Treatment of experimental endocarditis due to penicillin-resistant Streptococcus pneumoniae.

Using two strains of pneumococci for which MICs of penicillin were 1 and 4 micrograms/ml, those of cefotaxime were 0.01 and 0.5 micrograms/ml, and those of teicoplanin were 0.01 and 0.1 micrograms/ml, we studied the efficacy of different dosages of penicillin, cefotaxime, and teicoplanin in the treatment of experimental pneumococcal endocarditis in rabbits. Animals treated with dosages of penicillin G procaine needed to achieve levels in serum near the MIC for pneumococci showed a significant reduction in log10 CFU per gram of vegetation, as compared with the control (P < 0.001), although only 20% of the animals showed sterile vegetations. When levels of penicillin in serum were in the range of three- to fourfold the MIC, a greater reduction in log10 CFU per gram of vegetation was seen, and 88% of the animals showed sterile vegetations. Only the regimen of penicillin that provided concentrations in serum above the MIC throughout the interval between two doses provided constant sterilization of the cardiac vegetations. Dosages of cefotaxime and teicoplanin selected to achieve concentrations in serum equivalent to that obtained in humans during treatment resulted in levels of antimicrobial agents in serum hundreds or thousands of times higher than the MICs for the infecting strains. In terms of antimicrobial efficacy, cefotaxime and teicoplanin were equivalent to regimens with high dosages of penicillin.     

Ampicillin therapy of experimental enterococcal endocarditis.

In rabbits with experimental enterococcal endocarditis, subcutaneously implanted perforated polyethylene chambers were used for ampicillin administration by intra-chamber injection. A total of 21 days of intra-chamber ampicillin therapy sterilized vegetations of 14 out of 14 rabbits with experimental enterococcal endocarditis. In rabbits treated for less than 21 days, the duration of therapy and quantitative vegetation cultures were inversely related. Peak serum minimal bactericidal titers were greater than or equal to 1:8 in 94% of the determinations. Trough serum minimal bactericidal titers were less than or equal to 1:2. The mean trough serum ampicillin concentration (2.6 micrograms/ml) was greater than the minimal bactericidal concentration of ampicillin for the infecting enterococcus and less than the mean trough chamber fluid ampicillin concentration (3.7 micrograms/ml). Relatively prolonged therapy with intrachamber injections seemed to be well tolerated. Combination drug therapy of enterococcal endocarditis may not always be required. The maintenance of serum minimal bactericidal titers greater than or equal to 1:8 throughout the therapy of endocarditis, as is often recommended, may be unnecessary.     

Efficacy of teicoplanin in two dosage regimens for experimental endocarditis caused by a beta-lactamase-producing strain of Enterococcus faecalis with high-level resistance to gentamicin.

Optimal therapy for the treatment of infections caused by strains of enterococci demonstrating high-level resistance to gentamicin and other aminoglycosides has not been established. The present study examined the efficacy of teicoplanin, a glycopeptide antibiotic active against gram-positive bacterial infections in various animal models, in the treatment of experimental endocarditis due to a beta-lactamase-producing strain of Enterococcus faecalis with high-level resistance to gentamicin. Vancomycin was used as a comparative antibiotic. In the first set of experiments, both antimicrobial agents were administered by continuous intravenous infusion for 5 days at dosages which yielded comparable mean levels in serum (plus or minus the standard deviation) of 14.6 +/- 4.3 micrograms/ml for teicoplanin and 14.3 +/- 2.2 micrograms/ml for vancomycin. These regimens proved similarly effective in sterilizing cardiac vegetations (38 versus 50% of treated animals, respectively; P greater than 0.05). Mean (plus or minus the standard deviation) residual bacterial titers within vegetations were reduced to 3.2 +/- 1.2 log10 CFU/g and 3.4 +/- 1.7 log10 CFU/g, respectively. In separate experiments, the potential of teicoplanin to cure endocarditis was assessed, using two dosage regimens: (i) 30 mg/kg per day (mean level in serum, 13 micrograms/ml) for 10 days or (ii) 150 mg/kg per day (mean level in serum, 84 micrograms/ml) for 5 days. Surviving animals were sacrificed 10 days after the discontinuation of therapy. Both teicoplanin regimens were more effective than the comparative vancomycin (150 mg/kg per day) regimen: 92 versus 43% cured (P =0.025) in the standard-dose group, and 82 versus 37% cured (P = 0.015) in the high-dose group. Results in this rat model of enterococcal endocarditis show that teicoplanin may prove useful in the treatment of serious infections due to high level-gentamicin-resistant enterococci in humans.     

