For the median lethal dose (LD50) determination, one loop of
cells of B. licheniformis was transferred into 100 ml of LB medium
in a 500-ml flask. The cells of B. licheniformis were centrifuged to
precipitate the cells and then spread as a single-cell layer on a sterile
adhesive tape which was attached to a Petri dish and then
placed inside a sample holder, which also held a vacuum control.
Ion beam bombardment was carried out using the vertical bioengineering
ion beam facility at Chiang Mai University. In the experiment,
nitrogen (N) ions were used for ion irradiation with energy
of 30 keV and fluences in the range of 1014–1016 ions/cm2 at a normal
ion flux of an order of 1013 ions/cm2/sec. This flux level was
demonstrated to be low enough to maintain the cells survival [1].
After ion beam bombardment, the samples were separately
washed down with 10-ml LB solution and centrifuged for 1 min
at 9000 rpm. The precipitates were resuspended in 5 ml of LB medium
and incubated for 30 min at 37 C on a rotating shaker at a
speed of 220 rpm until the optical density at 600 nm (OD600)
reached 0.3–0.6. These cell suspensions were subsequently 10-fold
diluted to form samples at concentrations ranging from 101 to
1011 and grown on solid LB medium. All culture plates were then
incubated for 1 day at 37 C to ensure cell viability. Bacterial mutants
were screened for the phenotype which lost their antagonistic
ability against the fungi to be further tested. The mutant
screening was operated using the dual culture method as mentioned
above (Fig. 2).