In vivo activity of evernimicin (SCH 27899) against methicillin-resistant Staphylococcus aureus in experimental infective endocarditis.

Currently, there exist few satisfactory alternatives to vancomycin for therapy of serious methicillin-resistant Staphylococcus aureus (MRSA) infections. We employed a rat model of aortic valve endocarditis to assess the potential efficacy of evernimicin (SCH 27899) compared with vancomycin against infection with a strain susceptible to both agents (MICs of 0.25 and 0.50 microg/ml, respectively). Infected animals were assigned to one of three groups: controls (no treatment), evernimicin at 60 mg/kg of body weight by intravenous (i.v.) infusion once daily, or vancomycin at 150 mg/kg of body weight per day by continuous i.v. infusion. Therapy was administered for 5.5 days. At the start of therapy, colony counts in vegetations were 6.63 +/- 0.44 log(10) CFU/g. In both treatment groups, bacterial density within vegetations was significantly reduced in comparison with control animals that had not been treated. Final colony counts were as follows (mean +/- standard deviation): controls, 10.12 +/- 1.51 log(10) CFU/g of vegetation; evernimicin, 7.22 +/- 2.91 log(10) CFU/g of vegetation; vancomycin, 5.65 +/- 1.76 log(10) CFU/g of vegetation. The difference between the evernimicin and vancomycin groups was not significant. These results confirmed the bacteriostatic activity of evernimicin in vivo in an experimental model of severe MRSA infection.

In vivo activities of evernimicin (SCH 27899) against vancomycin-susceptible and vancomycin-resistant enterococci in experimental endocarditis.

To assess the potential efficacy of evernimicin (SCH 27899) against serious enterococcal infections, we used a rat model of aortic valve endocarditis established with either a vancomycin-susceptible Enterococcus faecalis or a vancomycin-resistant Enterococcus faecium strain. Animals infected with either one of the test strains were assigned to receive no treatment (controls) or 5-day therapy with one of the following regimens: evernimicin 60-mg/kg of body weight intravenous (i.v.) bolus once daily, 60-mg/kg i.v. bolus twice daily (b.i.d.), 60 mg/kg/day i.v. by continuous infusion, or 120 mg/kg/day i.v. by continuous infusion. These regimens were compared with vancomycin at 150 mg/kg/day. In animals infected with E. faecalis, evernimicin at 120 mg/kg/day by continuous infusion significantly reduced bacterial counts in vegetations (final density, 5.75+/-3.38 log(10) CFU/g) compared with controls (8.51+/-1.11 log(10) CFU/g). In animals infected with 0.5 ml of an 8 x 10(7)-CFU/ml inoculum of the vancomycin-resistant E. faecium, both 60-mg/kg bolus once a day and b.i.d. dose regimens of evernimicin were very effective (viable counts, 3.45+/-1.44 and 3.81+/-1.98 log(10) CFU/g, respectively). Vancomycin was unexpectedly active against infections induced with that inoculum. In animals infected with a 10(9)-CFU/ml inoculum of the vancomycin-resistant E. faecium, the evernimicin 60-mg/kg i.v. bolus b.i.d. reduced viable counts in vegetations compared with controls (6.27+/-1.63 versus 8.34+/-0.91 log(10) CFU/g; P<0.05), whereas vancomycin was ineffective. Although resistant colonies could be selected in vitro, we were not able to identify evernimicin-resistant clones from cardiac vegetations. An unexplained observation from these experiments was the great variability in final bacterial densities within cardiac vegetations from animals in each of the evernimicin treatment groups.

Activities of taurolidine in vitro and in experimental enterococcal endocarditis.

In vitro, the antimicrobial agent taurolidine inhibited virtually all of the bacteria tested, including vancomycin-resistant enterococci, oxacillin-resistant staphylococci, and Stenotrophomonas maltophilia, at concentrations between 250 and 2,000 microg/ml. Taurolidine was not effective in experimental endocarditis. While it appears unlikely that this antimicrobial would be useful for systemic therapy, its bactericidal activity and the resistance rates found (<10(-9)) are favorable indicators for its possible development for topical use.