Enoxacin compared with cefoperazone for the treatment of experimental Enterobacter aerogenes endocarditis.

This study compared enoxacin administered orally with cefoperazone administered intramuscularly for the treatment of Enterobacter aerogenes endocarditis in rabbits. The MICs and MBCs of both enoxacin and cefoperazone for an inoculum of 10(5) CFU/ml of the E. aerogenes strain used were 0.8 micrograms/ml, respectively. With an inoculum of 10(8) organisms per ml, enoxacin at 2 and 5 micrograms/ml and cefoperazone at 60 and 155 micrograms/ml were effective in reducing titers of E. aerogenes in broth. E. aerogenes endocarditis in rabbits was treated with enoxacin (100 or 25 mg/kg orally every 6 h) or cefoperazone (60 mg/kg intramuscularly every 6 h) for 5 or 10 days. Enoxacin at 100 and 25 mg/kg significantly reduced bacterial titers of vegetations compared with those of untreated controls. Enoxacin at 100 mg/kg was significantly more effective than enoxacin at 25 mg/kg and cefoperazone. Enoxacin at 25 mg/kg and cefoperazone did not differ significantly. Cefoperazone and controls did not differ significantly. In uninfected rabbits single doses of cefoperazone achieved much higher concentrations in serum than single doses of enoxacin (25 and 100 mg/kg). The half-lives of enoxacin at 25 and 100 mg/kg were approximately three times longer than that of cefoperazone.     

Critical influence of timing of administration of granulocyte colony-stimulating factor on antibacterial effect in experimental endocarditis due to Pseudomonas aeruginosa.

The effect of human recombinant granulocyte colony-stimulating factor (hrG-CSF) in rabbits with aortic endocarditis due to Pseudomonas aeruginosa was investigated. hrG-CSF significantly increased the number of polymorphonuclear neutrophils in blood and in cardiac vegetations and the expression of the adhesin molecule CD11b on the surface of polymorphonuclear neutrophils compared with those of animals that had not received hrG-CSF. When treatment was started 72 h after bacterial challenge, hrG-CSF alone had no antibacterial effect and did not enhance the efficacy of ciprofloxacin when used in combination, even with the higher dosing regimen used (50 micrograms/kg of body weight subcutaneously every 12 h for 4 days), in terms of number of positive blood cultures, bacterial counts in vegetations, and survival. In contrast, when treatment was started 30 min prior to bacterial challenge, hrG-CSF (50 micrograms/kg injected every 12 h) decreased bacterial titers in vegetations 72 h later (6.5 +/- 0.9 versus 7.9 +/- 0.9 log10 CFU/g of vegetation for hrG-CSF and controls, respectively; P = prophylactic administration of hrG-CSF did not increase the antibacterial effect of ciprofloxacin. We concluded that the antibacterial effect of hrG-CSF in experimental endocarditis was related to the timing of its administration since hrG-CSF demonstrated a significant but transient antimicrobial effect only when treatment was initiated before bacterial challenge.     

In vitro comparative antimicrobial activity of cefpimizole against clinical isolates from five medical centers.

Cefpimizole was compared with cefoperazone and cefotaxime against 6,599 clinical bacterial isolates from five medical centers. Cefoperazone and cefotaxime were both more active and provided a greater spectrum of antimicrobial coverage than cefpimizole. Some of the cefpimizole minimum concentrations inhibiting 50% of tested strains were as follows: Citrobacter freundii and Enterobacter cloacae, 16 micrograms/ml; Escherichia coli and Klebsiella pneumoniae, 2.0 micrograms/ml; Proteus mirabilis, 1.0 microgram/ml; Pseudomonas aeruginosa, 16 micrograms/ml; Staphylococcus spp., 32 micrograms/ml; and the enterococci, greater than 32 micrograms/ml.     

