Prediction and significance of synergy between antibiotics used for treatment for gram-negative sepsis

Combination therapy for treatment of infections with gram-negative bacteria

Combination antibiotic therapy for invasive infections with Gram-negative bacteria is employed in many health care facilities, especially for certain subgroups of patients, including those with neutropenia, those with infections caused by Pseudomonas aeruginosa, those with ventilator-associated pneumonia, and the severely ill. An argument can be made for empiric combination therapy, as we are witnessing a rise in infections caused by multidrug-resistant Gram-negative organisms. The wisdom of continued combination therapy after an organism is isolated and antimicrobial susceptibility data are known, however, is more controversial. The available evidence suggests that the greatest benefit of combination antibiotic therapy stems from the increased likelihood of choosing an effective agent during empiric therapy, rather than exploitation of in vitro synergy or the prevention of resistance during definitive treatment. In this review, we summarize the available data comparing monotherapy versus combination antimicrobial therapy for the treatment of infections with Gram-negative bacteria.

In vitro comparison of cefpirome and four other beta-lactam antibiotics alone and in combination with tobramycin against clinical isolates of Pseudomonas aeruginosa

In vitro susceptibility studies of cefpirome versus cefotaxime, ceftazidime, imipenem, and piperacillin alone and in combination with tobramycin were performed against 153 clinical isolates of Pseudomonas aeruginosa from four medical centers. The minimal inhibitory concentration (MIC) for each antibiotic alone was determined by a standardized dilution method. Antibiotic combination studies were performed using a modified checkerboard technique. Cefpirome alone was more active (MIC90 64 micrograms/ml) than piperacillin (MIC90 128 micrograms/ml) or cefotaxime (MIC90 256 micrograms/ml) but less active than imipenem (MIC90 2 micrograms/ml) or ceftazidime (MIC90 32 micrograms/ml). The addition of tobramycin reduced the MICs of all of the beta-lactam antibiotics except for imipenem. The MIC90 for cefpirome when combined with tobramycin was 8 micrograms/ml compared to 16 micrograms/ml for cefotaxime and piperacillin, 8 micrograms/ml for ceftazidime, and 4 micrograms/ml for imipenem. The combination of tobramycin and cefpirome proved to be additive or synergistic for 82% of the isolates (highest rate) compared to 31% with imipenem (lowest rate). The potent in vitro antipseudomonal activity of cefpirome alone and in combination with an aminoglycoside (tobramycin) suggests that this agent may play a useful role in the therapy of infections due to P. aeruginosa.

In vitro synergistic activities of aminoglycosides and new beta-lactams against multiresistant Pseudomonas aeruginosa

The in vitro interactions between amikacin, netilmicin, tobramycin, gentamicin, and various antipseudomonal beta-lactams were studied by the agar dilution checkerboard technique against 30 Pseudomonas aeruginosa strains resistant to all tested antibiotics. Amikacin produced more frequent synergy both at the total and clinically applicable level. Among the beta-lactams, clinically relevant synergistic interactions were obtained in the following order: ceftazidime and ceftriaxone greater than moxalactam greater than aztreonam greater than cefotaxime greater than azlocillin greater than cefoperazone greater than cefsulodin greater than carbenicillin.

Synergism between aminoglycosides and cephalosporins with antipseudomonal activity: interaction index and killing curve method

Combinations of gentamicin with cefotaxime, moxalactam, and ceftazidime were tested against 43 bacterial strains, most of them blood isolates. With an interaction index of less than or equal to 0.5 as borderline, synergism was demonstrated against 30 to 40% of the strains by the fractional inhibitory concentration index and against 50 to 70% by the fractional bactericidal concentration index. The reproducibility of the index was within +/- 0.2 for two-thirds of 40 repetitive assays and within +/- 0.4 to 0.5 for all of these assays. Similar results were obtained when netilmicin was substituted for gentamicin. The killing curve system for studying antibiotic synergism was standardized to give results comparable to those obtained with the interaction index. This was achieved when one-half of a previously determined minimum bactericidal concentration was used for single drugs and the amount of antibiotic was at least halved again when drugs were used in combination. An initial bacterial concentration of 10(5) to 10(6) colony-forming units per ml is recommended. Given these conditions, synergism could be defined as a 2-log 10 or more decrease in viable count given by both drugs together, as compared with the more active of the pair after 24 h. Prediction of killing curve results could then be obtained with the fractional bactericidal concentration index. When cephalosporins and gentamicin were combined from the start, the beta-lactam antibiotics were less susceptible to inactivation, as demonstrated in time-killing assays. If one of the antibiotics were added after 24 h, synergism was not demonstrable. The results indicate that the new cephalosporins may be advantageously combined with aminoglycosides.