The killing action of fleroxacin upon Bacteroides fragilis

The killing kinetics of fleroxacin against four strains of Bacteroides fragilis were investigated. Viable counts were determined from 0 to 24 h after exposure to fleroxacin at multiples of the MIC. A one to three log reduction in viable count was observed after 2 h at 8 mg/l fleroxacin, and a more rapid rate of killing was seen at greater concentrations of the antimicrobial agent. An increase in cell length was observed at concentrations at and above the MIC from 2 h after exposure. The presence of a bacteriostatic concentration of chloramphenicol had negligible influence on viability but inhibited cell elongation.     

Correlation between the growth and killing curves of Escherichia coli in the presence of cefotetan or piperacillin

Escherichia coli strains that were susceptible to multiple antibiotics were exposed to cefotetan and piperacillin. As with the majority of beta-lactam antibiotics, the growth curves showed an increase in optical density (OD) due to an increasing volume of cell-wall-deficient bacteria during the first hours before lysis. This increase in OD depended on the concentration of cefotetan and was less dependent on the concentration of piperacillin. A good correlation was found between the prelytic increase in OD and the killing curve. During the prelytic increase in OD, the number of CFU/ml remained more or less constant. The decrease in the number of CFU/ml depended on the concentration of cefotetan and was less dependent on the concentration of piperacillin.     

Comparison of two methods for determining in vitro postantibiotic effects of three antibiotics on Escherichia coli.

The postantibiotic effect (PAE) for 10 isolates of Escherichia coli was measured by two methods after 1 h of exposure to ampicillin, ciprofloxacin, or tobramycin. The reference method involved serial colony counting to determine growth after antibiotic exposure in relation to control growth. A spectrophotometric procedure was developed with the Abbott MS-2 research system. This method measured the time to detection of growth after exposure and compared this with the time for growth detection in control chambers having the same initial colony count. A reference curve of time to growth versus log initial CFU per milliliter was used to standardize control growth. PAE was determined after exposure to antibiotic at two and six times the MIC and with inocula ranging from 10(3) to 10(9) CFU/ml. There was a statistically significant correlation between PAE measured by the spectrophotometric and the reference methods, and the residuals about the regression line were normally distributed. The mean PAE determined by both methods was statistically different for tobramycin-exposed, but not for ampicillin- or ciprofloxacin-exposed, organisms. There was a concentration-dependent PAE for ciprofloxacin and tobramycin. The PAEs for ciprofloxacin (151 min) and tobramycin (108 min) at concentrations six times the MIC were prolonged compared with those measured at two times the MIC (69 and 66 min, respectively). PAE was inversely related to the exposed inoculum for ciprofloxacin and tobramycin. The PAE for E. coli exposed to ampicillin was minimal and was not affected by either concentration or inoculum. The MS-2 method for determining PAE yields similar results, but is less laborious than the reference method.     

Role of antibiotic penetration limitation in Klebsiella pneumoniae biofilm resistance to ampicillin and ciprofloxacin

