Efficacy of amikacin and ceftazidime in experimental aortic valve endocarditis due to Pseudomonas aeruginosa

Abstract

The in vivo efficacies of amikacin, ceftazidime, and their combination were evaluated in experimental aortic valve endocarditis due to Pseudomonas aeruginosa. Eighty catheterized rabbits were infected with a P. aeruginosa strain susceptible to both amikacin and ceftazidime and then received no therapy (controls), amikacin (15 mg/kg per day), ceftazidime (100 mg/kg per day), or amikacin-ceftazidime. Amikacin Morrison, Chemotherapy [Basel], in press). In that study, the amikacin-ceftazidime regimen was significantly more effective than the single-drug regimens in reducing the mean vegetation titers of P. aeruginosa and reducing the prevalence of bacteriologic relapses after therapy was terminated. In the current study, we examined this synergistic combination in experimental left-sided P. aeruginosa endocarditis, the most refractory syndrome in human cases (16 identification, serotyping, and resistance to rabbit serum have been previously described (5).Antibiotic susceptibility testing. MICs and MBCs of amikacin and ceftazidime were determined by the broth macrodilution technique (13) with Mueller-Hinton broth (MHB; BBL Microbiology Systems, Cockeysville, Md.). MHB has a concentration of 0.5 mg/100 ml of magnesium and 3.2 mg/100 ml of calcium. Logarithmic-growth-phase cultures were used to prepare inocula of PA-96 at -5 x 105 or -5 x 107 CFU/ml. The -107 inoculum was utilized to determine the antibiotic susceptibility at bacterial densities likely to be encountered within cardiac vegetations in vivo (1). The MIC was defined as the lowest antibiotic concentration causing no visible turbidity. The MBCs were determined by subculturing 10 ,ul from each clear tube onto antibiotic-free MH agar plates. The MBC was the lowest concentration of antibiotic that effected -99.9% reduction of the original inoculum. The MICs and MBCs of selected amikacin-or ceftazidime-resistant organisms isolated from endocardial vegetations were also determined for gentamicin, kanamycin, moxalactam, cefoperazone, azlocillin, mezlocillin, and ciprofloxacin (inoculum size, -5 x 105 CFU/ml).In vitro bactericidal interaction testing. Bactericidal interactions of ceftazidime and amikacin against PA-96 were examined with the time-kill assay system. As before, a logarithmic-phase inoculum of either -5 x 105 or -5 x 107 CFU/ml was used. The final antibiotic concentrations tested were amikacin alone at one-half the MBC, ceftazidime alone at one-half the MBC, or amikacin-ceftazidime, each drug at one-quarter its MBC. A synergistic interaction was considered present when the amikacin-ceftazidime combination caused a -2 log10 decrease in CFU per milliliter at 24 h in comparison with the more active single agent (10).

Bactericidal interactions of a beta-lactam and beta-lactamase inhibitors in experimental Pseudomonas aeruginosa endocarditis caused by a constitutive overproducer of type Id beta-lactamase

“…Recent experiences in our laboratory and others have demonstrated that in vivo development of P-lactam resistance is an important determinant of therapeutic efficacy in treating invasive pseudomonal infections (1,16,19,22,23). This problem has remained prevalent despite the development of cephalosporins with potent antipseudomonal activity in vitro and which are relatively resistant to P-lactamase hydrolysis (e.g., ceftazidime, cefoperazone).…”

“…One of the limiting factors in this regard, as exemplified in human as well as experimental Pseudomonas endocarditis, has been in vivo development of antibiotic resistahce. Such resistances have been variably directed towards the aminoglycoside or P-lactam components of the combination therapy regimens generally used to treat Pseudomonas endocarditis (1,19,23,26). The mechanism of P-lactam resistance in these situations has usually been associated with either inducible or constitutive P-lactamase overproduction (3,19,26).…”

“…Organism. The Pseudotnonas strain used in this study (PA-48) is a ceftazidime-resistant mutant strain isolated from cardiac vegetations during the therapy of experimental endocarditis with ceftazidime monotherapy (1). The identification, serotyping and rabbit serum resistance of the parental strain for PA-48 used to initially infect rabbits have been defined previously (5).…”

“…(Bristol). Transaortic valve catheterization was performed with a polyethylene catheter by the transcarotid approach as previously described (1). Each animal was then inoculated intravenously with _108 CFU of PA-48 at 24 h postcatheterization.…”

“…Animals were sacrificed by sodium pentobarbital overdosage after 3 or 6 days of treatment. At sacrifice, aortic valve vegetations were excised, weighed, and quantitatively cultured as previously described (1) [4]). …”