Synergy between amoxicillin and gentamicin in combination against a highly penicillin-resistant and -tolerant strain of Streptococcus pneumoniae in a mouse pneumonia model

In vivo synergy with b-lactam antibiotics and aminoglycosides has been studied only with penicillinsusceptible Streptococcus pneumoniae strains. We evaluated the interaction between amoxicillin (AMX) and gentamicin (GEN) on the basis of in vitro checkerboard and time-kill curves and offindings in a mouse model of acute bacteremic pneumonia due to a highly penicillin-resistant and -tolerant S. pneumoniae strain of serotype19(penicillinandAMXMICsof4 mg/ml;gentamicinMICof16 mg/ml).Checkerboardresultsat18h of incubation showed indifference. With regard to AMX alone, in vitro time-kill curves demonstrated synergy between AMX (1 mg/ml) and GEN (16 mg/ml) at 5 an d8ho fincubation and for AMX (16 mg/ml) in combination with GEN (16 mg/ml) at 3, 5, and8hofincubation. In leukopenic mice, pulmonary killing curves after a single drug injection demonstrated that AMX (100 mg/kg of body weight) with GEN (16 mg/kg) was more effective than AMX alone (P 510 24 ). With repeated-dose treatment, a synergy was apparent at 8 h after four injections with AMX (100 mg/kg) in combination with GEN (8 or 16 mg/kg) (P <0.05). The cumulative survivalratewithAMX(100mg/kg)every8h,combinedwithGEN(4or8mg/kg)every8,12,or24h,wasbetter than with AMX alone. Combined use of AMX and GEN may be a valuable therapeutic alternative for pneumococcal pneumonia due to highly penicillin-resistantS. pneumoniaestrains. The worldwide increase in resistance ofStreptococcus pneumoniaeto antibiotics has limited the number of antimicrobial agents that produce reliable results in the treatment of pneumococcal infections. Although experimental and clinical data indicate that increasing the dosage of b-lactam antibiotics seems adequate for pneumonia due to penicillin-resistant strains (7, 17), the management of infections caused by highly penicillin-resistant strains (MIC . 2 mg/ml) is challenging. Extended-spectrum cephalosporins (ceftriaxone, cefotaxime) maybeusefulalternatives,buthighlevelsofresistancetothese antibiotics are now found (MIC .1 mg/ml) (17). The combination of penicillin and gentamicin (GEN) has been shown to act synergistically in vitro and in vivo against enterococci, viridans streptococci, Staphylococcus aureus, and group A and B streptococci (5). Few studies of potential antipneumococcal synergy between b-lactam antibiotics and aminoglycosides have been reported, and in vivo synergy has been studied only with penicillin-susceptible strains (9, 12, 13). We evaluated the interaction between amoxicillin (AMX) and GEN on the basis of in vitro checkerboard and time-kill curves and offindings in a mouse model of acute pneumonia due to a highly penicillinresistantS. pneumoniaestrain of serotype 19 (AMX and penicillin MICs 54 mg/ml) from middle earfluid (strain P15986,