The staphylococcal typing phages require divalent cations for a stage in phage growth which is probably that of penetration. When phage is adsorbed to the cell surface of sensitive cocci and a chelating agent is added, there is a short period during which the phage particle may be inactivated and its lethal effect on the coccus prevented. Cocci attacked by phages of serological group A show penetration of the phages in distilled water + CaCl, or + MgCl, or to a less extent + SrCl. The phages of group B are unable to penetrate in presence of SrCl and in some cases MgCl is only partially active as compared with CaCl. Ca ions appear to be required specifically at a late stage in phage synthesis since the average yield of phage/coccus increases with increasing concentration of CaCl; the addition of a chelating agent towards the end of the latent period depresses the yield of phage.


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