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Berg, Gerald (Robert A. Taft Sanitary Engineering Center, Cincinnati, Ohio), Eugene K. Harris, Shih L. Chang, and Kenneth A. Busch . Quantitation of viruses by the plaque technique. J. Bacteriol. 85: 691–700. 1963.—This paper presents the results of a study on overcrowding as it occurred with several strains of enteroviruses on monkey kidney cell layers. Each cell sheet area was 4,500 mm 2 . Dispersion analysis of all plaque counts in the study revealed that the percentage error, in the absence of overcrowding, fluctuated within a range of 7 to 36% about a mean value of about 17%, which is close to what is expected from purely statistical variation in sampling. Overcrowding with Mahoney virus, a mixture of particles producing either rapidly or slowly expanding plaques, resulted primarily from the obscuring effect produced as rapidly expanding plaques obliterated infected foci produced by virus particles responsible for slow-forming plaques. Overcrowding occurred at plaque levels in excess of 35 to 40 per cell sheet. Overcrowding with Mahoney LP virus, a strain derived from a rapidly expanding Mahoney plaque which produced uniformly expanding plaques, did not occur until counts in excess of 60 to 70 per cell sheet were reached. Plaque obscuring was again responsible. Apparent overcrowding with Coxsackie A9 virus, when plaques were not permanently marked from the first day of counting, resulted from coalescence at levels above about 40 to 50 plaques per cell sheet. When plaques were permanently marked, overcrowding resulted from plaque obscuring which did not occur until levels in excess of about 120 per cell sheet were reached.

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