bifidum YIT 4007 and newly isolated resistant mutants. One of the
newly isolated erythromycin resistant mutants, EM11, had mutations
on two copies of three 23S ribosomal RNA genes unlike other mutants
which had mutations on all of three 23S ribosomal RNA genes (Table 2).
This mutant could not form colonies on erythromycin at 3000 μg/mL
whereas other mutants with three copies of the mutated genes could
grow at this concentration. Macrolide resistant mutants with two
mutated 23S ribosomal RNA genes out of three were also reported for
Campylobacter coli (Ladely et al., 2009). We found no mutants which
had only onemutated 23S ribosomal RNAgeneamong 15 newly isolated
mutants (Table 3). We just speculate that the mutantwith onemutated
and two wildtype 23S ribosomal RNA genes might not be resistant to
erythromycin at a high concentration.