In Vitro Activity of Ceftazidime and Meropenem in Combination with Tobramycin or Ciprofloxacin in A Clone of Multiresistant Pseudomonas Aeruginosa

Therapeutic options to fight against infections caused by mu ltiresistant (MR) Pseudomonas aeruginosa strains are restricted to a few antimicrobials such as colistimethate and amikacin. The purpose of this study was to compare in vitro synergy testing by epsilo metric (E-test) and the checkerboard (CB) methods with time-kill analysis in MR P. aeruginosa clin ical isolates. Four isolates belonging to a MR endemic clone were selected. Their resistance mechanisms were studied. Susceptibility to ceftazidime (CAZ) and meropenem (M EM ) in co mbination with tobramycin (TOB) and ciproflo xacin (CIP) were tested. Synergy was consistently detected in CAZ plus TOB as well as in M EM p lus TOB co mb inations. The E-test method was comparable to CB method. Synergy and bactericidal act ivity were observed at 1/4 or 1/ 8 MIC TOB concentration combined with 1 M IC of CAZ or 1 MIC of M EM by time kill curves, with slight differences in the two isolates tested. These findings indicate the possibility of designing therapies based on combinations of a β-lactam and an aminoglycoside as a therapeutic option in infections caused by MR P. aeruginosa.