An approach for the evaluation of synergy between antimicrobials.

Synergy can be assessed by a variety of microbiological techniques. Mathematical modelling of synergy data can accomplish similar results and may provide more information than currently available techniques. In this study a combination of 1/4 MIC of aztreonam (AZTR) and 1 MIC of ciprofloxacin (CIPX) showed synergy against Pseudomonas aeruginosa in killing curve studies (based on the two log different criteria) at the end of 24 h. However, re-growth was always observed even when bactericidal concentrations of antibiotics were present. The surviving organisms showed a two-fold increase of MIC to CIPX after the studies and this resistance was not reversible. A mathematical model to quantitatively describe this observation is proposed.