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Efficacy of beta-lactam antibiotics combined with gentamicin against penicillin-resistant pneumococcal pneumonia in CBA/J mice.

We examined the efficacy of gentamicin combined with beta-lactam antibiotics against penicillin-resistant Streptococcus pneumoniae (PRSP) in a noncompromised mouse model of pneumonia. In the presence of 8 mg/L (0.25 x MIC) and 16 mg/L (0.5 x MIC) of gentamicin, MICs of penicillin G against 23 strains of PRSP decreased from 1-4 mg/L to 0.03 mg/L in 14 (61%) and 23 strains (100%), respectively. A short-time killing study using strain 741 showed that 8 mg/L of gentamicin (0.25 x MIC) increased the killing activity of penicillin G, cefotaxime and imipenem (at 0.25, 1 and 4 x MIC) during a 6 h incubation period. Survival studies showed that the combined treatment of penicillin-G (160 mg/kg) and gentamicin (10 mg/kg), which commenced 2 days after infection (twice a day for 5 days), provided complete protection, while no animal survived when either antibiotic was used alone. A significant improvement in mortality was observed when a small dose of imipenem (2.5 and 10 mg/kg) was used with gentamicin. Our results suggest that gentamicin, when combined with beta-lactam antibiotics, especially imipenem, may be potentially useful against PRSP pneumonia in noncompromised individuals.