Isolates showed low levels of resistance to
ampicillin (4.0%) and penicillin (9.5%). The MIC90
of both antibiotics were 2 mg/mL, within the intermediate
breakpoint range. Intensive use of ceftiofur in
swine production resulted in higher resistance (22.1%)
than ampicillin and penicillin, but the mechanism of
resistance of S. suis strains to the three antibiotics was
unknown.