The penetration of two antibiotics, ampicillin and ciprofloxacin, through biofilms developed in an in vitro model system was investigated. The susceptibilities of biofilms and corresponding freely suspended bacteria to killing by the antibiotics were also measured. Biofilms of Klebsiella pneumoniae were developed on microporous membranes resting on agar nutrient medium. The susceptibilities of planktonic cultures and biofilms to 10 times the MIC were determined. Antibiotic penetration through biofilms was measured by assaying the concentration of antibiotic that diffused through the biofilm to an overlying filter disk. Parallel experiments were performed with a mutant K. pneumoniae strain in which beta-lactamase activity was eliminated. For wild-type K. pneumoniae grown in suspension culture, ampicillin and ciprofloxacin MICs were 500 and 0.18 microgram/ml, respectively. The log reductions in the number of CFU of planktonic wild-type bacteria after 4 h of treatment at 10 times the MIC were 4.43 +/- 0.33 and 4.14 +/- 0.33 for ampicillin and ciprofloxacin, respectively. Biofilms of the same strain were much less susceptible, yielding log reductions in the number of CFU of -0.06 +/- 0.06 and 1.02 +/- 0.04 for ampicillin and ciprofloxacin, respectively, for the same treatment. The number of CFU in the biofilms after 24 h of antibiotic exposure was not statistically different from the number after 4 h of treatment. Ampicillin did not penetrate wild-type K. pneumoniae biofilms, whereas ciprofloxacin and a nonreactive tracer (chloride ion) penetrated the biofilms quickly. The concentration of ciprofloxacin reached the MIC throughout the biofilm within 20 min. Ampicillin penetrated biofilms formed by a beta-lactamase-deficient mutant. However, the biofilms formed by this mutant were resistant to ampicillin treatment, exhibiting a 0.18 +/- 0.07 log reduction in the number of CFU after 4 h of exposure and a 1.64 +/- 0.33 log reduction in the number of CFU after 24 h of exposure. Poor penetration contributed to wild-type biofilm resistance to ampicillin but not to ciprofloxacin. The increased resistance of the wild-type strain to ciprofloxacin and the mutant strain to ampicillin and ciprofloxacin could not be accounted for by antibiotic inactivation or slow diffusion since these antibiotics fully penetrated the biofilms. These results suggest that some other resistance mechanism is involved for both agents.     

Influence of high-level gentamicin resistance and beta-hemolysis on susceptibility of enterococci to the bactericidal activities of ampicillin and vancomycin.

The bactericidal activities of ampicillin and vancomycin against 40 recent isolates of Enterococcus faecalis were examined by kill-kinetic studies at concentrations of 4 x the MIC and 20 micrograms/ml. Greater killing was seen with ampicillin (3.57 +/- 0.87 and 2.50 +/- 1.09 log10 CFU/ml, respectively; mean +/- standard deviation) than with vancomycin (1.23 +/- 0.65 and 1.05 +/- 0.57 log10 CFU/ml, respectively). Highly gentamicin-resistant strains showed a tendency toward reduced susceptibility to killing; beta-hemolytic strains were more susceptible than nonhemolytic strains when exposed to ampicillin at 20 micrograms/ml. Within each group, individual isolates demonstrated great variability in susceptibility to killing by the drugs.     

Activities and Time-Kill Studies of Selected Penicillins, β-Lactamase Inhibitor Combinations, and Glycopeptides against Enterococcus faecalis

The activities of piperacillin, piperacillin-tazobactam, ticarcillin, ticarcillin-clavulanate, ampicillin, ampicillin-sulbactam, vancomycin, and teicoplanin were tested against 212 Enterococcus faecalis strains (9 β-lactamase producers) by standard agar dilution MIC testing (10⁴ CFU/spot). The MICs at which 50 and 90% of the isolates were inhibited (MIC₅₀s and MIC₉₀s, respectively) were as follows (μg/ml): piperacillin, 4 and 8; piperacillin-tazobactam, 4 and 8; ticarcillin, 64 and 128; ticarcillin-clavulanate, 64 and 128; ampicillin, 2 and 2; ampicillin-sulbactam, 1 and 2; vancomycin, 1 and 4; and teicoplanin, 0.5 and 1. Agar dilution MIC testing of the nine β-lactamase-positive strains with an inoculum of 10⁶ CFU/spot revealed higher β-lactam MICs (piperacillin, 64 to >256 μg/ml; ticarcillin, 128 to >256 μg/ml; and ampicillin, 16 to 128 μg/ml); however, MICs with the addition of inhibitors were similar to those obtained with the lower inoculum. Time-kill studies of 15 strains showed that piperacillin-tazobactam was bactericidal (99.9% killing) for 14 strains after 24 h at four times the MIC, with 90% killing of all 15 strains at two times the MIC. After 12 and 6 h, 90% killing of 14 and 13 strains, respectively, was found at two times the MIC. Ampicillin gave 99.9% killing of 14 β-lactamase-negative strains after 24 h at eight times the MIC, with 90% killing of all 15 strains at two times the MIC. After 12 and 6 h, 90% killing of 14 and 13 strains, respectively, was found at two times the MIC. Killing by ticarcillin-clavulanate was slower than that observed for piperacillin-tazobactam, relative to the MIC. For the one β-lactamase-producing strain tested by time-kill analysis with a higher inoculum, addition of the three inhibitors (including sulbactam) to each of the β-lactams resulted in bactericidal activity at 24 h at two times the MIC. For an enzyme-negative strain, addition of inhibitors did not influence kinetics. Kinetics of vancomycin and teicoplanin were significantly slower than those of the β-lactams, with bactericidal activity against 6 strains after 24 h at eight times the MIC, with 90% killing of 12 and 14 strains, respectively, at four times the MIC. Slower-kill kinetics by both glycopeptides were observed at earlier periods.     

