A month ago I wrote a post about a planned experiment using the antibiotic bicyclomycin, to see if it induces H. influenzae cells to develop competence. At the time I couldn't remember why this was a reasonable question, but a commenter pointed me to this paper, which describes the induction of competence by bicyclomycin in Legionella pneumophila.
Bicyclomycin is expensive, and we're close to broke, but a generous colleague had given us 4 mg of it to use in a trial experiment. So I put our summer undergraduate to work on the project. She began by testing H. influenzae's ability to grow in different concentrations of bicyclomycin, since we wanted to use a semi-inhibitory (but not lethal) concentration for our experiment. We had found a paper that reported the minimum inhibitory concentration (MIC) for H. influenzae was 3.1 µg/ml, so she tested a wide range (up to 20 µg/ml). But she saw no inhibition of growth at all.
That MIC had been for a clinical strain, not the lab workhorse KW20, so she repeated the test (this time using the neighbour-lab's BioScreen system) for both a clinical strain (86-028NP) and KW20, and for a couple of E. coli strains (the same paper reported MICs for E. coli strains between 6 and 12 µg/ml), using bicyclomycin concentrations up to 50 µg/ml. Still no evidence of growth inhibition!
But now I think I've solved the mystery. Before making up our bicyclomycin stock we searched for solubility info. We learned that it's reasonably soluble in water, but that there's a related antibiotic called bicyclomycin benzoate that needs to be made up in ethanol. The colleague who gave us the 4 mg remimded me that she'd sent an email saying to dissolve it in ethanol. I'd forgotten about this email, but reading it now reminded me of the solubility difference, and when I checked with her I found out that what she'd given us was bicyclomycin benzoate.
The same paper that gave us the H. influenzae MIC for bicyclomycin tested a wide range of derivatives, one of which was bicyclomycin benzoate. It's MIC for H. influenzae was >100 µg/ml. No wonder our cells didn't care about the concentrations we tested!
Bicyclomycin is about 10 times more expensive that bicyclomycin benzoate ($280/mg) so I don't think we'll be doing this experiment after all.