SUSCEPTIBILITY TO ANTIMICROBIAL AGENTS OF STREPTOCOCCUS-SUIS CAPSULARTYPE-2 STRAINS ISOLATED FROM PIGS

The minimal inhibitory concentrations (MICs) for thirty-three epidemio logicaly unrelated clinical isolates of Streptococcus suis capsular ty pe 2 were determined in relation to ampicillin, ampicillin-sulbactam, amoxicillin, clavulanate-amoxicillin, penicillin G, cephalexin, gentam icin, streptomycin, erythromycin, tylosin and doxycycline, using the m icrotitre broth dilution procedure described by the U.S. National Comm ittee for Clinical Laboratory Standards (NCCLS). Gentamicin was the mo st active compound tested, with an MIC for 90% of the strains tested ( MIC(90)) of 0.4 mg/L. Overall, 70% of strains were resistant to doxycy cline (MIC(90) greater than or equal to 100.0 mg/L), followed by penic illin G (51% of strains) (MIC(90) 100.0 mg/L). Resistance to amoxicill in and ampicillin was 36.4% (MIC(90) 12.5 mg/L) and 33.3% (MIC(90) 50. 0 mg/L), respectively. 15.2% of S. suis strains were resistant to stre ptomycin, tylosin and cephalexin with MIC(90) values of 25.0 mg/L, 12. 5 mg/L and 25.0 mg/L, respectively. A combination of ampicillin and su lbactam (MIC(90) 6.3 mg/L) and a combination of amoxicillin and clavul anate (MIC(90) 3.1 mg/L) as well as erythromycin (1.6 mg/L) were of th e same efficacy, with a total of 9.1% resistant S. suis strains. This high percentage of resistance to doxycycline and penicillin G preclude s the use of these antibiotics as empiric therapy of swine diseases.