a Antimicrobial Research Laboratory, National Public Health Institute, PO Box 57, 20521 Turku; b Department of Ophthalmology, University of Turku, Turku, Finland
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Abstract |
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Introduction |
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The mechanisms of erythromycin resistance in S. pyogenes include target site modification and active drug efflux. Target site modification is mediated by an erythromycin resistance methylase, encoded by an erm gene, which reduces binding of macrolide, lincosamide and streptogramin B (MLSB) antibiotics to the target site in the 50S ribosomal subunit.4 The phenotypic expression of MLSB resistance can be inducible (IR) or constitutive (CR). In streptococci, MLSB resistance has commonly been mediated by genes belonging to the ermB class (ermB = ermAM genes).5 However, we have recently characterized a novel erm gene, ermTR, from an erythromycin-resistant clinical strain of S. pyogenes (A200) isolated in Finland.4 The ermTR gene shares only a 58% homology with the sequence of the ermB (= ermAM) gene, but an 82% homology with the sequence of ermA of Staphylococcus aureus.4 Hence, it has been proposed that the ermTR and the ermA genes may share a common origin,4 and they have been assigned to the same ermA class of genes (ermA = ermTR).5 Active drug efflux is mediated in S. pyogenes by the mefA (macrolide efflux) gene.6 It causes resistance to 14- and 15-membered macrolide compounds only; this phenotype is called the M-phenotype.1,3,6
In Finland, erythromycin resistance in S. pyogenes was shown to be caused by two prevalent genes: mefA and ermTR.1 In this study we have investigated the antibiotic resistance patterns and the distribution of the erythromycin resistance genes of S. pyogenes isolates with different erythromycin resistance phenotypes collected from eight different countries in Europe and in North and South America.
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Materials and methods |
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The 238 erythromycin-resistant S. pyogenes isolates studied were collected from 1986 to 1997 by nine local microbiology laboratories in Spain, Italy (two laboratories), England, Greece, Sweden, Bulgaria, Argentina and the USA (Table I). Each laboratory sent 472 erythromycin-resistant S. pyogenes isolates to the Antimicrobial Research Laboratory of the National Public Health Institute, Turku, Finland (Table I
), where identification of isolates was confirmed by a commercial latex agglutination technique (Streptex; Wellcome, Dartford, UK). The isolates were of pharyngeal origin, except those from England, which were from blood cultures. One hundred and seventy-four isolates (73%) expressed the M-phenotype, 55 (23%) the inducible (IR) and nine (4%) the constitutive (CR) MLSB resistance phenotypes as determined by the double-disc test using erythromycin and clindamycin discs as described previously (Table I
).1 The clonal relationships of a representative collection of isolates with different erythromycin resistance phenotypes from each laboratory were determined using two PCR-based genotyping techniques: random amplified polymorphic DNA analysis (RAPD) and restriction fragment length polymorphism (RFLP)PCR analysis of the mga regulon (Vir typing) as described previously (Table I
).7,8 For the purpose of the study, the isolates that shared the same RAPD pattern and Vir type were considered to be of the same clonal origin. The number of clones among the 92 isolates that were selected to undergo PCR-based detection of erythromycin resistance genes were as follows: 20 clones from the 54 M-phenotype isolates, whereas 16 and six clones were from the 30 and eight IR-phenotype and CR-phenotype isolates, respectively.
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The susceptibility of all 238 isolates to 14 different antimicrobial agents (Table II) was studied. The MICs of the antimicrobials tested were determined by the agar dilution method according to the recommendations of the NCCLS as described previously.1,9 The breakpoints for resistance and the controls used were as described previously.1,9
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Ninety-two isolates of known clonal origin and erythromycin resistance phenotype were subjected to PCR-based detection of the ermA, ermB, ermC, ermTR and mefA genes as described previously.1 PCR was thus performed on 54 isolates with the M-phenotype, 29 isolates with the IR-phenotype and nine isolates with the CR-phenotype (Table I).
