An increasing number of S. suis strains showing resistance to commonly used antibiotics have been identified over the last years. The aim of the study was to determine the antibiotic susceptibility of Polish field S. suis strains using the broth microdilution method. The following antibiotics were included in the test: penicillin, ampicillin, ceftiofur, gentamicin, neomycin, spectinomycin, erythromycin, tylosin, tiamulin, chlotretracycline, oxytetracycline, enrofloxacin, sulphadimetoxin, sulphachloropyridazine, sulphathiazole, sulphamethoxazole with trimethoprim and florphenicol. The S. suis strains tested were collected between 1996-2005 from pigs indicating nervous symptoms, respiratory tract disorders and/or arthritis. The necropsy hyperaemia of meninges revealed inflammation of lungs and joints and the presence of serofibrinous exudate on the pericardium. The results of the study demonstrate the high level of susceptibility of tested strains to b-lactams (about 100% of susceptible strains), florphenicol (98%), gentamycin and a combination of sulphamethoxazole and trimethoprim (95%), enrofloxacin and spectinomycin (90.37%). A large percentage of strains were resistant to chlortetracycline (49%), oxytetracycline (40%), erytromycin and tylosin (50%) and to sulphonamides (50.37%). Most strains were susceptible to the lowest concentrations of b-lactams and sulphametoxazole with trimethoprim. The greatest antimicrobial activity was obtained for florphenicol, enrofloxacin, spectinomycin and gentamicin when the highest concentration of antibiotics was used in the study. The presented study confirms the high susceptibility of Polish field S. suis strains to the antibiotics recommended for treating streptococci. Positive therapeutic effects should be expected after using b-lactams, florphenicol and sulphonamides with trimethoprim, gentamicin and fluoroquinolones.
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