Correlation of in vitro activities of cephalothin and ceftazidime with their efficacies in the treatment of Staphylococcus aureus endocarditis in rabbits.

Rabbits with Staphylococcus aureus endocarditis were treated with cephalothin or ceftazidime to determine whether differences in in vitro activity would result in differences in in vivo efficacy. Antibiotics were administered in doses equivalent to maximum recommended human doses, and results of laboratory tests to predict antimicrobial efficacy were determined during treatment. Cephalothin and ceftazidime MICs for the challenge strain were 0.5 and 8 micrograms/ml, respectively. MBCs were 32 and greater than 128 micrograms/ml, respectively. With peak sera, laboratory results (means) for cephalothin and ceftazidime were as follows: ratios of concentration in serum to MIC, 300 and 16; ratios of concentration in serum to MBC, 4.8 and less than 1; bacteriostatic antibacterial activity titers in serum, 1:256 and 1:16; and bactericidal antibacterial activity titers in serum, 1:16 and 1:4, respectively. Trough sera contained little or no measurable antibiotic and had no antibacterial activity. Both cephalothin and ceftazidime were efficacious in the treatment of infected rabbits. There were no statistically significant differences in efficacy as defined by survival, eradication of bacteremia, or sterilization of cardiac vegetations. Results of laboratory tests which quantitated antimicrobial activity did not correlate with efficacy, either independent of antibiotic or adjusted for antibiotic. Despite their lesser in vitro activities, the new cephalosporins may be equivalent to the older cephalosporins for treating staphylococcal infections in humans, when administered in maximum recommended doses.     

Development of beta-lactam resistance and increased quinolone MICs during therapy of experimental Pseudomonas aeruginosa endocarditis.

The in vivo efficacies of pefloxacin, a new fluoroquinolone, and amikacin-ceftazidime were compared in 50 rabbits with experimental aortic endocarditis caused by Pseudomonas aeruginosa. Animals were randomly chosen to receive 4 or 10 days of no therapy (controls), pefloxacin (40 mg/kg [body weight] per day, intramuscularly [i.m.]), or amikacin (30 or 80 mg/kg per day, i.m.)-ceftazidime (150 mg/kg per day, i.m.). Pefloxacin and both amikacin regimens significantly reduced vegetation bacterial densities compared with controls at days 4 and 10 of treatment (P less than 0.0005). By day 10 of therapy, between 33 and 40% of vegetations from amikacin-ceftazidime recipients contained ceftazidime-resistant bacteria (MICs, greater than 25 micrograms/ml); nitrocefin agar overlay confirmed that these ceftazidime-resistant variants were constitutive overproducers of beta-lactamase. At therapy days 4 and 10, approximately 30% of vegetations sampled from pefloxacin recipients contained bacteria for which pefloxacin MICs were four- to eightfold higher than the MIC for the parental strain used to initially induce endocarditis (MIC, 0.19 microgram/ml). These variants also exhibited increases in ciprofloxacin and ticarcillin MICs, as well as pleotropic resistance to chloramphenicol (but not to amikacin, ceftazidime, or tetracycline). Amikacin-ceftazidime, as well as pefloxacin, was effective in this model of aortic pseudomonal endocarditis. However, in vivo development of ceftazidime resistance and step-ups in pefloxacin MICs among intravegetation isolates were associated with inability to completely eradicate P. aeruginosa from aortic vegetations.     

Treatment of experimental endocarditis due to methicillin-susceptible or methicillin-resistant Staphylococcus aureus with trimethoprim-sulfamethoxazole and antibiotics that inhibit cell wall synthesis.

