Synergism of aminoglycosides and carbenicillin against resistant strains of Serratia marcescens

Synergism at clinically attainable concentrations of aminoglycoside and beta-lactam antibiotics

We evaluated the in vitro synergistic activity at clinically attainable concentrations of combinations of aminoglycoside and beta-lactam antibiotics against 30 gentamicin-resistant clinical isolates of gram-negative bacilli. All 56 pairs of 4 aminoglycosides and 14 beta-lactams were evaluated. Combinations with amikacin demonstrated inhibitory synergistic activity in 29% of the assays, as compared with 22% for netilmicin (P = 0.018), 17% for gentamicin (P less than 0.001), and 13% for tobramycin (P less than 0.001). Among the beta-lactams, combinations with cefoperazone, ceftriaxone, or cefpiramide (SM-1652) demonstrated inhibitory synergistic activity most often (39, 38, and 35% of the assays, respectively) and with ceforanide, cefsulodin, and imipenem least often (less than or equal to 8% each). The most active combination was amikacin and ceftriaxone, with which 67% of the assays demonstrated inhibitory synergism. Isolates with high-level resistance to either antibiotic in a combination were unlikely to be inhibited synergistically by the combination. Further, combinations generally demonstrated little synergistic activity against isolates highly susceptible to beta-lactams.

Comparative in vitro synergistic activity of new beta-lactam antimicrobial agents and amikacin against Pseudomonas aeruginosa and Serratia marcescens

The in vitro synergistic activities of moxalactam, cefoperazone, or cefotaxime in combination with amikacin or piperacillin were compared against aminoglycoside-susceptible and aminoglycoside-resistant isolates of Pseudomonas aeruginosa and Serratia marcescens by the checkerboard agar dilution method. All antimicrobial combinations demonstrated some synergy, and no antagonism was observed. Moxalactam plus amikacin and piperacillin plus amikacin were most frequently synergistic (two-thirds of the isolates inhibited synergistically by each combination), whereas combinations of moxalactam, cefotaxime, or cefoperazone with piperacillin were synergistic against only 18 to 25% of the isolates. Moxalactam plus amikacin was the combination most often synergistic for amikacin-susceptible P. aeruginosa, and piperacillin plus amikacin was the combination most frequently synergistic for amikacin-resistant P. aeruginosa and amikacin-susceptible S. marcescens. These results demonstrate frequent in vitro synergistic activity between the new beta-lactam agents and amikacin (especially moxalactam or piperacillin with amikacin), but comparative clinical trials are needed to establish the relative efficacy and toxicity of these combinations.

Comparative susceptibilities of clinical isolates of Serratia marcescens to newer cephalosporins, alone and in combination with various aminoglycosides

We examined 100 clinically significant isolates of Serratia marcescens for susceptibility to newer cephalosporin and cephamycin antibiotics, alone and in combination with various aminoglycosides. Moxalactam and cefotaxime were the most effective agents; all isolates were inhibited by 25 and 50 micrograms/ml, respectively. All strains were susceptible to amikacin at concentrations safely achievable in serum, whereas gentamicin, netilmicin, and tobramycin inhibited 63, 63, and 16% of the isolates, respectively. Moxalactam, cefotaxime, and amikacin were active against gentamicin-susceptible and gentamicin-resistant strains. Studies of synergy revealed that moxalactam and cefotaxime, in combination with netilmicin or amikacin, were often synergistic and infrequently antagonistic against cephalothin- and gentamicin-resistant strains. These results suggest that moxalactam and cefotaxime, alone or in combination, may be efficacious in treating infections due to multiply antibiotic-resistant S. marcescens.