The synergistic effect of gentamicin and ceftazidime against Pseudomonas fluorescens

along with the fractional inhibitory concentration index (FICI) to test for synergy. A value of ≤0.5 defined synergy; 0.5 < FICI < 4 defined no interaction; ≥4 defined antagonism. No results of synergy were found; there were five results of no interaction and six results of antagonism. The MIC of ceftazidime was 3 µg/ml and the MBC was 4 µg/ml. The MIC of gentamicin was 0.25 µg/ ml and the MBC was 3 µg/ml. The combination of gentamicin and ceftazidime is optimal at a volume ratio of 1:1, in this case 25 µl gentamicin/25 µl ceftazidime, where gentamicin has a concentration of 0.5 µg/ml and ceftazidime has a concentration of 0.25 µg/ml, when used against 50 µl of 1–2 × 10 6 colony forming units per millilitre of P. fluorescens in vitro. This study recommends that this combination therapy be studied in depth in vivo, and that clinicians understand that this combination of antibiotics does not have a synergistic effect when treating patients in this manner.