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Antimicrobial Agents and Chemotherapy, June 2001, p. 1889-1891, Vol. 45, No. 6
Service de Microbiologie, Hôpital
Robert Debré, 75019 Paris,1 and
Service de Microbiologie Hôpital Delafontaine, 93200 Saint Denis,2 France
Received 12 January 2001/Returned for modification 19 February
2001/Accepted 17 March 2001
Macrolide susceptibility was investigated in clinical group B
streptococci obtained from neonates or pregnant women in 2000 in
France. Of 490 consecutive isolates, 18% were resistant to erythromycin. The erm(B), erm(A) subclass
erm(TR), and mef(A) genes were harbored by 47, 45, and 6% of these strains, respectively. Two isolates did not harbor
erm or mef genes.
Group B streptococci (GBS) are a
leading cause of neonatal infections. Intrapartum antibiotic
prophylaxis is now recommended for colonized women to prevent neonatal
GBS disease, with penicillin G being the drug of choice
(1). Women allergic to The known mechanisms of macrolide resistance in streptococci are
targets of modification by a ribosomal methylase associated with
erm genes (17, 26, 33), a macrolide-specific
efflux mechanism encoded by the mef(A) gene
(7), and mutations in the 23S rRNA and ribosomal L4 and
L22 proteins (9, 30, 31; A. Canu, B. Malbruny, M. Coquemont, T. A. Davies, P. C. Appelbaum, and R. Leclercq,
Abstr. 40th Intersci. Conf. Antimicrob. Agents Chemother., abstr. 1927, p. 118, 2000). The prevalences and mechanisms of macrolide resistance
have been widely reported worldwide for group A streptococci (GAS) and
Streptococcus pneumoniae (4, 5, 8, 11, 12, 14, 16, 22,
28); relevant data on GBS are rare (3). The aims of
this study were to assess the macrolide sensitivity of clinical GBS
strains recently isolated in France and determine the genetic
mechanisms of resistance.
In 2000, 88 erythromycin-resistant GBS isolates were identified among
490 consecutive isolates in the Paris (France) area. The isolates were
recovered from genital specimens of pregnant women (n = 67) or from gastric fluid or ear specimens of colonized or
infected newborns (n = 21). The detection of erythromycin-resistant GBS isolates and determination
of resistance phenotypes were performed as previously described
(11, 27). The MICs of erythromycin azithromycin, josamycin, spiramycin, clindamycin, and streptogramin B were determined for all isolates with erythromycin inhibition zone diameters of less
than 21 mm (20, 21). MICs were determined by the agar dilution method in Mueller-Hinton medium supplemented with 5% defibrinated sheep blood. The plates were incubated overnight at 35°C
in air.
All erythromycin-resistant isolates were screened for erythromycin
resistance genes. The mef and erm genes were
detected by multiplex PCR amplification with previously described
primers (5, 15, 26, 29). The internal PCR control was the
mreA gene. The primers used to detect the mreA
gene were 5'-AGA CAC CTC GTC TAA CCT TC-3' and 5'-TCT
GCA GGT AAG TAA GTG CG-3' (6). Streptococcus
agalactiae BM 132, S. agalactiae SBI, and
Streptococcus pyogenes 02 C1110 were used as positive PCR
controls for the erm(B), mef(A), and
erm(A) subclass erm(TR) genes, respectively
(3, 5, 7). Five erythromycin-susceptible GBS isolates were
used as negative controls. Amplification of DNA from the positive
controls with the corresponding primers yielded PCR products of the
expected sizes [616, 490, 348, and 206 bp for erm(B),
mreA, erm(A) subclass erm(TR), and
mef(A), respectively] (Fig.
1). These PCR products were used for
direct sequencing in an Applied Biosystems model 373 DNA sequencer by a
modification of Sanger's method (25). The amplimers were
found to be identical to the erm(B), erm(A) subclass erm(TR), and mef(A) genes (7, 26,
33).
0066-4804/01/$04.00+0 DOI: 10.1128/AAC.45.6.1889-1891.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Mechanisms of Macrolide Resistance in Clinical
Group B Streptococci Isolated in France
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ABSTRACT
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-lactam antibiotics can receive
intravenous clindamycin or erythromycin (1). Although
penicillin resistance in GBS has not yet been reported, isolates
resistant to erythromycin and related antibiotics have been previously
described (2, 10, 18, 19, 23, 24, 32).
-hemolytic colonies and
suspected nonhemolytic colonies were identified as GBS by using a
commercial agglutination technique (Murex Diagnostics, Dartford, United
Kingdom). The GBS serotypes were as follows: serotype Ia, n = 2; serotype Ib, n = 9; serotype II, n = 6; serotype III, n = 28; serotype IV,
n = 10; serotype V, n = 26; and
nontypeable, n = 7.

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FIG. 1.
PCR analysis of erythromycin-resistant control strains.
