Investigating the requirement for calcium during lactococcal phage infection

• Many lactococcal phages do not appear to require calcium for adsorption or plaque formation.
• Ca2 + may in many cases be replaced by Mg2 + or Mn2 +, highlighting phage adaptability.
• The rare lactococcal phage 1706, specifically requires calcium for plaque formation.
• Calcium expedites the phage lytic cycle with implications for dairy fermentations.

Calcium is widely used in the study and successful propagation of virulent bacteriophages infecting lactic acid bacteria; however, it has not been assessed if and why this divalent cation is required for the infection process. Lactococcal phages are a persistent problem in the calcium-rich dairy environment and therefore were used as the model for this study. Using representative members of nine of the currently recognized ten lactococcal phage groups, encompassing phages of the Podoviridae and Siphoviridae families, we present data to suggest that calcium is not an explicit requirement for many of these phages. However, calcium expedited the pace of the lytic cycle for certain phages. Additionally, for calcium-dependent phages belonging to the 936 siphogroup, we could substitute this cation with magnesium or manganese, indicating that these phages are more adaptable than lactococcal phages of other groups. We postulate that the ability of phages to adapt to their environment and to harness the available mineral content may ultimately decide the success of a given phage infection. This may explain, in part, why 936 phages are one of the most frequently isolated phages in the dairy industry.