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Antimicrobial Agents and Chemotherapy, July 2000, p. 1838-1841, Vol. 44, No. 7
Servicio de Microbiología
Clínica, Hospital Clínico San Carlos, 28040 Madrid,
Spain
Received 13 December 1999/Returned for modification 25 March
2000/Accepted 18 April 2000
The in vitro activities of the new agents linezolid,
quinupristin-dalfopristin, moxifloxacin, and trovafloxacin were
determined and compared with those of penicillin, clindamycin, and four
macrolides against 53 erythromycin-resistant Streptococcus
pneumoniae, 117 S. pyogenes (64 erythromycin-susceptible and 53 -resistant), and 101 S. agalactiae (53 erythromycin-susceptible and 48 -resistant) isolates. Differentiation of macrolide resistance phenotypes was performed by the double-disk method. The genetic basis for macrolide resistance in 52 strains was also determined. The M phenotype was found
in 84.9, 6.3, and 1.9% of S. pyogenes, S. agalactiae, and S. pneumoniae isolates, respectively.
These strains were susceptible to miocamycin and clindamycin. Strains
with the inducible phenotype accounted for 27.1% of S. agalactiae isolates and 9.4% each of S. pyogenes and
S. pneumoniae isolates. All erythromycin-resistant isolates
were also resistant to the 14- and 15-membered macrolides tested.
Strains with all three phenotypes were susceptible to Erythromycin has been considered to
be the principal alternative agent for the treatment of gram-positive
streptococcal infections in penicillin-allergic patients. However,
increased incidence of erythromycin resistance in Streptococcus
pyogenes has been reported in several parts of the world (6,
7, 19, 21, 25). In our area, erythromycin resistance in S. pyogenes increased from 2.6% in 1995 to 17.1% in 1996 (4). The prevalence of infections caused by
penicillin-resistant and multidrug-resistant pneumococci has been
increasing over the last 2 decades (2, 9, 10). Erythromycin
resistance is also increasing in Streptococcus agalactiae in
some countries (29; M. K. H. McGavin, A. McGeer, J. C. de Azavedo, L. Trpeski, S. Pong-Porter, C. Duncan,
and D. E. Low, Abstr. 39th Intersci. Conf. Antimicrob. Agents
Chemother., abstr. 1224, p. 159, 1999). The current rates of
erythromycin resistance in our area are about 40% for
Streptococcus pneumoniae, 26% for S. pyogenes,
and 15% for S. agalactiae.
Two mechanisms of macrolide resistance in streptococci have been
described: (i) target site modification mediated by erythromycin resistance methylases encoded by erm genes, whose phenotypic
expression can be inducible or constitutive, and (ii) an efflux
mechanism encoded by mef genes (mefA or
mefE). Because macrolide resistance in streptococci appears
to have increased worldwide over the past decade, new therapeutic
alternatives are needed.
Compared with older quinolones, moxifloxacin and trovafloxacin have
enhanced activity against gram-positive organisms and anaerobes and
maintain the activity of older quinolones against gram-negative
organisms. However, because of hepatotoxicity, the European Union has
banned trovafloxacin. Quinupristin-dalfopristin is a new streptogramin
antibiotic with activity against gram-positive aerobes. Previous
studies have shown that this compound is highly active against
streptococci, including multidrug-resistant strains (3, 14).
Linezolid belongs to a new class of synthetic agents, the
oxazolidinones, and has activity against multidrug-resistant gram-positive bacteria, including methicillin-resistant staphylococci, penicillin-resistant pneumococci, and vancomycin-resistant enterococci (27, 30).
The purpose of this study was to determine the in vitro activities of
the new antimicrobials linezolid, quinupristin-dalfopristin, moxifloxacin, and trovafloxacin against erythromycin-resistant streptococci and to compare these activities with those of six other
antibiotics. The mechanisms of macrolide resistance were also examined.
