Characterization of a new infectious bronchitis virus isolate. III. Cell-culture adaptation of Clark 333

The Clark 333 strain of infectious bronchitis virus (IBV) was substantially resistant to primary chicken cell-culture adaptation. More than 40 serial embryo passages were required before the virus would produce cytopathic alterations upon cell-culture inoculation. The cytopathic effect was characteristic of the effect of reported for IBV. Adaptation was not accomplished by alternating serial passages in embryo and cell-cultured systems. A careful monitoring of cell-culture fluid infectivity by embryo inoculation was found to be essential because absence of adaptation is accompanied by a loss in virus titer (3 to 5 serial passages). Helpful additional aids were immunofluorescence and cytopathology.