Although both B. bifidum YIT 4001 and YIT 4007 show resistance to
neomycin at a concentration lower than 1000 μg/mL, the MIC of B.
bifidumYIT 4007 for neomycinwas superior to 2000 μg/mL, and theMIC
of B. bifidum YIT 4001 for neomycin was inferior to 2000 μg/mL. For the
isolation of neomycin resistant mutants, cells of B. bifidum YIT 4001
were grown from nine independent colonies at 37 °C on MILS lactose
broth and spread on MILS lactose agar containing 5000 μg/mLneomycin
trisulfate (Sigma). Neomycin resistant colonies were isolated after
incubation at 37 °C for 3 days at the frequency of 10−8 to 10−9, and
confirmed their neomycin susceptibility after single colony isolation on
MILS lactose neomycin agar.