(This study was presented in part at the 39th Interscience Conference
on Antimicrobial Agents and Chemotherapy, San Francisco, Calif., 26 to
29 September 1999.)
A total of 271 streptococcal isolates were tested. These
included 53 erythromycin-resistant S. pneumoniae, 117 S. pyogenes (64 erythromycin-susceptible and 53 -resistant),
and 101 S. agalactiae (53 erythromycin-susceptible and 48 -resistant) isolates. Only one isolate per patient was studied to avoid
duplication. Organisms were identified by standard methods, including
agglutination with latex (Slidex pneumo-kit, Slidex Strepto A, and
Slidex Strepto B; bioMérieux, Marcy L'Etoile, France).
Susceptibility testing was performed by the agar dilution method
according to the guidelines of the National Committee for Clinical
Laboratory Standards (15, 16). Mueller-Hinton agar supplemented with 5% sheep blood was used. Approximately
104 CFU was inoculated per spot with a Steers replicator.
In each test, Staphylococcus aureus ATCC 29213, Enterococcus faecalis ATCC 29212, and S. pneumoniae ATCC 49619 were used as internal controls. Plates were
incubated overnight at 35°C in air. The MIC was defined as the lowest
concentration of each antimicrobial which completely inhibited
bacterial growth.
The following antimicrobial agents were used and obtained from the
indicated manufacturers: penicillin, Compañía
Española de la Penicilina y Antibióticos, S.L., Madrid,
Spain; erythromycin, Abbott, Madrid, Spain; roxithromycin, Hoechst
Marion Roussel, S.A., Madrid, Spain; azithromycin, Farmasierra, S.A.,
Madrid, Spain; miocamycin, Menarini, S.A., Barcelona, Spain;
clindamycin and linezolid, Pharmacia & Upjohn, Barcelona, Spain;
quinupristin-dalfopristin, Rhône-Poulenc Rorer, Madrid, Spain;
moxifloxacin, Bayer, Barcelona, Spain; and trovafloxacin, Pfizer, New
York, N.Y.
Erythromycin resistance phenotypes were determined by the double-disk
test (20) with erythromycin (15 µg) and clindamycin (2 µg) disks on Mueller-Hinton agar supplemented with 5% sheep blood.
Plates were incubated overnight at 35°C in air. The presence of the
erythromycin resistance genes in 52 (13 S. pneumoniae, 25 S. pyogenes, and 14 S. agalactiae) randomly
selected isolates was investigated by PCR (22, 24). Primer
sets specific for the ermA, ermB,
ermC, and ermTR genes were used for the
identification of erythromycin resistance genes. The presence of the
ermTR gene was confirmed by HinfI digestion. PCR
with mefA and mefE primers was performed to
confirm the presence of the efflux system.
Results of MIC determinations are listed in Table
1. Intermediate susceptibility and
resistance to penicillin were found in 52.8 and 37.8%, respectively,
of S. pneumoniae isolates. Both S. pyogenes and
S. agalactiae, including those strains which were erythromycin resistant, were uniformly susceptible to pencillin. Of the
14- and 15-membered macrolides, erythromycin showed slightly better
activity than roxithromycin and azithromycin against
erythromycin-susceptible streptococci. All erythromycin-resistant
isolates were also resistant to the 14- and 15-membered lactone ring
agents tested, roxithromycin and azithromycin, respectively.
Miocamycin, the 16-membered macrolide tested, had the greatest activity
against erythromycin-resistant strains, retaining full activity against
M-phenotype isolates of S. pyogenes, S. pneumoniae, and S. agalactiae. The widespread use of
macrolides might contribute to the high incidence of erythromycin resistance among streptococci (mainly S. pneumoniae and
S. pyogenes) observed in many countries (6, 19, 21,
29). Thus, new agents are needed as alternatives to penicillin
for the prophylaxis or treatment of infections caused by streptococci
in penicillin-allergic patients, as well as those caused by
penicillin-resistant pneumococci.
