Effect of dosing and dosing frequency on the efficacy of ceftizoxime and the emergence of ceftizoxime resistance during the early development of murine abscesses caused by Bacteroides fragilis and Enterobacter cloacae mixed infection.

The efficacy of beta-lactams is thought to be dependent on the time that the unbound concentrations exceed the MIC (fT>MIC). However, the pharmacokinetic/pharmacodynamic index (PDI) that correlates best to the selection of resistance is not yet clear. The selection of ceftizoxime (CZX)-resistant Enterobacter cloacae mutant strains during the development of murine mixed-infection abscesses was studied to determine the PDI that is important for the emergence of resistance and the PDI value needed for the prevention of resistance. Studies were carried out 24 h after inoculation with Bacteroides fragilis ATCC 23745 and E. cloacae 22491. Six to 1,536 mg of CZX/kg of body weight/day given every 2 h (q2h), q4h, q6h, or q8h was started 30 min before inoculation and continued for 24 h. Resistant mutants were isolated to determine mutant frequencies (MF). The fT>MIC varied from 9 to 98% for E. cloacae, the peak concentration (unbound fraction) was 0.6 to 578 mg/liter, and the area under the concentration-time curve (unbound fraction) (fAUC) was 1.9 to 553 mg.h/liter. The fAUC-to-MIC ratio best explained the in vivo efficacy. CZX-resistant B. fragilis and E. cloacae mutants were isolated from untreated controls at an MF of 10(-5) to 10(-7). The MF of resistant B. fragilis did not increase during therapy. The selection of resistant E. cloacae strains at an MF of 10(-1) to 10(-2) was related to the fT>MIC and the ratio of fAUC to MIC following an inverse U shape. However, the ratio of fAUC to MIC was the stronger driver of resistance. The highest MFs were 0.7 to 0.9 at an fAUC-to-MIC ratio of approximately 250. We conclude that the ratio of fAUC to MIC is the PDI that correlated best to the in vivo efficacy of CZX and probably also to the emergence of resistant E. cloacae mutants. An fAUC-to-MIC ratio of 1,000 was needed to prevent the emergence of this resistance.

Effect of recombinant murine granulocyte colony-stimulating factor with or without fluoroquinolone therapy on mixed-infection abscesses in mice.

The aim of the study was to determine if immunomodulation of host defense with recombinant murine granulocyte colony-stimulating factor (G-CSF) improves the efficacy of trovafloxacin or moxifloxacin in abscesses containing Bacillus fragilis ATCC 23745 and different Escherichia coli strains varying in virulence. Treatment of mice inoculated with 10(7) CFU B. fragilis and 10(5) CFU low-virulence E. coli with either trovafloxacin (150 mg/kg/day every 24 hours, days 3 to 7) or moxifloxacin (96 mg/kg/day every 12 hours, days 3 to 7), significantly reduced the number of B. fragilis to 6.9 +/- 0.35 and 5.8 +/- 0.10 and that of E. coli to 4.9 +/- 0.09 and 4.2 +/- 0.07 log CFU/abscess for trovafloxacin and moxifloxacin, respectively, compared to controls (B. fragilis 8.7 and E. coli 7.4 log CFU/abscess) on day 8. Also, moxifloxacin was more potent than trovafloxacin. Addition of G-CSF prophylaxis (1 mug once on day -1) or therapy (1 mug/day on days 3 to 7) to fluoroquinolone treatment did not improve the efficacy of fluoroquinolone therapy alone. The effect of moxifloxacin with or without G-CSF prophylaxis on abscesses with a virulent hemolytic E. coli strain was also studied. In moxifloxacin-treated mice, 75% survived infection compared to 10% of controls. Combining moxifloxacin with G-CSF prophylaxis significantly decreased survival (30%) compared to moxifloxacin alone. In addition, G-CSF prophylaxis resulted in a threefold (E. coli) to 100-fold (B. fragilis) increased outgrowth in the abscesses of surviving mice. In conclusion, the addition of G-CSF to a fluoroquinolone is not advisable since, depending on the virulence of the E. coli strains, this might detrimentally influence the outcome of therapy.

Comparative study of the effects of ceftizoxime, piperacillin, and piperacillin-tazobactam concentrations on antibacterial activity and selection of antibiotic-resistant mutants of Enterobacter cloacae and Bacteroides fragilis in vitro and in vivo in mixed-infection abscesses.