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Results and discussion |
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All the 54 M-phenotype isolates were positive with primers specific for the mefA gene (Table I), which is in accordance with previous studies.13 Thus, the mefA gene was widely distributed and was found in isolates derived from every country studied (Table I
). The mefA gene was found among 20 different clones of S. pyogenes that were determined by combining RAPD and Vir typing.
Of the 30 IR-phenotype isolates, 25 and four were positive with primers specific for the ermTR gene and the ermB gene, respectively. The remaining isolate was not positive with any of the primers used. Of the IR-phenotype isolates, two isolates carrying either the ermTR or ermB gene also carried the mefA gene by PCR. The MICs of 14- and 15-membered macrolides for these two isolates were no different from those isolates that carried only the erm gene. Of the eight CR-phenotype isolates, seven and one, respectively, were positive with primers specific for the ermB gene and ermTR. The ermTR and ermB genes were found in 14 and seven clones, respectively.
The ermTR gene was found in five European countries: Sweden, England, Bulgaria, Italy and Greece, and also outside Europe in Argentina and the USA. The ermB gene was found in four countries: Italy, England, Sweden and the USA. The higher prevalence of the ermTR gene as compared with the ermB gene among isolates with the IR-resistance phenotype of different clonal origin parallels the situation in Finland and Canada,1,10 and indicates that this recently sequenced methylase gene is indeed widespread among S. pyogenes.
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Acknowledgments |
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Notes |
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Members are listed in the Acknowledgements
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References |
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2
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Giovanetti, E., Montanari, M. P., Mingoia, M. & Varaldo, P. E. (1999). Phenotypes and genotypes of erythromycin-resistant Streptococcus pyogenes strains in Italy and heterogeneity of inducibly resistant strains. Antimicrobial Agents and Chemotherapy 43, 193540.
3 . Perez-Trallero, E., Urbieta, M., Montes, M., Ayestaran, I. & Marimon, J. M. (1998). Emergence of Streptococcus pyogenes strains resistant to erythromycin in Gipuzkoa, Spain. European Journal of Clinical Microbiology and Infectious Diseases 17, 2531.[ISI][Medline]
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Seppälä, H., Skurnik, M., Soini, H., Roberts, M. C. & Huovinen, P. (1998). A novel erythromycin resistance methylase gene (ermTR) in Streptococcus pyogenes. Antimicrobial Agents and Chemotherapy 42, 25762.
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Roberts, M. C., Sutcliffe, J., Courvalin, P., Jensen, L. B., Rood, J. & Seppälä, H. (1999). Nomenclature for macrolide and macrolide-lincosamide-streptogramin B resistance determinants. Antimicrobial Agents and Chemotherapy 43, 282330.
6 . Clancy, J., Petitpas, J., Dib-Hajj, F., Yuan, W., Cronan, M., Kamath, A. V. et al. (1996). Molecular cloning and functional analysis of a novel macrolide-resistance determinant, mefA, from Streptococcus pyogenes. Molecular Microbiology 22, 86779.[ISI][Medline]
7 . Seppälä, H., He, Q., Österblad, M. & Huovinen, P. (1994). Typing of group A streptococci by random amplified polymorphic DNA analysis. Journal of Clinical Microbiology 32, 19458.[Abstract]
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Gardiner, D. L., Goodfellow, A. M., Martin, D. R. & Sriprakash, K. S. (1998). Group A streptococcal Vir types are M-protein gene (emm) sequence type specific. Journal of Clinical Microbiology 36, 9027.
9 . National Committee for Clinical Laboratory Standards. (1999). Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria that Grow AerobicallyFourth Edition: Approved Standard M7-A4. NCCLS, Wayne, PA.
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De Azavedo, J. C., Yeung, R. H., Bast, D. J., Duncan, C. L., Borgia, S. B. & Low, D. E. (1999). Prevalence and mechanisms of macrolide resistance in clinical isolates of group A streptococci from Ontario, Canada. Antimicrobial Agents and Chemotherapy 43, 21447.
Received 1 March 2000; returned 12 May 2000; revised 28 June 2000; accepted 24 July 2000