Using two strains of Staphylococcus aureus, one susceptible and one heterogeneously resistant to methicillin, for which MICs and MBCs of trimethoprim-sulfamethoxazole (TMP-SMX) were 0.06 and 0.06 micrograms/ml and 0.06 and 0.25 microgram/ml, respectively (concentrations are those of TMP), we studied the efficacies of TMP-SMX and cloxacillin, teicoplanin, and vancomycin for treatment of experimental staphylococcal endocarditis. Rabbits were treated with dosages of TMP-SMX selected to achieve concentrations in serum equivalent to that obtained in humans treated for Pneumocystis carinii pneumonia. The overall mortality rate of rabbits treated with TMP-SMX was 84% at day 3, not different from that of the control groups (P > 0.1). No sterile vegetations were observed to be present in control groups or in animals treated with TMP-SMX. However, 26, 60, and 75% of rabbits treated with teicoplanin, cloxacillin, and vancomycin, respectively, showed sterile vegetations. For methicillin-susceptible S. aureus (MSSA), the mean vegetation counts were not significantly different between the control group and the group treated with TMP-SMX (P > 0.1). For methicillin-resistant S. aureus (MRSA), treatment with TMP-SMX was more effective than no therapy, decreasing the number of organisms in vegetations (P < 0.01). For both strains, therapy with cloxacillin and therapy with teicoplanin or vancomycin were significantly more effective than therapy with TMP-SMX. Despite high concentrations of teicoplanin in serum which exceeded MBCs for staphylococci more than 50 times at the peak and 10 times at the trough, therapy with cloxacillin or vancomycin was superior to therapy with teicoplanin against both MSSA and MRSA. These data do not support the use of TMP-SMX in treatment of endocarditis and other severe staphylococcal infections with high bacterial counts.     

Effect of serum on the in vitro activities of 11 broad-spectrum antibiotics.

We evaluated the effect of serum on the in vitro activities of 11 antimicrobial agents against gram-negative isolates obtained from 100 patients with nosocomial bacteremia. The test organisms included 25 stains of Pseudomonas aeruginosa and 75 strains of the family Enterobacteriaceae. MICs were determined by broth microdilution with Mueller-Hinton broth alone or supplemented with 25 or 50% pooled, heat-inactivated human serum (25S or 50S, respectively). Among the antibiotics evaluated, the protein binding ranged from 9 to 95%. The antibiotics tested and their MICs for 90% of the strains tested in 50S included ciprofloxacin (0.12 micrograms/ml), ceftazidime (1 micrograms/ml), imipenem (1 micrograms/ml), aztreonam (4 micrograms/ml), cefpirome (4 micrograms/ml), cefotaxime (16 micrograms/ml), cefoperazone (16 micrograms/ml), desacetylcefotaxime plus cefotaxime (32 micrograms/ml), ceftriaxone (greater than 32 micrograms/ml), ticarcillin (128 micrograms/ml), and desacetylcefotaxime (greater than 128 micrograms/ml). MICs for 90% of the strains tested were calculated with 95% confidence intervals to show the precision of the MICs for these strains. With the exceptions of ceftriaxone (greater than 95% protein bound) and cefoperazone (90% protein bound), serum had no significant effect on the in vitro activities of various agents. A fourfold-or-greater increase in the MIC of ceftriaxone was observed in 45 of 100 isolates with 50S and in 30 of 100 isolates with 25S. With cefoperazone, 17 of 100 isolates demonstrated more than 2 twofold dilution increases in 50S. Testing of antibiotics which were less protein bound illustrated minor effects primarily with members of the Enterobacteriaceae. The presence of serum did not adversely affect the in vitro activities of broad-spectrum agents against these nosocomial isolates.     

Enoxacin compared with vancomycin for the treatment of experimental methicillin-resistant Staphylococcus aureus endocarditis.