0066-4804/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Comparative In Vitro Activities of Linezolid,
Quinupristin-Dalfopristin, Moxifloxacin, and Trovafloxacin against
Erythromycin-Susceptible and -Resistant Streptococci
ABSTRACT
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Abstract
Introduction
Materials and Methods
Results and Discussion
References
2 µg of
linezolid per ml. Quinupristin-dalfopristin exhibited good in vitro
activity against all strains, irrespective of their resistance to
erythromycin (MICs at which 90% of the isolates tested were inhibited
[MIC90s], 0.2 to 1 µg/ml). Against the
erythromycin-resistant S. pyogenes and S. agalactiae strains, moxifloxacin and trovafloxacin were the most
active agents (MIC90s, 0.1 µg/ml). The new antimicrobials evaluated may be alternative agents to treat infections caused by
macrolide-resistant as well as macrolide-susceptible streptococci.
INTRODUCTION
Top
Abstract
Introduction
Materials and Methods
Results and Discussion
References
MATERIALS AND METHODS
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Abstract
Introduction
Materials and Methods
Results and Discussion
References
RESULTS AND DISCUSSION
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Abstract
Introduction
Materials and Methods
Results and Discussion
References
TABLE 1.
In vitro activities of 10 antimicrobial agents against
271 streptococcal strains
Quinupristin-dalfopristin exhibited good in vitro activities against all strains tested, irrespective of their resistance to erythromycin. Although against macrolide-susceptible isolates quinupristin-dalfopristin had 8- to 16-fold less activity than erythromycin, S. pyogenes, and S. agalactiae were highly susceptible to this new streptogramin, with MICs at which 90% of the tested isolates were inhibited (MIC90s) ranging from 0.2 to 0.5 µg/ml. The MIC50 and MIC90 of this antibiotic for S. pneumoniae were similar to those described by other authors (3, 14). Among S. pyogenes and S. agalactiae we found quinupristin-dalfopristin MIC90s that were slightly lower than those reported by Barry et al. (3).
Linezolid was active against all streptococci tested at 2 µg/ml.
For S. pyogenes, as well as S. pneumoniae and
S. agalactiae, the linezolid MIC90 was 2 µg/ml. The results of this study agree with those of previous reports
(27, 30) on in vitro susceptibility to linezolid. This
suggests that linezolid may be a therapeutic option for the treatment
of infections due to erythromycin-resistant or penicillin-resistant streptococci.
The new fluoroquinolones included in this study, moxifloxacin and trovafloxacin, showed high activities against both the erythromycin-resistant and erythromycin-susceptible streptococcal strains tested. Moxifloxacin showed excellent in vitro activity against all strains, with MIC90s of 1 µg/ml for S. pneumoniae and 0.1 µg/ml for both S. pyogenes and S. agalactiae. Like Woodcock et al. (28), we found that against S. pyogenes and S. agalactiae the activities of moxifloxacin were comparable to those of trovafloxacin. Among macrolide-resistant pneumococci, both fluoroquinolones showed potent activity, as several authors have already reported (9, 26), with the trovafloxacin MIC50s and MIC90s being slightly lower than those of moxifloxacin. Nevertheless, an increasing prevalence of pneumococci with reduced susceptibility to fluoroquinolones (5, 13), as well as the isolation of penicillin-resistant S. pneumoniae strains that were highly resistant to trovafloxacin (9), has recently been reported. Therefore, caution in the use of these antibiotics must be exercised and surveillance is necessary in order to detect the emergence of or increase in resistance to these new fluoroquinolones.
The so-called M phenotype was found in 84.9, 6.3, and 1.9% of S. pyogenes, S. agalactiae, and S. pneumoniae isolates, respectively. These strains were susceptible to clindamycin and miocamycin and resistant at a low level to erythromycin. The constitutive macrolide-lincosamide-streptogramin B (cMLS) phenotype was observed for S. pneumoniae (88.7%), S. agalactiae (66.6%), and S. pyogenes (5.7%) strains which showed high-level resistance to erythromycin, clindamycin, and miocamycin. Strains with the inducible MLS (iMLS) phenotype accounted for 27.1% of S. agalactiae isolates and 9.4% each of S. pyogenes and S. pneumoniae isolates.
