Prepare host cells for infection
1. Pick a single colony of host cell (e.g., XL1-blue MRF' for a ZAP II library on LB-tetracycline plate).

2. Grow overnight in 50 ml of LB/0.2% maltose/10 mM MgSO4 at 30 C.

3. Pellet cells. And resuspend cells in 10 ml of 10 mM MgSO4.

4. Dilute cells with 10 mM MgSO4 to OD4 ~ 0.5.

5. Cells can be stored at 4 C for less than 1 month.