a Department of Bacteriology, Hellenic Pasteur Institute, 127 Vass. Sofias, 11521 Athens; b Microbiology Laboratory of the P. & A. Kyriakou Children's Hospital of Athens, Athens, Greece
Sir,
Group A ß-haemolytic streptococci (GABHS) remain highly susceptible to penicillin. Most isolates are also susceptible to erythromycin, which is the principal alternative to penicillin as therapy of group A streptococcal pharyngo-tonsillitis in patients allergic to ß-lactams. However, reports of the failure of penicillin to eradicate GABHS in vivo and, more importantly, the emergence of erythromycin resistance amongst GABHS during the past decade, have focused attention on the antibiotic susceptibility patterns of these pathogens. 1 The aim of the present study was to determine the frequency and phenotypes of macrolide-resistant GABHS in a large sample of strains isolated in Greece; the in-vitro susceptibilities of these strains to clindamycin and ß-lactams were also determined.
Altogether, 534 consecutive non-replicate GABHS isolates were recovered from throat swabs obtained from children (age range, 1 month to 14 years) with pharyngo-tonsillitis who attended the P. & A. Kyriakou Children's Hospital as outpatients between August 1996 and July 1997. The isolates were identified according to standard laboratory techniques and Lancefield grouping was performed with the Streptococcal Grouping Kit (Oxoid Ltd, Basingstoke, UK). All strains were screened for susceptibility to erythromycin by a disc diffusion test according to a method and breakpoints recommended by the National Committee for Clinical Laboratory Standards (NCCLS). 2 MICs of erythromycin, clindamycin, penicillin G, cefaclor and cefprozil for all of the erythromycin-resistant GABHS and 40 randomly selected erythromycin-susceptible strains were determined by the Etest method (AB Biodisk, Solna, Sweden) on Mueller-Hinton agar supplemented with 5% sheep blood (SBMH). The plates were inoculated with bacterial suspensions with turbidities equivalent to that of a 0.5 McFarland standard and incubated overnight at 35°C in an atmosphere containing 5% CO 2. The isolates were assigned to susceptibility categories according to MIC breakpoints recommended by the NCCLS. 2 The phenotypes of the erythromycin-resistant strains were determined by the double-disc induction test with erythromycin and clindamycin, as described previously; 3 SBMH agar was the medium used and discs containing 15 IU of erythromycin or 2 IU of clindamycin were obtained from Sanofi-Diagnostics Pasteur (Marnes la Coquette, France).
Eighty-one (15.2%) of the 534 GABHS were resistant to erythromycin and a further 15 (2.8%) exhibited intermediate susceptibility. On the basis of the double-disc induction test, 39 (48.1%) of the 81 erythromycin-resistant strains exhibited inducible resistance to clindamycin (IR phenotype) 3,4 and 42 (51.9%) exhibited indifferent susceptibility, consistent with the so-called novel or M-resistance (MR) phenotype. 3,5 The MICs for the 81 erythromycin-resistant and 40 erythromycin-susceptible (ES) isolates are shown in the Table. All of the resistant isolates were either resistant or exhibited intermediate susceptibility to clarithromycin, but were susceptible to clindamycin. The MICs of clindamycin and the macrolides for the IR strains were marginally higher and, curiously, lower respectively than those for the MR strains. Predictably, the ß-lactam antibiotics tested were highly active against all of the GABHS (data not shown), with no differences in the MICs of these agents for the IR, MR and ES strains (MIC 90s of penicillin G, cefprozil and cefaclor, 0.016 mg/L, 0.023 mg/L and 0.125 mg/L, respectively).
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All of the isolates in the present study were susceptible to clindamycin. However, it has been documented that pressure by either macrolides or lincosamides on the IR population may select variants expressing high-level constitutive resistance to macrolide, lincosamide and streptogramin B antibiotics. 4
In conclusion, we have demonstrated that erythromycin-resistant GABHS are firmly established in this Greek community; this underscores the need to closely monitor trends in the susceptibility patterns of these pathogens.
Notes
* Corresponding author. Tel: 30-1-646-2281; Fax: 30-1-642 3498;
E-mail: evatzelepi{at}hotmail.com
References
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2 . National Committee for Clinical Laboratory Standards. (1995). Performance Standards for Antimicrobial Susceptibility TestingSixth Informational Supplement: Approved Standard M100-S6. NCCLS, Wayne, PA.
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6 . Cocuzza, C. E., Mattina, R., Mazzariol, A., Orefici, G., Rescaldani, R., Primavera, A. et al (1997). ( High incidence of erythromycin-resistant Streptococcus pyogenes in Monza (North Italy) in untreated children with symptoms of acute pharyngo-tonsillitis: an epidemiological and molecular study). Microbial Drug Resistance 3, 3718.