Using λ JL259 (makes blue colonies)

This phage contains a sulA::lacZ fusion in its b2 region. The phage was obtained from John Little (University of Arizona). He writes the following: "I don't know where the fusion is. I think it has about 1 kb of Mu sequence between the sulA promoter and lacZ; somebody told me that, and I think it makes sense due to the way it was made in the 1970's (using Casadaban's Mud::lac phage I think). [...] the fusion phage is cIind-; the mutation is E117K in cI, I think."

JL259 makes blue plaques on X-Gal plates. It also makes blue colonies. The latter must be irradiated shortly, to ensure a reliable development of the blue color. Lysogens can either be spotted (preferably a large volume, say four spots of 50 µL each on one plate) or plated. Depending on the density, colonies can be incubated at 28 or 37 °C overnight. They are then exposed to 5-10 sec of UV light (just place the plates upside down on a UV table and switch it on). Plates are then incubated at 37 °C for another 2-5 hours) to allow for the coloration to develop.

Blue colonies can be detected easily against a white background, white colonies against a dark background. Blue colonies can be very faint, yet the difference to a white colony is still quite obvious.

Figure 1: Blue colonies are reliably detected in a background of up to 10'000 white colonies.