	7	1	24	1/25	96
10–5	0	8	1	17	1/18	94
10–6	2	6	3	9	3/12	75
10–7	5	3	8	3	8/11	27

      Although