Primers specific for the detection of erm(B) (lane 1),
mef(A) (lane 2), and erm(A) subclass
erm(TR) (lane 3) were used, followed by three representative
clinical isolates (lanes 4, 5, and 6). Lanes 1 to 6, internal PCR
control, the mreA gene, Lane M, DNA molecular size marker
VIII (Boehringer Mannheim).
Among the 88 GBS erythromycin-resistant isolates, 71, 23 and 6%
expressed the inducible macrolide-lincosamide-streptogramin B
(MLSB), constitutive MLSB, and M resistance
phenotypes, respectively. Table 1 shows
MICs for the isolates according to erythromycin resistance genotype.
PCR amplification showed that all the resistant isolates with the
constitutive MLSB, inducible MLSB, and M
phenotypes harbored the erm(B) or erm(A) subclass
erm(TR) and mef(A) genes, respectively. All
strains carried the mreA gene, but two
erythromycin-resistant strains did not yield amplified products with
the erm and mef primers tested; the mechanisms of
resistance are under investigation. The MICs of various drugs for these
two isolates were as follows:
128 µg/ml for all macrolides and
clindamycin and 16 µg/ml for streptogramin B for the first isolate
and 32 µg/ml for macrolides,
128 µg/ml for clindamycin, and 8 µg/ml for streptogramin B for the second isolate. The distributions
of the erythromycin resistance genes are shown in Table
2 according to serotype.
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Erythromycin resistance in GBS has mainly been investigated in North America. In the most recent studies, the rates of resistance ranged from 4 to 25% (2, 10, 18, 19, 23, 24, 32). In our study of GBS isolates of similar origins collected in the Paris area in 2000, the prevalence of erythromycin resistance was 18%. A previous North American study has shown an increase in GBS erythromycin resistance from 1995 to 1998, which could be related to the implementation of American guidelines recommending intrapartum antibiotic prophylaxis for GBS infection (1). In our institutions, the level of GBS erythromycin resistance varied only from 16% in 1997 to 18% in 2000, with no significant change in the consumption of macrolides during the last 5 years (E. Bingen, unpublished data).
While the prevalence and mechanisms of erythromycin resistance in S. pneumoniae and GAS have been widely investigated (4, 5, 8, 12, 14, 22, 28), to our knowledge such data are not available for GBS. In our study, erythromycin resistance in GBS was mainly associated with the erm(B) and erm(A) subclass erm(TR) genes (47 and 45% of isolates, respectively), with only 6% of isolates harboring the mef(A) gene. None of the strains carried both erm(A) and erm(B) or both mef and erm, as previously observed with GAS isolates (13). The mreA gene, initially considered a novel macrolide efflux gene, was detected for all our strains (6). Indeed, the mreA gene is now considered a housekeeping gene for the GBS species (G. Clarebout, and R. Leclercq, Abstr. 39th Intersci. Conf. Antimicrob. Agents Chemother., abstr. 840, p. 115, 1999). Erythromycin resistance in two of our strains was not associated with either the mef or the erm genes. Similar results have recently been reported with GAS isolates (22). Such resistance in beta-hemolytic streptococci may be related to mutations in ribosomal proteins, as previously reported for S. pneumoniae (9, 30, 31).
Interestingly, the mechanisms of macrolide resistance in our GBS isolates differed from those previously described for pneumococcal and GAS isolates in France (5, 11). While erythromycin resistance in pneumococci is mainly associated with erm(B), erythromycin-resistant GAS strains bore the erm(B) or mef(A) gene and, sporadically, the erm(A) subclass erm(TR) gene. Insufficient data are available to compare the genetic mechanisms underlying erythromycin resistance in GBS in France and elsewhere. However, several recent North American studies showed a rate of erythromycin- and clindamycin-resistant GBS of 4 to 16% (2, 10, 18, 19), pointing to the involvement of the erm(B) and/or erm(A) subclass erm(TR) genes.
Our study shows that erythromycin resistance is not equally distributed among the different GBS serotypes, with higher rates being associated with serotypes III and V. This is a matter of concern, as these serotypes are usually associated with invasive strains. Thus, antibiotic intrapartum prophylaxis for patients allergic to penicillin must be guided by macrolide susceptibility testing of each GBS isolate.
Surveillance of macrolides and patterns of resistance in GBS, associated with a survey of macrolide consumption, should continue.
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ACKNOWLEDGMENTS |
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We thank Joyce Sutcliffe for providing reference strains S. pyogenes 02C1110, Corinne Arpin for providing S. agalactiae SB1 and BM 132, and R. Leclercq for helpful discussion.
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FOOTNOTES |
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* Corresponding author. Mailing address: Service de Microbiologie, Hôpital R. Debré, 48 Bd Sérurier, 75019 Paris, France. Phone: 33 (1) 40 03 23 40. Fax: 33 (1) 40 03 24 50. E-mail: edouard.bingen{at}rdb.ap-hop-paris.fr.
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