Correlation between growth curve and killing curve of Escherichia coli after a brief exposure to suprainhibitory concentrations of ampicillin and piperacillin.

Escherichia coli strains that were susceptible to multiple antibiotics were exposed to suprainhibitory concentrations of ampicillin and piperacillin. As with the majority of beta-lactam antibiotics, the growth curves showed an increase in optical density (OD) before lysis during the first hours. This increase in OD depended on the concentration of ampicillin and was independent of the concentration of piperacillin. A good correlation was found between the prelytic increase in OD and the killing curve. During the prelytic increase in OD, the number of CFU per milliliter remained constant. The decrease in the number of CFU per milliliter depended on the concentration of ampicillin and was independent of the concentration of piperacillin. pH variations gave rise to similar effects on growth curves and killing curves.     

Comparative killing activity and postantibiotic effect of streptomycin combined with ampicillin, ciprofloxacin, imipenem, piperacillin or vancomycin against strains of Streptococcus faecalis and Streptococcus faecium

Nine strains of Streptococcus faecalis and Streptococcus faecium were studied with respect to ampicillin, ciprofloxacin, imipenem, piperacillin, vancomycin and streptomycin. Two strains were highly resistant (MIC greater than or equal to 2,000 micrograms/ml) to streptomycin. Evaluation and comparison of the killing activity with killing curves, and duration of the postantibiotic effect (PAE) after exposure for 1 h with regrowth curves was done with combinations of antibiotics or alone. The overall killing effect of ciprofloxacin with streptomycin was antagonistic, whereas synergism (greater than or equal to one log10 decrease in viable counts) was observed in streptomycin-susceptible strains with combinations of streptomycin and ampicillin, imipenem, piperacillin or vancomycin. In addition, prolongation of PAE (greater than or equal to 0.5 h) was seen only in streptomycin-susceptible strains. Thus, seven (100%) strains showed a synergistic increase in PAE to combinations with ampicillin and vancomycin, three (43%) to imipenem, four (57%) to piperacillin, but none to the combination of streptomycin and ciprofloxacin. A significant correlation was observed between the magnitude of increased killing and the extent of increase in recovery period with combinations of streptomycin with either ampicillin or vancomycin.     

Effect of Ampicillin and Chloramphenicol Alone and in Combination on Ampicillin-Susceptible and -Resistant Haemophilus influenzae Type B

