Amy E. Morgan

With antibiotic resistance becoming a significant problem in recent years, methodologies to overcome resistance have quickly become a necessity. One such mechanism to overcome resistance is to use antibiotics in combination. Clinical advice recommends the use of gentamicin and ceftazidime in combination to treat severe Pseudomonas aeruginosa infections. However, there is little evidence to support this recommendation. This study proposed that this recommendation is due to a synergistic effect and aimed to determine the optimum combination treatment, using Pseudomonas fluorescens as a model organism. Potentially, this research could give reason to a medical recommendation and even instigate a change in this treatment strategy in a clinical setting. To find the minimal inhibitory concentration (MIC) of the antibiotics used singularly, varying concentrations of each antibiotic and P. fluorescens were placed in each well of a microtitre plate and incubated at 30°C for 24 h. Wells determined to have no growth were re-plated on nutrient agar and incubated at 30°C for 22 h for minimal bactericidal concentration (MBC) testing. When testing gentamicin and ceftazidime in combination, the checkerboard method was employed 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 × 10⁶ 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.