Enoxacin administered orally was compared with vancomycin administered intravenously for the treatment of experimental methicillin-resistant Staphylococcus aureus endocarditis. The MICs and MBCs of both enoxacin and vancomycin for an inoculum of 5.0 X 10(5) CFU of the methicillin-resistant S. aureus strain per ml were 1.56 microgram/ml. With an inoculum of 10(8) CFU/ml, enoxacin at 6 micrograms/ml and vancomycin at 180 micrograms/ml resulted in similar decreases in numbers of methicillin-resistant S. aureus in broth. Methicillin-resistant S. aureus endocarditis in rabbits was treated with enoxacin at 100 mg/kg orally every 12 h or vancomycin at 30 mg/kg intravenously every 12 h for 3 or 5 days. Enoxacin treatment for 3 or 5 days and vancomycin treatment for 5 days significantly reduced bacterial counts of vegetations compared with those in untreated control rabbits after 1 day of infection. Bacterial counts of vegetations after vancomycin treatment for 3 days did not differ significantly from those of untreated controls. Bacterial counts of vegetations in the four therapeutic groups did not differ significantly from one another. In uninfected rabbits single doses of vancomycin at 30 mg/kg administered intravenously achieved much higher concentrations in serum than did single doses of enoxacin at 100 mg/kg administered orally. Enoxacin had an elimination half-life in serum that was approximately 1.5 times longer than that of vancomycin. This study demonstrated that enoxacin administered orally is as effective as vancomycin administered intravenously for the treatment of experimental methicillin-resistant S. aureus endocarditis.     

In vitro activity of ciprofloxacin (Bay o 9867).

The in vitro activities of ciprofloxacin (Bay o 9867) and seven comparative antimicrobial agents against 664 aerobic and facultatively anaerobic bacterial isolates were studied. Minimal inhibitory concentrations (MICs) of ciprofloxacin were less than or equal to 2 micrograms/ml for Enterobacteriaceae, less than or equal to 8 micrograms/ml for nonfermentative gram-negative bacilli, less than or equal to 4 micrograms/ml for gram-positive cocci, less than or equal to 0.03 micrograms/ml for Aeromonas hydrophila and Pasteurella multocida, and less than or equal to 1 microgram/ml for Listeria monocytogenes. MICs for multi-drug-resistant Enterobacteriaceae and Pseudomonas aeruginosa were less than or equal to 4 micrograms/ml. Ciprofloxacin MICs were consistently 0 to 4 (usually 2 to 3) dilution steps lower than those of a related drug, norfloxacin (P less than 0.0001). For most species, they were lower than MICs of cefotaxime, aztreonam, theinamycin, mezlocillin, trimethoprim-sulfamethoxazole, and amikacin. With all eight drugs, increasing the inoculum size by 100-fold had a variable effect on MICs which was species related. Ciprofloxacin is a potent broad-spectrum new antimicrobial agent.     

Efficacy of telavancin in the treatment of experimental endocarditis due to glycopeptide-intermediate Staphylococcus aureus

The efficacy of telavancin, a novel lipoglycopeptide, was evaluated in experimental endocarditis in rabbits using two clinical isolates of glycopeptide-intermediate Staphylococcus aureus: ATCC 700788 and HIP 5836. Infected rabbits were treated for 2 days with telavancin (10 mg/kg of body weight once daily intravenously) or vancomycin (1 g twice daily intravenously), administered with a computer-controlled infusion pump system simulating human serum kinetics. Vegetations were harvested at 16 h postinoculation in the control group and at the end of treatment in the drug-treated group. For ATCC 700788, MICs and minimal bactericidal concentrations (MBCs), respectively, were 1 mg/liter and 4 mg/liter for telavancin and 8 mg/liter and 128 mg/liter for vancomycin. For HIP 5836, MICs and MBCs, respectively, were 4 mg/liter and 8 mg/liter for telavancin and 8 mg/liter and 128 mg/liter for vancomycin. Peak and trough levels were 90 microg/ml and 6 microg/ml, respectively, for telavancin and 46 microg/ml and 6 microg/ml, respectively, for vancomycin. In glycopeptide-intermediate S. aureus ATCC 700788, telavancin sterilized 6 of 16 vegetations (37%), whereas vancomycin sterilized 4 of 20 (20%) (P = 0.29) compared with 0 of 17 in the control group. In HIP 5836 experiments, telavancin and vancomycin sterilized 5 of 16 (31%) and 1 of 15 (7%) vegetations (P = 0.17), respectively, compared with none in the control group. Telavancin reduced vegetation titers by 2.0 and 2.3 logs greater than vancomycin for the ATCC 700788 (4.6 [2.0 to 5.8] versus 6.6 [2.0 to 6.9] log CFU/g vegetation; P = 0.05) and HIP 5836 (4.4 [2.0 to 7.4] versus 6.7 [4.5 to 8.7] log CFU/g vegetation; P = 0.09) strains, respectively; these differences did not reach statistical significance. All isolates from vegetations remained susceptible to telavancin after therapy. The results suggest that telavancin may be an effective treatment for endocarditis caused by glycopeptide-intermediate S. aureus.     

