The requirement for calcium in infection with Lactobacillus phage.

Calcium ions were specifically required for a stage in the formation of phageinfected cells in the growth cycle of PL-I phage active against Lactobacillus casei ATCC 27092. Both the adsorption of phage on to the host cells and the intracellular multiplication of phage were independent of the presence or absence of calcium ions. The optimum pH for successful infection was 5"5 to 6.0 and the optimum temperature was approximately 3 ° °C. Among the divalent salts tested SrC12 was as effective as CaC12 in allowing infection, whereas MgSO4, MnC12, BaCI~, BeSO4, ZnSO4 and COSO4 were not. The impairment of infection by lack of calcium ions could not be reversed by the delayed addition of CaCI~. An experiment with host cells infected with [3~P]-labelled phage showed that calcium ions were required for the penetration of phage genomes into the host cells. The host cells, which had adsorbed phage but not permitted the penetration of phage genomes owing to the absence of calcium ions, were not killed by the phage particles. PL-I phage infection was, therefore, abortive in the absence of calcium ions owing to the inhibition of a stage of the formation of phage-infected cells involving penetration.