To evaluate ampicillin (Amp) and chloramphenicol (Cm) alone and in combination against Haemophilus influenzae type b, we examined the viability of 5 log(10) colony-forming units (CFU) of early-log-phase organisms per ml after 4 and 8 h of incubation with the drug(s). Nine Amp-susceptible (Amp(s)) and five Amp-resistant (Amp(r)) systemic isolates were examined. Antibiotic concentrations included: the minimum inhibitory concentration (MIC) of Amp, 50% of the MIC of Amp, 25% of the MIC of Amp, the MIC of Cm, 50% of the MIC of Cm, 25% of the MIC of Cm, and nine combinations of these concentrations. Both Amp and Cm at their MIC significantly reduced bacterial titers of Amp(s)H. influenzae type b after 8 h of incubation (1.36 and 1.47 log(10) CFU/ml, respectively; both p < 0.01); only Cm at its MIC significantly reduced the number of viable organisms after 4 h (0.91 log(10) CFU/ml; P < 0.001). With Amp(r) organisms, significant reductions in mean bacterial titers occurred after 4 and 8 h of incubation in the presence of Amp at its MIC (1.66 and 2.06 log(10) CFU/ml, respectively; both P < 0.02); smaller but significant reductions were noted after 4 and 8 with Cm at its MIC (0.56 and 0.87 log(20) CFU/ml, respectively; both P < 0.025). Antagonism with Amp(s) or Amp(r) strains was not seen. We conclude that combinations of Amp and Cm have indifferent effects on Amp(s) and Amp(r)H. influenzae type b.     

Comparative antibacterial effects of amoxycillin, amoxycillin/clavulanic acid and cefotaxime against Enterobacteriaceae as determined by turbidimetry, morphology and viable count

Cefotaxime has extremely low MIC values in comparison with amoxycillin against Enterobacteriaceae, but studies of antibacterial activity using turbidimetric and viable count methods show no advantage for cefotaxime. Superior rates of killing have been demonstrated for amoxycillin and amoxycillin/clavulanic acid using both constant antibiotic concentrations and changing concentrations to simulate conditions in vivo. Slow bactericidal activity was seen with cefotaxime even at levels greatly in excess of the MIC and could be correlated with filament formation by this compound over a wide range of concentrations.     

In vitro activities of fleroxacin against clinical isolates of Legionella spp., its pharmacokinetics in guinea pigs, and use to treat guinea pigs with L. pneumophila pneumonia.

The activities of fleroxacin against 22 clinical Legionella isolates were determined by agar and broth microdilution susceptibility testing. The fleroxacin MIC required to inhibit 90% of strains tested on buffered charcoal yeast extract agar medium supplemented with 0.1% alpha-ketoglutarate was 0.64 micrograms/ml and was 0.04 microgram/ml when testing was done with buffered yeast extract broth supplemented with 0.1% alpha-ketoglutarate. Fleroxacin (0.25 microgram/ml) reduced the bacterial counts of two L. pneumophila strains grown in guinea pig alveolar macrophages by 1 log10 CFU/ml, but regrowth occurred over a 3-day period; fleroxacin was significantly more active than erythromycin in this assay. Single-dose (10 mg/kg of body weight given intraperitoneally) pharmacokinetic studies performed in guinea pigs with L. pneumophila pneumonia revealed peak levels in plasma and lungs to be 3.3 micrograms/ml and 3.5 micrograms/g, respectively, at 0.5 h and 0.8 microgram/ml and 0.8 microgram/g, respectively, at 1 h. The half-life of the terminal phase of elimination from plasma and lung was approximately 2 h. All 17 infected guinea pigs treated with fleroxacin (10 mg/kg/day) for 2 days survived for 14 days post-antimicrobial therapy, as did all 16 guinea pigs treated with the same dose of fleroxacin for 5 days. Only 1 of 16 animals treated with saline survived. The animals treated with fleroxacin for 2 days lost more weight and had higher temperatures than those treated with the antibiotic for 5 days. Fleroxacin is effective against L. pneumophila in vitro and in a guinea pig model of Legionnaires' disease. Fleroxacin should be evaluated as a treatment for human Legionnaires' disease.     

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.     

