Evaluation of synergistic enhancement of antimicrobial activities of piperacillin and amikacin against Pseudomonas aeruginosa using FIC index and disc diffusion method

A preliminary study was carried out to see whether or not the synergistic enhancement of antimicrobial activity of piperacillin (PIPC) and amikacin (AMK) in combinations against Pseudomonas aeruginosa can be evaluated using combination discs of these antibiotics. A synergistic antimicrobial effect was observed between these antibiotics, when evaluated using FIC index, against piperacillin-sensitive strains with MIC less than or equal to 12.5 micrograms/ml under the conditions of the present investigation using the agar dilution method. However, no synergy was found between these antibiotics against piperacillin-resistant strains with MIC greater than or equal to 25 micrograms/ml, even when a higher concentrations of the drugs were used. Using strains of P. aeruginosa against which the synergistic effect between these antibiotics was observed, it was found that inhibitory diameters obtained with combination discs of PIPC/AMK at concentrations of 30 micrograms/30 micrograms or 30 micrograms/15 micrograms were more than 3 mm larger than those obtained with discs containing either PIPC or AMK alone. Using strains against which no synergistic effects were observed, however, the above mentioned enlargement of inhibitory zone diameters with combination discs were not clearly found. Further studies are deemed necessary to determine whether or not the combination discs of PIPC and AMK are useful in routine clinical work.