Ultrastructure and Life Cycle of the Lipid-containing Bacteriophage 6

An electron microscopic study of the lipid-containing bacteriophage phi 6 revealed an electron dense compact inner core of 30 nm in diam., which apparently contains the nucleic acid of the virus. This inner particle is surrounded by a complex polyhedral capsid with an outer diam. of 50 nm. Outside this is the envelope, which gives the virus a total diam. of 65 to 75 nm. The envelope, which has a thickness of a unit membrane, could be removed by treating the phage with Triton X-100. A definite structure is seen inside the envelope of the phage tail. In infection, phages are attached by their tails to the host cell pili. Occasional pili with a few attached phages were seen in a phage resistant mutant. In the course of the infection phages were also seen attached to the outer membrane of the cell. In a phage-tolerant mutant many normal-looking pili with adsorbed phages were visible, but we could never see phage-cell membrane associations. The membrane of the phage appears to fuse with the bacterial outer membrane and 50 nm virus particles could be seen in the periplasmic space of the bacterium, probably attached to the cytoplasmic membrane. Newly formed 50 nm particles appeared 45 min post infection (p.i.) cnetrally in the host cell. Assembly of the envelope also began at this time and by 80 minutes p.i. all the 50 nm particles were covered by the virus membrane. At no stage were phages seen in the periphery of the bacterium. Mature phages were finally released by a rupture of the host cell without spheroplast formation.