Objectives:

Streptococcus pyogenes, a major human pathogen, is still considered susceptible to betalactams, but for other relevant antibiotics highly variable resistance rates have been reported. Since the EC funded Strep-EURO project started in September 2002, a total number of 301 invasive and 500 non-invasive control strains were collected from different region of Sweden. One main objective was to study antibiotic susceptibility and type distribution of resistant strains.

Methods:

The strains used in this study were clinical isolates from different diagnostic laboratories in Sweden. The invasive strains were mainly blood isolates, but also from other sterile sites, whereas control strains were throat or skin isolates. Strains were maintained frozen at -80 °C in calf serum. The in vitro susceptibility to antibiotics was tested by disk diffusion on PDM agar following the instruction provided by the Swedish Reference Group for Antibiotics (homepage: http://www.srga.org). MICs of resistant strains were determined by the E-test (Biodisk AB) following the recommendations of the manufacturer. T typing was performed by slide agglutination according to previously documented methods using sera from Sevapharma, Prague.

Results:

Erythromycin resistance was uncommon (0.6%), whereas an overall high rate of tetracycline resistance was found (25–30%). MIC for tetracycline resistant strains varied between 8 and 64 mg/L with a clustering at 24 mg/L. The tetracycline resistant strains belonged to more than 10 different T types, the majority being types 3/3264, 13, 28 and B3264. Among Invasive strains T-types 3/3264, 3/13/3264 and 28 predominated.

Conclusion:

Since in Sweden tetracycline is used in the treatment of chlamydial and mycoplasmal rather than streptococcal infections the level of tetracycline resistance among GAS clinical isolates, 25–30%, appeared comparatively high. However, in certain countries much higher rates were recently reported. To account for resistance development, horizontal gene transfer from chlamydia or mycoplasma seems more unlikely than from other streptococci (groups B, C, D, G) especially with regard to increasing veterinary usage of tetracycline.

Acknowledgments:

The Strep-EURO project is funded by the European Commission. We are grateful to diagnostic laboratories providing strains and patient data to Strep-EURO.