Activities of the oxazolidinones linezolid and eperezolid in experimental intra-abdominal abscess due to Enterococcus faecalis or vancomycin-resistant Enterococcus faecium.

The in vivo effectiveness of oxazolidinones eperezolid (U-100592) and linezolid (U-100766) against one strain each of Enterococcus faecalis and vancomycin-resistant Enterococcus faecium was examined in a rat model of intra-abdominal abscess. MICs of both drugs were 2 microg/ml for each strain. At doses of 25 mg/kg of body weight twice daily intravenously or orally, linezolid produced small but statistically significant reductions in abscess bacterial density for E. faecalis. The reduction in viable cells observed would not likely be clinically relevant. Eperezolid was ineffective at this dose. At a dosage of 100 mg/kg/day, linezolid treatment led to an approximately 100-fold reduction in viable cells per gram of abscess. Against E. faecium infections, intravenous eperezolid and oral linezolid were effective, reducing densities approximately 2 log(10) CFU/g. Both oxazolidinones demonstrated activity against enterococci in this model. However, results were modest with the dosing regimens employed.

A cluster of VanD vancomycin-resistant Enterococcus faecium: molecular characterization and clinical epidemiology.

VanD-mediated glycopeptide resistance has been reported for an isolate of Enterococcus faecium, BM4339. Three clinical isolates of vancomycin-resistant E. faecium collected from 3 patients during a 6-week period in 1993 had agar dilution MICs of vancomycin and teicoplanin of 128 and 4 microg/mL, respectively. Polymerase chain reaction (PCR) using degenerate primers complementary to genes encoding d-Ala-d-X ligases yielded a 630-bp product that was similar to the published partial sequence of vanD. By use of inverse PCR, vanD, vanHD, and two partial flanking open-reading frames were sequenced. The deduced amino acid sequence of VanD showed 67% identity with VanA and VanB. vanD appeared to be located on the chromosome and was not transferable to other enterococci. The 3 isolates were indistinguishable by pulsed-field gel electrophoresis and differed from BM4339. No other isolates carrying vanD were found in a subset of 875 recent US isolates of vancomycin-resistant enterococci.

Efficacies of piperacillin-tazobactam and cefepime in rats with experimental intra-abdominal abscesses due to an extended-spectrum beta-lactamase-producing strain of Klebsiella pneumoniae.

The in vivo activities of piperacillin-tazobactam and cefepime were compared with those of ticarcillin-clavulanate, ceftazidime, cefotaxime, and imipenem in a rat model of intra-abdominal abscess with a strain of Klebsiella pneumoniae elaborating an extended-spectrum beta-lactamase (TEM-26). With the exception of ceftazidime, all of the antimicrobial agents significantly reduced bacterial counts within abscesses at the end of therapy compared with those in untreated controls. Residual viable cell counts (mean +/- standard deviation in log10 CFU/gram) were as follows: control, 8.76 +/- 0.97; ceftazidime, 8.00 +/- 0.76; piperacillin-tazobactam, 3.87 +/- 1.72; ticarcillin-clavulanate, 3.74 +/- 1.34; cefepime, 3.15 +/- 1.19; cefotaxime, 2.61 +/- 0.77; imipenem, 2.41 +/- 0.93. Imipenem was more effective than either of the inhibitor combinations (P < 0.05). Cefotaxime was unexpectedly effective given its poor in vivo activity against this organism in our earlier studies, which used a different dose and total duration of therapy (L. B. Rice, J. D. C. Yao, K. Klimm, G. M. Eliopoulos, and R. C. Moellering, Jr., Antimicrob. Agents Chemother. 35:1243-1244, 1991). These observations suggest that the effectiveness of cephalosporins in the treatment of experimental infections caused by extended-spectrum beta-lactamase-producing K. pneumoniae may be highly dependent on dosing regimens, even for a specific organism and site of infection.

