Fifty-three clinical Streptococcus pneumoniae isolates from Hungary and Spain which were either moderately susceptible to penicillin G (MIC 0.25-1 mg/l) or highly resistant (MIC greater-than-or-equal-to 2 mg/l) were studied for their antibacterial susceptibility to the following 14 agents: penicillin G, amoxicillin, mezlocillin, cefotaxime, ceftriaxone, amikacin, cotrimoxazole, erythromycin, rifampicin, vancomycin, fosfomycin, doxycycline, ciprofloxacin, and the dual-action compound Ro 23-9424. Rifampicin was the most active compound (MIC90 less-than-or-equal-to 0.015 mg/l) followed by imipenem (MIC90 0.25 mg/l), vancomycin (MIC90 1 mg/l), cefotaxime and ceftriaxone (MIC90 1 mg/l), the dual-action compound Ro 23-9424 and amoxicillin (MIC90 2 mg/l). Most of the isolates were less sensitive to ciprofloxacin (MIC90 4 mg/l), doxycycline and erythromycin (MIC90 16 mg/l) and resistant to cotrimoxazole (MIC90 640 mg/l). Furthermore, synergism between penicillin G or ceftriaxone on the one hand, amikacin or rifampicin on the other hand was studied in all 53 isolates. A combination of a beta-lactam with amikacin resulted mostly in an additive effect, as was the case for the rifampicin combinations. The selection of rifampicin-resistant clones can be repressed by the addition of a beta-lactam thus suggesting such a combination of rifampicin with a beta-lactam as a possible therapeutic approach.
