Fractional maximal effect method for in vitro synergy between amoxicillin and ceftriaxone and between vancomycin and ceftriaxone against Enterococcusfaecalis and penicillin-resistant Streptococcus pneumoniae

In the present study we assessed the use of a new in vitro testing method a nd graphical representation of the results to investigate the potential eff ectiveness of combinations of amoxicillin (AMZ) plus ceftriaxone (CRO) and of CRO plus vancomycin (VAN) against strains of Streptococcus pneumoniae hi ghly resistant to penicillin and cephalosporins (PRP strains). We used the fractional maximal effect (FME) method of time-kill curves to calculate ade quate concentrations of the drugs to be tested rather than relying on arbit rary choices. The concentrations obtained, each of which corresponded to a fraction of the maximal effect, were tested alone and in combination with t he bacterial strains in a broth medium. Synergy was defined as a ratio of o bserved effect/theoretical effect, called FME, of greater than 1, additivit y was defined as an FME equal to 1, and antagonism was defined as an observ ed effect lower than the best effect of one of the antibiotics used alone. The area between antagonism and additivity is the indifference zone. The we ll-known synergy between amoxicillin and gentamicin against a reference str ain of Enterococcus faecalis was confirmed, with a best FME equal to 1.07. Two strains of PRP, strains PRP-1 and PRP-2, were studied. The MICs for PRP -1 and PRP-2 were as follows: penicillin, 4 and 16 mug/ml, respectively; AM Z, 2 and 8 mug/ml, respectively, CRO, 1 and 4 mug/ml, respectively; and VAN , 0.5 and 0.25 mug/ml, respectively. For PRP-1 the best FME for the combina tion AMZ-CRO was 1.22 with drug concentrations of 1.68 mg/liter for AMZ and 0.17 mg/liter for CRO; the best FME for the combination VAN-CRO was 1.75 w ith VAN at 0.57 mg/liter and CRO at 0.17 mg/liter. For PRP-2 the best FME o btained for the combination AMZ-CRO was 1.05 with drug concentrations of 11 .28 mg/liter for AMZ and 0.64 mg/liter for CRO; the best FME obtained for t he combination VAN-CRO was 1.35 with VAN at 0.25 mg/liter and CRO at 1.49 m g/liter. These results demonstrated the synergy of both combinations, AMZ-C RO and VAN-CRO, against PRP strains at drug concentrations achievable in hu mans. Consequently, either of the combinations can be proposed for use for the treatment of PRP infections.