In vitro activity of multiple antimicrobial combinations against Pseudomonas cepacia isolates

For 16 isolates of 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 x T)]. All 16 isolates were resistant to Ti, To, Ri, and PB; 11 were sensitive to S x 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.