In Vitro Effects of Fluorescent Brightener on the Efficacy of Occlusion Body Dissolution and Polyhedral-Derived Virions

The fluorescent brightener calcofluor white M2R (M2R) increased the efficacy of a nuclear polyhedrosis virus of the gypsy moth Lymantria dispar. This increase was seen with both larval-produced and cell culture-produced occlusion bodies (OBs), but only when the OBs were continuously exposed to the M2R. No enhancement was observed when OBs were treated for an hour and then rinsed to remove M2R. M2R enhanced the activity of virions that were released from OBs by alkaline treatment and stabilized the released virions for at least 5 days. When M2R was removed from the virions after OB dissolution followed by dilution in water or dialysis, no infectivity was detected. Some activity was regained when M2R was added back to these virions. Finally, M2R also enhanced per os infectivity of nonoccludded virions. Our investigations suggest that enhancement of M2R only occurs when both brightener and virus are present concurrently in the insect′s midgut.