Importance of the aminoglycoside dosing regimen in the penicillin-netilmicin combination for treatment of Enterococcus faecalis-induced experimental endocarditis.

The penicillin-aminoglycoside combination is recommended for the treatment of systemic enterococcal infections. However, the optimal dosing regimen of the aminoglycoside remains to be elucidated. We evaluated the efficacy of penicillin, alone or in combination with various dosing regimens of netilmicin, for the treatment of experimental left-sided Enterococcus faecalis endocarditis in rabbits. Animals were injected intramuscularly for 4 days with penicillin alone or in combination with netilmicin in one of the following regimens: netilmicin at a low dose (2 mg/kg of body weight every 8 h), netilmicin at a high dose (4 mg/kg every 8 h), or netilmicin at a single daily high dose (12 mg/kg every 24 h). MICs and MBCs were 3.1 and 6.2 micrograms/ml and 8 and 8 micrograms/ml for penicillin and netilmicin, respectively. A netilmicin concentration of 4 micrograms/ml was the lowest concentration that achieved synergism with penicillin, as shown by the kill-curve method. Mean peak levels of netilmicin in serum were 5.6 (netilmicin at 2 mg/kg), 9.8 (netilmicin at 4 mg/kg), and 20.6 (netilmicin at 12 mg/kg) micrograms/ml. Mean penicillin levels in serum were constantly above the MIC. Penicillin plus netilmicin at a high dose given three times daily was more effective (P less than 0.05) than any other regimen in reducing bacterial titers in vegetations and was the only treatment that induced a significant bactericidal activity in rabbit serum during the trough. We concluded that divided doses of aminoglycoside are more effective than the same total dose given once daily in combination with penicillin. Our data suggest that prolonged levels of aminoglycoside in serum might be important to exhibit the greatest in vivo efficacy of the combination against E. faecalis. They also indicate that use of a reduced total daily dose of aminoglycoside or an increase in the interval between each dose might reduce the efficacy of therapy in animals with this type of infection.     

Lack of homology of enterococci which have high-level resistance to trimethoprim with the dfrA gene of Staphylococcus aureus.

Multiresistant enterococci were tested for susceptibility to trimethoprim (TMP). Although most enterococci are inhibited by less than or equal to 1.0 microgram/ml, the MICs for 7 of 29 selected multiresistant isolates were greater than or equal to 8 micrograms/ml, including for two beta-lactamase positive (Bla+) strains, for which the MICs of TMP were greater than 1,000 micrograms/ml, and for another Bla+ strain, for which the MIC was 128 micrograms/ml. None of five isolates tested transferred TMP resistance and none of the resistant isolates hybridized to the dfrA gene of Staphylococcus aureus. Whether TMP resistance in enterococci is due to a mutation(s) or to acquisition of a new gene is not known. Acquisition of resistance to TMP is another example of the multiple antimicrobial resistance typically displayed by enterococci.     

