Microbiology experiment: UV light seemed to have no effect on bacteria. Why?

Question

Our lab group had to measure the lethal effects of UV light on bacteria. We used nutrient agar plates with bacteria on them, and put them in a UV exposure box. One plate of Bacillus megaterium was exposed for 15 minutes, and another for 30 minutes. One plate of Staphylococcus epidermis was exposed for 2.5 minutes, the other for 5 minutes. B. megaterium is endospore-forming so we knew it would take longer to kill the bacteria. Half of each plate was covered with an index card when exposed (so one side could be the control). After exposure the plates were incubated for 31 hours. The problem is that there was no difference between the control and exposure sides. All of the plates showed growth, the UV light seeming to have no effect. Other lab groups had pretty much the same results (some groups exposing B. meg for up to 1 hour, staph for 10 minutes). What went wrong? Too little exposure? Faulty UV box? Human error?

Answer 1

Have you considered that these bacteria are immune to UV radiation?

If UV rays were supposed to kill them, then natural selection solves the problem. UVs can kill most of the bacteria, even 99.9%, but a mutant version which can survive will and will reproduce.

Answer 2

Usually Staph Epi exposure time is about 2 mins. to kill all organisms, so I would say it was probably a malfunctioning exposure boxes.  You did take the clear covers off the plates, right?  UV won't even go through them, even though they're clear.

Answer 3

If a bunch of independant groups all had the same results, it's notlikely that you all made the same random human error.

I'd guess that the box doesn't work right.  Maybe if you bought that photosensitive paper, the kind that's blue and turns white in teh sun, that might show you if its working.