PLAQUE ASSAY PROCEDURE FOR COLORADO TICK FEVER VIRUS.

Deig, E. Frank (University of California, Berkeley), and H. M. S. Watkins. Plaque assay procedure for Colorado tick fever virus. J. Bacteriol. 88:42-47. 1964.-A reproducible plaque assay procedure is described for the quantitation of Colorado tick fever virus in a cell line established from embryonic hamster tissue. Under the best conditions, plaques approximately 4 mm in diameter were formed after incubation at 37 C of 4 to 6 days. Several environmental variables in the procedure were studied. Efficiency was increased markedly by combining the virus during adsorption with serum proteins, and by carrying out this step at 25 C rather than at 37 C. The overlay medium used contained metabolites which promoted cell viability for periods greater than 1 week, and allowed plaques to develop. Plaque formation was relatively insensitive to a variation in pH between 7.1 and 8.1 (with optimal concentrations of bicarbonate). However, plaque development was inhibited with medium containing greater than 0.22% bicarbonate (at optimal pH), or when the initial pH was less than 7.0.