Resistance to macrolides in our S. pneumoniae isolates is mainly due to the constitutive MLS phenotype, as recently described by Baquero et al. (2) in Spain and Oster et al. (18) in Italy. By contrast, the M phenotype predominates in pneumococcal strains isolated in Japan (17), Canada (11), and the United States (23). The erythromycin-resistant S. pyogenes isolates from our study were predominantly (84.9%) of the M phenotype, whereas the erythromycin-resistant S. agalactiae isolates accounted only for 1.9% of the isolates with this phenotype. The incidence of the M phenotype of erythromycin resistance among isolates of S. pyogenes in this study is similar to that reported by other groups in Spain (1, 19) but different from that (around 50%) reported in Italian surveys (8) and in Greece (25). The 16-membered macrolides and clindamycin might be considered alternative agents for the treatment of infections caused by strains showing this phenotype of macrolide resistance.
The genotypes of the 13 S. pneumoniae, 25 S. pyogenes, and 14 S. agalactiae isolates are summarized
in Table 2. All the S. pneumoniae isolates with a cMLS phenotype were positive for the ermB gene. No ermTR gene was detected in
pneumococcal strains with a cMLS phenotype. Of the six cMLS-phenotype
S. agalactiae isolates, two were positive with primers
specific for the ermTR gene and the remaining isolates were
positive with primers for the ermB gene. Three isolates (1 S. pneumoniae, 1 S. pyogenes, and 1 S. agalactiae isolate) bearing the iMLS phenotype possessed both the
ermTR and ermB genes.
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Among S. pyogenes isolates, we found the mefA gene in all isolates with the M phenotype and the ermTR gene in the three isolates tested with the iMLS phenotype. This corresponds with recent reports by Kataja et al. (12) and de Azavedo et al. (7). We did not find the mefA gene in the iMLS S. pyogenes isolates tested, whereas Giovanetti et al. (8) detected this gene in 30 of 60 iMLS isolates tested. However, we found one S. pyogenes isolate bearing the iMLS phenotype which possessed both the ermB and ermTR genes. The MIC of erythromycin for these isolate (8 µg/ml) was higher than those for the remaining two iMLS isolates (1 µg/ml each) harboring only the ermTR gene.
This study shows that the 14- and 15-membered macrolides had poor activities against clinical isolates of erythromycin-resistant streptococci. The 16-membered macrolide tested, miocamycin, as well as clindamycin, showed good activity against streptococci with the M erythromycin resistance phenotype. In areas where streptococcal resistance to erythromycin is common, determination of erythromycin resistance phenotypes can be helpful in the selection of an appropriate alternative therapy for penicillin-allergic patients.
The new agents tested, linezolid, quinupristin-dalfopristin, moxifloxacin, and trovafloxacin, showed excellent activities against the S. pneumoniae, S. pyogenes, and S. agalactiae isolates studied, irrespective of their susceptibility to erythromycin. On the basis of these results, we suggest that any of the new antimicrobials evaluated might offer a good therapeutic option in infections caused by macrolide-susceptible as well as macrolide-resistant streptococcal isolates. Further evaluations of these agents are warranted.
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ACKNOWLEDGMENT |
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This work was supported by grant 99/0434 from the Fondo de Investigación Sanitaria, Madrid, Spain.
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FOOTNOTES |
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* Corresponding author. Mailing address: Servicio de Microbiología Clínica, Hospital Clínico San Carlos, Plaza Cristo Rey s/n, 28040 Madrid, Spain. Phone: 34 913303478. Fax: 34 913303478. E-mail: cbetriu{at}efd.net.
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Antimicrobial Agents and Chemotherapy, July 2000, p. 1838-1841, Vol. 44, No. 7
0066-4804/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
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