Comparative in vitro activities of AC98-6446, a novel semisynthetic glycopeptide derivative of the natural product mannopeptimycin alpha, and other antimicrobial agents against gram-positive clinical isolates

AC98-6446 is a novel semisynthetic cyclic glycopeptide antibiotic related to the natural product mannopeptimycin alpha (AC98-1). In the present study the activity of AC98-6446 was evaluated against a variety of recent clinical gram-positive pathogens including multiply resistant strains. AC98-6446 demonstrated similar potent activities against methicillin-susceptible and methicillin-resistant staphylococci and glycopeptide-intermediate staphylococcal isolates (MICs at which 90% of isolates are inhibited [MIC(90)s], 0.03 to 0.06 microg/ml). AC98-6446 also demonstrated good activities against both vancomycin-resistant and -susceptible strains of enterococci (MIC(90)s, 0.12 and 0.25 microg/ml, respectively) as well as against streptococcal strains (MIC(90)s, 3 log(10) CFU/ml) of staphylococcal and streptococcal isolates and a marked decrease in the viable counts of most enterococcal strains (from 0.2 to 2.5 log(10) CFU/ml). Unlike vancomycin, which demonstrates time-dependent killing, AC98-6446 demonstrated concentration-dependent killing. The potent activity, novel structure, and bactericidal activity demonstrated by AC98-6446 make it an attractive candidate for further development.     

Synergic post-antibiotic effect of mecillinam, in combination with other beta-lactam antibiotics in relation to morphology and initial killing.

The synergic in-vitro post-antibiotic effect (PAE) of mecillinam, in combination with either ampicillin, aztreonam, ceftazidime or piperacillin, on a reference strain of Escherichia coli was evaluated by bioluminescence assay of bacterial ATP. Ampicillin, ceftazidime and mecillinam alone induced a concentration dependent PAE (greater than 3 h) on E. coli, whereas aztreonam and piperacillin alone induced a short (less than 1 h) non-dose dependent PAE. At most concentrations, the combination of mecillinam and ampicillin, aztreonam, ceftazidime or piperacillin induced longer PAEs on E. coli than the sum of the individual antibiotics' PAEs. Long PAEs were seen concomitantly with the presence of spheroplasts. In addition to the synergistic PAE, the decrease in colony counts and changes in ATP values after a 2 h exposure to mecillinam, in combination with the other beta-lactam antibiotics, were more prominent than the respective values after exposure to the individual antibiotics. The change in ATP was generally less pronounced than the decrease in colony counts. This could be due to lysis of spheroplasts on agar plates, leading to an over-estimation of the initial killing when assayed by viable counting. Mecillinam, which induced long PAEs on E. coli at almost all concentrations in this study, has a high affinity for penicillin binding protein 2 (PBP 2) and induced spheroplast formation at all concentrations. However, mechanisms other than the affinity for PBP 2 and spheroplast formation are involved in the PAE of beta-lactam antibiotics on Gram-negative bacteria; since the PAE was prolonged when mecillinam was combined with ampicillin, aztreonam, ceftazidime or piperacillin, which bind preferentially to PBP 1 and 3.     

Rapid assessment of antibiotic effects on Escherichia coli by bis-(1,3-dibutylbarbituric acid) trimethine oxonol and flow cytometry.

The effects of selected antibiotics on Escherichia coli were studied by flow cytometry with the fluorescent anionic membrane potential probe bis-(1,3-dibutylbarbituric acid) trimethine oxonol [DiBAC4(3)]. The actions of azithromycin, cefuroxime, and ciprofloxacin at five times the MIC on E. coli were compared by the traditional CFU assay and flow cytometry. Changes in viable counts of bacteria determined with DiBAC4(3) and by flow cytometry following treatment with the antibiotics showed trends similar to those found by the CFU assays. However, viable counts determined by flow cytometry following antibiotic treatment were 1 to 2 logs higher than those determined by the corresponding CFU assays. All the results obtained by flow cytometry were provided within 10 min after sampling, whereas the conventional CFU assay results took at least 18 h. The results indicated that flow cytometry is a sensitive analytical technique that can rapidly monitor the physiological changes of individual microorganisms following antibiotic action and can provide information on the mode of action of a drug. The membrane potential probe DiBAC4(3) provides a robust flow cytometric indicator for bacterial cell viability.     

