In situ investigations of single living cells infected by viruses

In this paper we report the direct observation of biological processes on living cells. The experiments were done in situ by scanning force microscopy on a scale inaccessible by other techniques under physiological conditions. Living monkey-kidney cultured cells were imaged under normal growth conditions and showed reproducible features on the 10 nm scale. Upon adding a suspension of pox viruses, characteristic changes of the cell membrane were repeatedly observed in different experiments on different cells. Almost immediately, a pronounced softening of the cell surface occurred which lasted only for a few minutes. More than two hours later very significant protrusions appeared to grow out of the cell membrane. These protrusions abruptly disappeared again. These events were only observed after infection and we interpret them as the exocytosis of proteins related to viral reproduction. After almost 20 h, a different type of event occurred which we interpret as the exocytosis of the progeny viruses themselves.