Activity of fleroxacin alone and in combination with clindamycin or metronidazole in experimental intra-abdominal abscesses.

To assess the potential efficacy of fleroxacin in combination with clindamycin or metronidazole in mixed aerobic and anaerobic infections, we used a rat model of intra-abdominal abscesses in which the inoculum consisted of pooled rat feces mixed with BaSO4. Two hours after bacterial challenge, antimicrobial therapy was begun intravenously with regimens designed to stimulate human pharmacokinetics. A combination of clindamycin and gentamicin was included as an established treatment regimen. After 8.5 days of therapy, final bacterial counts in abscesses showed that fleroxacin alone or combined with metronidazole or clindamycin effectively eradicated Escherichia coli, with bacterial densities of < or = 2.84 +/- 0.1, < or = 2.9 +/- 0.1, and < or = 2.9 +/- 0.1 (mean +/- standard error of the mean) log10 CFU/g, respectively. The addition of either clindamycin or metronidazole to fleroxacin substantially enhanced the effectiveness of the regimens against Bacteroides fragilis, with bacterial counts of < or = 3.0 +/- 0.1 or < or = 2.9 +/- 0.1 log10 CFU/g, respectively, versus 9.2 +/- 0.2 log10 CFU/g for fleroxacin alone. The combination of metronidazole and fleroxacin also resulted in a significantly greater reduction of peptostreptococci and Bacteroides thetaiotaomicron than fleroxacin alone (< or = 2.9 +/- 0.1 versus 6.1 +/- 0.9 log10 CFU/g and 3.3 +/- 0.4 versus 8.3 +/- 0.1 log10 CFU/g, respectively). Except for those of B. fragilis, counts of other anaerobes were reduced to a greater extent by metronidazole plus fleroxacin than by clindamycin plus fleroxacin, although differences were not always significant. Metronidazole plus fleroxacin was at least as active a clindamycin plus gentamicin against all species and was significantly more active against Clostridium spp. No regimen effectively eradicated enterococci from the abscesses. These results suggest that the addition of either metronidazole or clindamycin would effectively enhance the spectrum of fleroxacin for treatment of mixed aerobic and anaerobic infections.

Efficacy of ceftriaxone plus tazobactam in a rat model of intraabdominal abscess due to Bacteroides fragilis.

Using a rat model of intraperitoneal abscess due to Bacteroides fragilis, we evaluated therapy with the combination of ceftriaxone plus the beta-lactamase inhibitor tazobactam in comparison with ceftriaxone or cefotaxime alone. When treatment was begun five hours after bacterial challenge, final bacterial counts within abscesses at 3.5 days of treatment were as follows (mean +/- S.D., log10 cfu/g): ceftriaxone plus tazobactam, 4.15 +/- 1.25; cefotaxime, 4.77 +/- 1.80; ceftriaxone alone, 5.68 +/- 1.04; untreated controls, 9.14 +/- 1.13. In spite of pharmacokinetic differences between the two drugs, coadministration of tazobactam significantly enhanced activity of ceftriaxone in this model.

A gene conferring resistance to vancomycin but not teicoplanin in isolates of Enterococcus faecalis and Enterococcus faecium demonstrates homology with vanB, vanA, and vanC genes of enterococci.