Norfloxacin penetration into subcutaneous tissue cage fluid in rabbits and efficacy in vivo.

The activity of norfloxacin was studied in vivo with steel net cages implanted subcutaneously in rabbits. Four weeks after implantation, two of four cages in each animal were inoculated with a strain of Escherichia coli (seven animals) or Klebsiella pneumoniae (six animals). Four animals in each group received oral treatment with norfloxacin for 7 days. Treatment was started 18 h after inoculation of the cages. Peak concentrations above the in vitro minimal inhibitory concentrations for the strains used were achieved in the fluid of 14 of 16 of the infected cages after the first norfloxacin dose. The penetration of norfloxacin into both infected and uninfected tissue cage fluid was significantly higher on treatment days 3 and 7 than on treatment day 1. No difference was observed between the concentrations in uninfected and infected cage fluids or between cage fluids infected with different organisms. The viable counts of E. coli and K. pneumoniae decreased from 2 X 10(3) to 2 X 10(8) CFU/ml of cage fluid to less than 10 CFU/ml in 10 of the infected cage fluids 12 h after the last dose of norfloxacin. Fluid from four cages still containing low numbers of viable bacteria at that time became free from bacteria (less than 10 CFU/ml) 1 to 4 days later. No regrowth was found in any cage fluid 7 days after the treatment period. The viable counts of E. coli or K. pneumoniae in five untreated control animals did not decrease during 8 to 14 days after inoculation of cage fluid. In comparison with cephalosporins and aminoglycosides studied previously with the same experimental method, norfloxacin penetrated better into cage fluid and more effectively reduced the viable counts of the organisms.     

Activities of β-Lactams and Macrolides against Helicobacter pylori

A continuous-culture system (chemostat) was used to study the activities of beta-lactam antimicrobial agents, clarithromycin, and 14-OH-clarithromycin against slowly growing Helicobacter pylori NCTC 11637. H. pylori was grown to steady state before exposure to these antimicrobial agents at x8 the MIC. The bactericidal actions of combinations of amoxicillin and clarithromycin were also studied. Viable counts (numbers of CFU per milliliter) were determined at 2-h intervals for 12 h and at 20 h after the addition of antibiotics. The effects of pH changes (6.5 to 7.4) on the activities of amoxicillin, clarithromycin, and the combination of these against H. pylori NCTC 11637 were also studied. Viable counts following exposure to ampicillin, cefixime, ceftazidime, cefuroxime, cefotaxime, azlocillin, and piperacillin at 20 h showed bacteriostatic activity. Imipenem, meropenem, amoxicillin, clarithromycin, and 14-OH-clarithromycin reduced the viable counts by 3 log10 CFU/ml (>/=99.9% killing). Imipenem was the most rapidly bactericidal against H. pylori NCTC 11637. Results of the pH experiments showed that amoxicillin was bactericidal at pHs 6.5 to 7. 4. Clarithromycin was bactericidal at pH 7.0 to 7.4 but was bacteriostatic at pH 6.5. The combination of amoxicillin and clarithromycin was bactericidal at pHs 6.5 and 7.0. A batch culture (flask system) was also used to investigate 12 strains of H. pylori for their susceptibilities to beta-lactams, clarithromycin, and/or 14-OH-clarithromycin in order to determine whether results from the chemostat model can be reproduced with batch cultures. Results of the chemostat time-kill kinetic study were reproducible in our batch culture flask system. The role of carbapenems in the eradication of H. pylori should be investigated.     

Postantibiotic effect of ciprofloxacin compared with that of five other quinolones.

