Abstract
The macrolide-lincosamide-streptogramin B (MLS
B
) resistance determinants have been detected among
Clostridia
in both
C. perfringens
and
C. difficile
strains. Previous studies
have shown that MLS
B
-resistant
C. difficile
strains can be differentiated by specific hybridizing bands using an
erm
(B) probe. A recent study has demonstrated that
C. difficile
630, a strain highly resistant to clindamycin and erythromycin (MIC ≥ 256 ml/L), showing a hybridizing band at 9.7 kb, contains two copies of an
erm
(B) gene. It was also hypothesized that
C.
difficile
630
erm
(B) determinant has arisen from a progenitor, represented by the
C. perfringens
CP592 determinant, which contains only one copy of an
erm
(B) gene that differs from
C. difficile
630
erm
(B) for seven nucleotide substitutions. To investigate the possibility that
C. difficile
strains with hybridizing fragments of different molecular size have an
erm
(B)
determinant not identical to the one described in
C. difficile
630, we performed a genetic analysis on the
erm
(B) determinant in 18
C. difficile
strains, isolated from different sources.
The results showed a heterogeneity in
erm
(B) determinant:
C. difficile
strains with hybridizing bands at 7.3 or 3.7 kb contained only one
erm
(B) copy, whereas strains with a band at
9.7 kb had two copies. The majority of the toxigenic strains examined was characterized by only one
erm
(B) copy with a sequence identical to the one found in
C. difficile
630 and a lower resistance
level for erythromycin (MICs ranging from 16 to 24 ml/L). Differently, some strains had an
erm
(B) gene identical to the one found in
C. perfringens
CP592. PCR ribotyping and clustering analysis
indicate that the examined resistant strains, except one, belong to the same genetic lineage. These results seem to support the hypothesis of the evolution of the
C. difficile
630
erm
(B) determinant.
The functional significance of one or two copies of
erm
(B) gene in
C. difficile
strains should be further investigated.