As for erythromycin resistance, the type of mutations and copy
numbers seemingly affects the level of resistance. B. bifidum YIT 4007
with C residue at the 2090th position could form single colonies on
erythromycin at 4000 μg/mL while newly isolated resistant mutants
with G residue at the same position could not at this concentration.
Therefore, the range of erythromycin resistance could be affected by
the nucleotide at the 2090th position of 23S ribosomal RNA genes
though we cannot exclude the possibility that it might result from
minor possible differences of nucleotide sequences between B.