High-Yield Method for Isolation of λ DNA

Isolation of bacteriophage λ DNA is among the most common procedures used by those working with recombinant DNA methodologies. Although there are many protocols for λ DNA isolation (1,2,4), some are cumbersome and time-consuming, and others result in poor DNA yield. Among the most important parameters for successful phage DNA isolation is estimating the correct phage:host cell ratio for infection (3). If this ratio is too high, lysis occurs too quickly, before the phage population builds to an optimal level. If the ratio is too low, the host bacteria will outgrow the phage. In both cases, the result is a poor DNA yield. With the method described here, we have found it is not necessary to titrate the phage. An innovation of this procedure is supplementation of the LB growth medium with glycerol. Although the exact reason that this improves the yield is not known, the glycerol may serve as an additional carbon source for the host cells.