lambda plaque assay - Mg+ maltose?

[Gerd ]

Hei,
As a stand-in, I am teaching a microbiology labcourse, where one of the
experiments involves titration of Lambda phages (NK1316) by plaque assay
using E.coli LE392. The protocol says: grow the cells in medium with maltose
and MgSO4, before infecting with phages diluted in buffer containing Mg. I
remember having used the same additions screening a genomic library in
Lambda, years ago, so these additions seems to be common.
My question is: What is the effect of maltose, and Mg? 
There really is not time enough to figure that out by experimentation.

Gerd