3.2. Resistance to streptomycin
B. bifidum YIT 4007 is also resistant to streptomycin at the concentration
higher than 50 μg/mL, whereas the progenitor B. bifidum YIT
4001 is susceptible. It is well documented that resistance to streptomycin
is caused bymutations of rpsL gene in many bacteria (Spahn and
Prescott, 1996). We amplified and sequenced DNA of rpsL genes of B.
bifidum YIT 4001 and YIT 4007. Among 372 nucleotides of rpsL gene,
we found one nucleotide replacement of A (YIT 4001) to G (YIT 4007) at
the 128th position of rpsL genes. A128G change results in the change of
an amino acid lysine (YIT 4001) to arginine (YIT 4007) at the 43rd
position of amino acid sequence. Then we isolated spontaneous
streptomycin resistant mutants from B. bifidum YIT 4001 and determined
the sequence of rpsL gene. All three independent resistant
mutants had G on 128th nucleotide of rpsL genes as B. bifidum YIT 4007
did. It was suggested that G residue at the 128th position of the rpsL
gene is a causative mutation of streptomycin resistance of B. bifidumYIT
4007. The same nucleotide replacement was also found for streptomycin
resistant strains of B. breve (Kiwaki and Sato, 2009). Alignment of
rpsL genes of B. bifidum YIT 4007 and E. coli highlighted the 43rd
amino acidwhere lysine of B. bifidumYIT 4001 is replacedwith arginine
of B. bifidum YIT 4007. This site is located within the streptomycin
interaction site of the E. coli rpsL gene (http://www.ncbi.nlm.nih.gov/
entrez/viewer.fcgi?db=protein&val=16131221).