We report the sequence of a 630-bp fragment of a gene associated with resistance to high levels of vancomycin in a clinical isolate of Enterococcus faecalis which retained susceptibility to teicoplanin. This gene was similar to the recently sequenced vanB and partially homologous with vanA, but it showed less-marked similarity to vanC. A DNA probe, derived from this polymerase chain reaction-amplified gene fragment, hybridized specifically with genomic DNA from Enterococcus faecium and E. faecalis isolates which were vancomycin resistant (MICs ranged from 8 to 512 micrograms/ml) but susceptible to teicoplanin. Curing of vancomycin resistance was associated with loss of DNA hybridization with the gene probe. Transfer of DNA which hybridized with the probe accompanied transfer of vancomycin resistance to a susceptible recipient strain. Neither curing nor transfer of vancomycin resistance was consistently related to loss or acquisition, respectively, of plasmid DNA.

Activity of ampicillin-sulbactam and oxacillin in experimental endocarditis caused by beta-lactamase-hyperproducing Staphylococcus aureus.

Using a rat model of aortic valve infective endocarditis, we previously found that oxacillin was equally effective against an oxacillin-susceptible strain of Staphylococcus aureus and a beta-lactamase-hyperproducing borderline oxacillin-susceptible strain of S. aureus; also, ampicillin-sulbactam was less effective than oxacillin against both isolates and at low doses was less effective against the borderline-susceptible strain than against the fully oxacillin-susceptible strain (C. Thauvin-Eliopoulos, L. B. Rice, G. M. Eliopoulos, and R. C. Moellering, Jr., Antimicrob. Agents Chemother. 34:728-732, 1990). In the present study, we extended this work, using alternative treatment schedules and additional bacterial strains. Extending treatment with low doses of ampicillin-sulbactam (500 and 250 mg/kg of body weight per day, respectively) to 6.5 days resulted in equalization of effectiveness against the previously studied strains BOSSA-1 and OSSA-1 (3.75 +/- 1.61 log10 and 4.71 +/- 1.79 log10 CFU of residual viable bacteria per g, respectively). Against the borderline oxacillin-susceptible strain BOSSA-1, increasing the sulbactam dosage from 500 to 2,000 mg/kg/day while maintaining a fixed dose of ampicillin (1,000 mg/kg/day) by continuous infusion resulted in lower bacterial counts (4.93 +/- 1.84 log10 versus 3.65 +/- 1.26 log10 CFU of residual viable bacteria per g, respectively), but this difference was of only borderline significance; differences in efficacy between the low-dose and high-dose sulbactam regimens were exaggerated when intermittent intravenous administration was used (6.19 +/- 1.90 log10 versus 3.37 +/- 1.41 log10 CFU/g, respectively; P < 0.001). However, for any individual sulbactam dosage, the model of administration (continuous versus intermittent infusion) did not affect the activity of the regimen. When additional strains were used in the model, oxacillin and ampicillin-sulbactam (1,000 plus 2,000 mg/kg/day) were equally effective against both oxacillin-susceptible and borderline oxacillin-resistant strains of S. aureus. These results support the predictions that oxacillin would be clinically effective in the treatment of infections caused by borderline oxacillin-susceptible strains of S. aureus and that, except at very low doses, ampicillin-sulbactam would also be as effective against borderline-susceptible strains as against fully oxacillin-susceptible strains of S. aureus.

Contribution of animal models in the search for effective therapy for endocarditis due to enterococci with high-level resistance to gentamicin.

Earlier studies suggest that ampicillin and amoxicillin are more effective than other beta-lactam agents in killing enterococci, although beta-lactam agents are slowly and incompletely bactericidal against most strains of Enterococcus faecalis. We previously showed that continuous infusion of ampicillin is more effective than intermittent administration in decreasing the number of enterococci in valvular vegetations of rats with catheter-induced endocarditis that are treated for 5 days. In this model, we found ampicillin plus sulbactam more effective than ampicillin alone against a beta-lactamase-producing enterococcal strain with high-level resistance to gentamicin. Daptomycin therapy produced results approximately equal to those of ampicillin plus sulbactam. Vancomycin and teicoplanin given for 5 days at doses producing equivalent serum levels had approximately equal efficacy. However, 10-day therapy with low-dose teicoplanin was considerably more effective than similar treatment with vancomycin. High-dose teicoplanin for 5 days produced sterile valves in 82% of the animals studied.

