This is an iphone snap of my test plate after overnight incubation. The first and third streaks are a recA+ control strain and the second and fourth are of a known recA- strain. Because I was working at an odd angle, the green lines I drew to guide the exposures aren't lined up with the actual exposures. The recA+ cells grew fine after 5 seconds of UV, and gave a few colonies even after 1 minute (probably cells that were shielded in some way from the UV). The recA- cells grew fine when they were not exposed to any UV, but didn't grow at all even after only 5 seconds UV.
So today I've streaked all the cells I want to test, each on at least 2 different plates. I also reduced the UV dose. Yesterday I used a high dose range (5, 15, 30 and 60 seconds) to make sure I had a dose that would kill the recA- cells; today I used 2, 5 10 and 15 seconds.