Antimicrobial susceptibility of Streptococcus suis isolates.

The antimicrobial activities of penicillin (PEN), ampicillin (AMP), cephalothin (CT), trimethoprim-sulphamethoxazole (TMP-SMX), streptomycin (STM), and gentamicin (GM) against 122 representative strains of Streptococcus suis, were compared by the agar dilution procedure. The current US National Committee for Clinical Laboratory Standards (NCCLS) breakpoints for non-enterococcal streptococci were used for PEN, AMP, CT, and TMP-SMX. Overall, 50% of strains were not fully susceptible to PEN, whereas these percentages for AMP and CT were 9% and 6% respectively. One strain was resistant to TMP-SMX. High-level GM resistance could not be detected, but more than 46% of strains were highly resistant to STM (MIC > 2000 mg l-1). This high percentage of resistance to STM precludes the use of this aminoglycoside-penicillin combination as empiric therapy in severe S. suis infections. These results should prompt microbiology laboratories to carry out antimicrobial susceptibility tests on a routine basis on S. suis isolates.