Failure of trimethoprim-sulfamethoxazole therapy in experimental enterococcal endocarditis.

To assess the potential efficacy of trimethoprim-sulfamethoxazole (TMP-SMX) against serious enterococcal infections, we used a rat enterococcal endocarditis model comparing TMP-SMX therapy (500 mg of TMP plus 2,500 mg of SMX per kg of body weight per day given every 8 h by intragastric gavage) with intravenous ampicillin therapy (1,000 mg/kg per day). Despite concentrations of active drug in serum well in excess of the MIC and MBC, the mean residual vegetation bacterial titer in TMP-SMX-treated rats was similar to that in untreated controls (8.4 +/- 1.1 versus 8.6 +/- 1.3 log10 CFU/g) and significantly higher than that in the ampicillin-treated group (3.6 +/- 1.5 log10 CFU/g; P less than or equal to 0.001). This demonstrates discordance between in vitro activity and in vivo efficacy of TMP-SMX in serious enterococcal infection.

Efficacy of oxacillin and ampicillin-sulbactam combination in experimental endocarditis caused by beta-lactamase-hyperproducing Staphylococcus aureus.

Optimal therapy of infections caused by borderline oxacillin-susceptible, beta-lactamase-hyperproducing Staphylococcus aureus has not been established. We used a rat model of aortic valve endocarditis to examine efficacies of antibiotic regimens against a borderline oxacillin-susceptible strain as compared with a fully susceptible S. aureus strain. Animals were treated with oxacillin alone or in combination with sulbactam or with ampicillin-sulbactam combinations at two dose levels. Infections caused by the borderline susceptible and fully susceptible strains responded equally well to oxacillin alone, with residual bacterial titers in vegetations falling to 4.8 +/- 1.6 and 4.4 +/- 1.7 (mean +/- standard deviation) log10 CFU/g, respectively. Addition of sulbactam to oxacillin (1:2) did not enhance the efficacy of oxacillin against either strain in the animal model. A high-dose regimen of ampicillin-sulbactam (2:1) yielding mean (+/- standard deviation) levels in serum of 16.8 +/- 7.4 and 9.5 +/- 1.1 micrograms/ml, respectively, proved equally effective against both strains (bacterial titers, 6.6 log10 CFU/g). However, at lower doses (8.3 +/- 2.6 and 5.9 +/- 2.4 micrograms/ml, the combination showed greater efficacy against the fully susceptible strain, with residual titers of 7.1 +/- 2.0 versus 9.0 +/- 1.6 log10 CFU/g (P less than 0.05). In vitro studies revealed that the beta-lactamase inhibitor sulbactam was also a potent inducer of staphylococcal beta-lactamase at clinically relevant concentrations. Based on this short-term in vivo therapy study, oxacillin would be predicted to be clinically effective in the therapy of infections caused by borderline oxacillin-susceptible strains of S. aureus, while the combination of ampicillin with sulbactam appears to be inferior to oxacillin alone against such infections.

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.

Treatment of experimental Escherichia coli pyelonephritis in rat by ciprofloxacin in comparison with tobramycin.

The ciprofloxacin efficacy was compared to that of tobramycin in an Escherichia coli pyelonephritis model in rat. Treatments started 48 h after ligation of the left ureter and inoculation of the bladder and continued for 5 days. Ciprofloxacin (2.5 mg/kg/d and 10 mg/kg/d) was administered intravenously either in a single daily dose or in 2 divided doses at 12 h intervals. Tobramycin (2.5 mg/kg/d and 10 mg/kg/d) was administered by the intramuscular route, in a single daily dose. Ciprofloxacin 10 mg/kg/d was as efficacious as tobramycin irrespective of dosage schedule. Ciprofloxacin 2.5 mg/kg/d was more effective when given twice a day than once.