Effect of calcium ions on the infection of Bacillus subtilis by bacteriophage SF 6.

Infection of Bacillus subtilis 168Wt by SF 6 resulted in a rapid reduction in the number of phages. This could be counteracted by the addition of calcium, barium or strontium ions. At the optimum concentration of 7.5 x 10(-2) M, the number of p.f.u. remained constant until lysis began. Although cultures of another host. B. subtilis 31 try- his-, at the end of the logarithmic growth phase produced a substance which inactivated free phages, this was not the major cause of the reduction in the numbers of p.f.u. during infection experiments at low Ca2+ concentrations. The diminution of the number of p.f.u. was therefore attributed to the fact that at least one of the steps of the lytic cycle was calcium dependent. Adsorption of SF 6 was equally effective in media containing high or low concentrations of calcium ions. Infection experiments with phages whose DNA had been labelled radioactively revealed that, at high concentrations of calcium ions, the label remained associated with the host cells until lysis commenced. At low concentrations, however, a dissociation between phage DNA and the host was found, although adsorption took place at a normal rate. From these experiments we concluded that a high concentration of calcium ions was required for the penetration of phage DNA. Similar experiments with phages whose protein coat had been labelled showed the same results, indicating that desorption of the inactivated phages occurred. Both electron microscopy and column chromatogarphy with hydroxyapatite showed that a considerable fraction of the inactivated phages had ejected their DNA into the medium. A hypothesis explaining these results is presented.