DIFFERENCES IN MAXIMUM AND MINIMUM PLAQUE-FORMING TEMPERATURES AMONG SELECTED GROUP A ARBOVIRUSES,

The parent strain of Venezuelan quine encephalitis VEE and of Eastern equine encephalitis EEE viruses show distinct plaque fronts at maximum temperatures t sub f-max of 46 C. Two attenuated variants of VEE T and A show a t sub f-max of 43 C, and 45 C, respectively. All four viruses show indistinct plaque fronts at a minimum temperature t sub f-min of 24.5 C as much as one per cent of the inoculum formed plaques that were distributed between the t sub f-min and 20 C. From VEE-inoculated plates, the progeny derived from one of 31 plaques selected below the t sub f-min was avirulent and resembled T. T had a selective advantage over VEE in passage experiments in liquid cultures at low temperatures. Virulent mutants of T, resembling VEE, were isolated from plaques beyond the t sub f-max of T only after overcoming a reversible interference effect in which T interfered with plaque formation of VEE above the t sub f-max. The possible relation of t sub f-max and t sub f-min to virulence among these viruses is discussed. Author