Bifidobacterium bifidum strain Yakult (abbreviated as B. bifidum ) is a strain
and resistant to erythromycin, neomycin, and streptomycin. Resistances to these antibiotics were endowed
by sequential isolation of resistant mutants from its susceptible progenitor strain YIT 4001. Comparison of
nucleotide sequences of various candidate genes of both strains led us to find that B. bifidum YIT 4007 had
mutations on three copies of 23S ribosomal RNA genes, an 8 bp deletion of the rluD gene for pseudouridine
synthase, and a mutation on the rpsL gene for ribosomal protein S12. The responsibility of these mutations to
antibiotic resistances was supported by analyses of newly isolated mutants resistant to these antibiotics. The
antibiotic resistances of B. bifidum were evidently acquired by mutations of the structural genes on
the chromosome and not associated with mobile genetic elements like insertion sequences, phages, and
plasmids.