  Ever since Hershey and Chase used phages to establish DNA as the carrier of
genetic information in 1952, the precise mechanisms of phage DNA translocation
have been a mystery. While bulk measurements have set a time scale for in vivo
DNA translocation during bacteriophage infection, measurements of DNA ejection
by single bacteriophages have only been made in vitro. Here, we present direct
visualization of single bacteriophages infecting individual Escherichia coli
cells. For bacteriophage lambda, we establish a mean ejection time of roughly 5
minutes with significant cell-to-cell variability, including pausing events. In
contrast, corresponding in vitro single-molecule ejections take only 10 seconds
to reach completion and do not exhibit significant variability. Our data reveal
that the velocity of ejection for two different genome lengths collapses onto a
single curve. This suggests that in vivo ejections are controlled by the amount
of DNA ejected, in contrast with in vitro DNA ejections, which are governed by
the amount of DNA left inside the capsid. This analysis provides evidence
against a purely intrastrand repulsion based mechanism, and suggests that
cell-internal processes dominate. This provides a picture of the early stages
of phage infection and sheds light on the problem of polymer translocation.
