In vitro Activity of Multiple Antimicrobial Combinations against Pseudomonas cepacia Isolates

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Abstract
For 16 isolates for Pseudomonas cepacia from patients with cystic fibrosis, synergism was determined by the killing curve method, utilizing various combinations of antibiotics [ticarcillin (Ti), rifampin (Ri), tobramycin (To), imipenem (Im), ofloxacin (Of), polymyxin B (PB), and trimethoprim/sulfamethoxazole (S .times. T)]. All 16 isolates were resistant to Ti, To, Ri, and PB; 11 were sensitive to S .times. T, 13 were sensitive to Im, and 2 were sensitive to OF. The combination of IM/Ri was synergistic for 8 isolates. Several combinations (Ti/Ri/Im), (Ti/Im/To), (Ri/Im/To) also demonstrated synergism for these 8 isolates. The Im/Ri combination did not demonstrate synergism against the other 8 isolates. The Ti/Im/To combination was synergistic for 6 of these. The Ri/Im/To combination was synergistic for 5 and the Ti/Im/Ri combination was synergistic for 3. Synergism could also be demonstrated with several 4-drug combinations. This study suggests that multi-drug combinations may have a role in the treatment of P.cepacia infections if proven in clinical studies.