Activity and diffusion of tigecycline (GAR-936) in experimental enterococcal endocarditis

The activity of tigecycline (GAR-936), a novel glycylcycline, was investigated in vitro and in experimental endocarditis due to the susceptible Enterococcus faecalis JH2-2 strain, its VanA type transconjugant BM4316, and a clinical VanA type strain, E. faecium HB217 resistant to tetracycline. MICs of GAR-936 were 0.06 micro g/ml for the three strains. In vitro pharmacodynamic studies demonstrated a bacteriostatic effect of GAR-936 that was not enhanced by increasing concentrations to more than 1 micro g/ml and a postantibiotic effect ranging from 1 to 4.5 h for concentrations of 1- to 20-fold the MIC. Intravenous injection of [(14)C]GAR-936 to five rabbits with enterococcal endocarditis sacrificed 30 min, 4 h, or 12 h after the end of the infusion evidenced a lower clearance of GAR-936 from aortic vegetations than from serum and a homogeneous diffusion of GAR-936 into the vegetations. In rabbits with endocarditis, GAR-936 (14 mg/kg of body weight twice a day [b.i.d.]) given intravenously for 5 days was bacteriostatic against both strains of E. faecalis. Against E. faecium HB217, bacterial counts in vegetations significantly decreased during therapy (P < 0.01), and the effect was similar with GAR-936 at 14 mg/kg b.i.d., 14 mg/kg once a day (o.d.), and 7 mg/kg o.d., which provided concentrations in serum constantly above the MIC. Mean serum elimination half-life ranged from 3.3 to 3.6 h. No GAR-936-resistant mutants were selected in vivo with any regimen. We concluded that the combination of prolonged half-life, significant postantibiotic effect, and good and homogeneous diffusion into the vegetations may account for the in vivo activity of GAR-936 against enterococci susceptible or resistant to glycopeptides and tetracyclines, even when using a o.d. regimen in rabbits.     

Minimal concentrations of aminoglycoside that can synergize with penicillin in enterococcal endocarditis

Minimal concentrations of aminoglycoside that could produce a synergistic effect with penicillin were investigated in broth cultures containing 10(8) enterococci per ml, in vitro in vegetations infected with ca. 10(8) enterococci per g, and in vivo in an experimental model of enterococcal endocarditis. Penicillin G plus gentamicin (1.5 or 0.75 microgram/ml) sterilized a broth culture of a streptomycin-resistant strain (E1) at 48 h. In contrast, penicillin G plus gentamicin (1.5 or 0.75 microgram/ml) sterilized only 2 of 15 in vitro vegetations at 5 days. Similarly, doses of gentamicin that resulted in peak serum levels of 1.5 microgram/ml failed after 10 days of therapy with penicillin G plus gentamicin to sterilize in vivo vegetations infected with E1, and doses of gentamicin that resulted in peak serum levels of about 8 micrograms/ml sterilized four of six vegetations. Similar results were obtained with a streptomycin-susceptible strain. These studies indicated that the rate of bactericidal activity in broth cultures is greater than the bacteriological response in infected vegetations and that aminoglycoside concentrations that appear efficacious on the basis of synergy studies in broth cultures may not be satisfactory clinically.     

Efficacy of ampicillin plus ceftriaxone in treatment of experimental endocarditis due to Enterococcus faecalis strains highly resistant to aminoglycosides

The purpose of this work was to evaluate the in vitro possibilities of ampicillin-ceftriaxone combinations for 10 Enterococcus faecalis strains with high-level resistance to aminoglycosides (HLRAg) and to assess the efficacy of ampicillin plus ceftriaxone, both administered with humanlike pharmacokinetics, for the treatment of experimental endocarditis due to HLRAg E. faecalis. A reduction of 1 to 4 dilutions in MICs of ampicillin was obtained when ampicillin was combined with a fixed subinhibitory ceftriaxone concentration of 4 micrograms/ml. This potentiating effect was also observed by the double disk method with all 10 strains. Time-kill studies performed with 1 and 2 micrograms of ampicillin alone per ml or in combination with 5, 10, 20, 40, and 60 micrograms of ceftriaxone per ml showed a > or = 2 log10 reduction in CFU per milliliter with respect to ampicillin alone and to the initial inoculum for all 10 E. faecalis strains studied. This effect was obtained for seven strains with the combination of 2 micrograms of ampicillin per ml plus 10 micrograms of ceftriaxone per ml and for six strains with 5 micrograms of ceftriaxone per ml. Animals with catheter-induced endocarditis were infected intravenously with 10(8) CFU of E. faecalis V48 or 10(5) CFU of E. faecalis V45 and were treated for 3 days with humanlike pharmacokinetics of 2 g of ampicillin every 4 h, alone or combined with 2 g of ceftriaxone every 12 h. The levels in serum and the pharmacokinetic parameters of the humanlike pharmacokinetics of ampicillin or ceftriaxone in rabbits were similar to those found in humans treated with 2 g of ampicillin or ceftriaxone intravenously. Results of the therapy for experimental endocarditis caused by E. faecalis V48 or V45 showed that the residual bacterial titers in aortic valve vegetations were significantly lower in the animals treated with the combinations of ampicillin plus ceftriaxone than in those treated with ampicillin alone (P < 0.001). The combination of ampicillin and ceftriaxone showed in vitro and in vivo synergism against HLRAg E. faecalis.     