The effects of ceftizoxime (CZX), piperacillin (PIP), and PIP-tazobactam (PT) concentrations on the antibacterial activity and selection of resistant mutants of Bacteroides fragilis and Enterobacter cloacae were investigated in vitro in a mixed-culture anaerobic time-kill study and in vivo in a mixed-infection abscess model. Mixed cultures were incubated for 24 h with 0.125 to 512 micro g of CZX per ml or 0.125 to 2,048 micro g of PIP or PT per ml. Mice were treated every 2 h for 24 h with CZX at 6 to 1,536 mg/kg/day or with PIP or PT at 24 to 6,144 mg/kg/day starting 30 min before inoculation with different B. fragilis-E. cloacae combinations. There was a good correlation between the in vitro and in vivo activities of the antibiotics and their MICs obtained with high inocula (10(8) CFU/ml). The respective 50% effective doses (milligrams per kilogram per day) with B. fragilis and E. cloacae 22491 were 771 and 521 for CZX, 416 and 643 for PIP, and 85 and 554 for PT, and with the B. fragilis-E. cloacae 032349 combination, they were 81 and 21 for CZX and 77 and 766 for PT. Resistant mutants of E. cloacae 22491 were preferentially selected in vitro with 2 to 64 micro g of CZX per ml and in vivo with CZX at 12 to 384 mg/kg/day. There was no preferential selection of CZX-resistant B. fragilis or E. cloacae 032349. For CZX-resistant E. cloacae 22491, we found a 16- to 512-fold increase in the MIC of CZX and increased MICs of other expanded-spectrum cephalosporins, owing in part to the production of a stably derepressed cephalosporinase. In vitro and in vivo, PT did not select resistant mutants of E. cloacae and B. fragilis. Results demonstrate the adverse microbiological outcome of choosing an expanded-spectrum cephalosporin like CZX for empirical treatment of mixed infections involving a susceptible Enterobacter strain.

Effect of a single percutaneous abscess drainage puncture and imipenem therapy, alone or in combination, in treatment of mixed-infection abscesses in mice.

The importance of supplementary imipenem therapy after a single percutaneous abscess drainage puncture was studied in a mouse model of established mixed-infection abscesses. Animals were treated for 3 days with daily dosing regimens of 384 to 1,536 mg/kg of body weight that took into account the short half-life of this antibiotic in mice. Imipenem therapy in conjunction with abscess drainage was significantly better than drainage alone in reducing the Escherichia coli and Bacteroides fragilis counts in the mixed infections. Furthermore, the killing of B. fragilis by the combination of imipenem therapy and abscess drainage was significantly better than that by imipenem treatment alone. The maximum reductions in E. coli and B. fragilis counts were 1.1 and 2.2 log(10) CFU/abscess, respectively. In contrast, the in vitro activity of imipenem was significantly better (maximum reduction, > or =6.2 log(10) CFU/ml) against mixed cultures of the same strains even when bacterial numbers similar to those found in the abscesses were used. Comparable in vivo activity was achieved only when treatment was started 30 min before inoculation (reduction for both strains, > or =6.1 log(10) CFU/abscess), but this killing was significantly diminished if the start of treatment was delayed until > or =12 h after inoculation. Imipenem concentrations in abscess tissue reached levels above the MIC for E. coli for >60% of the dosing interval. Possible reasons for the reduced activity of imipenem in vivo are discussed, and we conclude that standard susceptibility tests overestimate the efficacy of this antibiotic against the organisms present in these abscesses.

Biodistribution of long-circulating PEG-liposomes in a murine model of established subcutaneous abscesses.

The biodistribution of long-circulating PEG-liposomes in a subcutaneous mouse model of established mixed infection abscesses was investigated to assess their possible role as drug carriers in the treatment of small, undrainable intra-abdominal abscesses. There was a 10-30-fold greater localisation of (67)Ga-labelled PEG-liposomes in abscesses compared to uninfected normal skin samples. Over 3% of the injected dose (ID) of liposomes was present in the abscesses 24 h after liposome administration in contrast to 0.1% in normal skin sections. The percentage ID present in the liver, spleen and kidneys was 17%, 4% and 2% per organ respectively. Five days after liposome injection, 2% ID could still be recovered from the abscesses. Using colloidal gold-labelled PEG-liposomes, it was shown that there was a 4-fold greater density of liposome clusters in the subcutaneous tissue surrounding the capsule than in the core of the abscesses. The clusters within the abscesses were distributed evenly. We conclude that PEG-liposomes localise to a significant degree at the infection focus in our mouse model and may provide a new approach to the antimicrobial treatment of intra-abdominal abscesses.