Comparison of penicillin and vancomycin, individually and in combination with gentamicin and amikacin, in the treatment of experimental endocarditis induced by nutritionally variant streptococci.

Six different antibiotic treatment regimens were compared for efficacy in rabbits with endocarditis induced by inoculation with a nutritionally variant strain of streptococcus. Seven untreated animals, sacrificed at day 11, had vegetations containing 8.89 +/- 1.35 log10 CFU/g, none of which was sterile. The vegetations from the rabbits in all treated groups had bacterial titers significantly lower than those of the controls (P less than 0.001). Vegetations from penicillin-treated animals averaged 5.14 +/- 1.00 log CFU/g, and no vegetations were sterile. Treatment with penicillin plus gentamicin or amikacin was more effective than treatment with penicillin alone, resulting in 3.99 +/- 0.94 log CFU/g of vegetation and sterile lesions in 5 of 12 animals. Treatment with vancomycin alone was as least as efficient as that with penicillin plus an aminoglycoside, resulting in an average of 3.33 +/- 0.96 log CFU/g of vegetation and sterile lesions in five of eight animals. Treatment with vancomycin plus an aminoglycoside was not superior to treatment with vancomycin alone, resulting in an average of 3.68 +/- 1.37 log CFU/g of vegetation and sterile lesions in 8 of 13 animals. These in vivo results correlated poorly with the in vitro susceptibility of the strain to the various antibiotics, as measured by the time-kill method. These results support the current practice of using vancomycin as alternative therapy when a penicillin-aminoglycoside combination is ineffective or contraindicated in patients with endocarditis caused by nutritionally variant streptococci.     

Staphylococcal endocarditis in rabbits treated with a low dose of cloxacillin.

Rabbits with established staphylococcal endocarditis, injected twice at an interval of 2 h with either 0.5 mg of cloxacillin per kg or saline, were sacrificed 2.5 h after the second injection. Vegetations were excised, weighed, and cultured, and ultrathin sections were prepared and examined by light microscopy, transmission electron microscopy, and scanning electron microscopy. Several affected valves were examined histologically. Concentrations of cloxacillin in serum were determined 1 and 3 h after dosage. Staphylococci grown on membranes placed on agar containing 0.09 micrograms of cloxacillin per ml and in broth at the same cloxacillin concentration (one-third of the MIC) were examined by transmission electron microscopy. The mean numbers of CFU per gram of vegetations from control and treated rabbits were 2.28 X 10(10) and 1.31 X 10(10), respectively. Vegetations of treated rabbits contained staphylococci of normal size and form as well as organisms two to six times larger than normal with multiple cross walls. Larger bacterial cells were usually located in areas close to blood; cells of normal size were usually embedded in fibrin. The structures of these staphylococci and those grown on membranes in the presence of 0.09 micrograms of cloxacillin per ml were comparable but were different from those grown in broth containing this concentration of cloxacillin. Concentrations of cloxacillin in serum were 0.166 micrograms/ml at 1 h and 0.286 micrograms/ml at 3 h after dosage. The similarities in ultrastructure between staphylococci in vegetations of treated rabbits and staphylococci grown on membranes suggest that the vegetations contained approximately 0.09 micrograms of cloxacillin per g. Thus, antibiotic penetration from blood into vegetations and diffusion into fibrin were limited.