The antimicrobial activity (minimal inhibitory concentration, MIC, and killing kinetics) and postantibiotic effect (PAE) of different concentrations (MIC and 6 mg/1) of ciprofloxacin, norfloxacin, ofloxacin, pefloxacin, fleroxacin and lomefloxacin on pure cultures of Staphylococcus aureus and Escherichia coli were compared in vitro. The MIC, killing kinetics and PAE were determined by standard methods. Ciprofloxacin displayed the lowest MICs, while the highest were those of norfloxacin against S. aureus and lomefloxacin against E. coli. The killing curves showed ciprofloxacin to be the most and norfloxacin the least active. The bactericidal power was dependent on the concentration. At MIC, the fluoroquinolones, with the exception of norfloxacin, induced PAEs of 1-2 h. The effect was, in all cases, greater against E. coli. When assayed at 6 mg/l the PAEs were increased to 2-5 h, the best results being obtained by ciprofloxacin followed by ofloxacin. Norfloxacin produced no PAE on S. aureus and scarcely reached 1.3 h against E. coli. There was a close relationship between bactericidal power and PAE.     

Development and Validation of an In Vitro Pharmacokinetic/Pharmacodynamic Model To Test the Antibacterial Efficacy of Antibiotic Polymer Conjugates

This study describes the use of a novel, two-compartment, static dialysis bag model to study the release, diffusion, and antibacterial activity of a novel, bioresponsive dextrin-colistin polymer conjugate against multidrug resistant (MDR) wild-type Acinetobacter baumannii. In this model, colistin sulfate, at its MIC, produced a rapid and extensive drop in viable bacterial counts (<2 log₁₀ CFU/ml at 4 h); however, a marked recovery was observed thereafter, with regrowth equivalent to that of control by 48 h. In contrast, dextrin-colistin conjugate, at its MIC, suppressed bacterial growth for up to 48 h, with 3 log₁₀ CFU/ml lower bacterial counts after 48 h than those of controls. Doubling the concentration of dextrin-colistin conjugate (to 2× MIC) led to an initial bacterial killing of 3 log₁₀ CFU/ml at 8 h, with a similar regrowth profile to 1× MIC treatment thereafter. The addition of colistin sulfate (1× MIC) to dextrin-colistin conjugate (1× MIC) resulted in undetectable bacterial counts after 4 h, followed by suppressed bacterial growth (3.5 log₁₀ CFU/ml lower than that of control at 48 h). Incubation of dextrin-colistin conjugates with infected wound exudate from a series of burn patients (n = 6) revealed an increasing concentration of unmasked colistin in the outer compartment (OC) over time (up to 86.3% of the initial dose at 48 h), confirming that colistin would be liberated from the conjugate by endogenous α-amylase within the wound environment. These studies confirm the utility of this model system to simulate the pharmacokinetics of colistin formation in humans administered dextrin-colistin conjugates and further supports the development of antibiotic polymer conjugates in the treatment of MDR infections.     

Escherichia coli resistant to cephalosporins and quinolones is still susceptible to the cephalosporin-quinolone ester Ro 23-9424.

Ro 23-9424 is a broad-spectrum antibacterial agent consisting of a cephalosporin (desacetylcefotaxime) linked through an ester bond to a fluoroquinolone (fleroxacin). Its activity against mutants of Escherichia coli TE18 resistant to both antibacterial components was examined. E. coli TE18 overproduces the AmpC beta-lactamase and is resistant to several cephalosporins, including desacetylcefotaxime (MIC, 50 micrograms/ml), although it is still susceptible to Ro 23-9424 (MIC, 0.2 microgram/ml). Thirty-five spontaneous, two-step mutants of E. coli TE18 which were resistant to fleroxacin (MIC, 50 micrograms/ml) were isolated. In the mutants, replicative DNA biosynthesis (permeabilized cells) was resistant to fleroxacin, and some mutants had porin abnormalities. However, all remained susceptible to Ro 23-9424 (MIC, 0.5 microgram/ml). Examination of beta-lactamase activity in the parent strain revealed that it hydrolyzes desacetylcefotaxime more rapidly than it does Ro 23-9424. Thus, Ro 23-9424 may be less susceptible to the gram-negative, chromosomal beta-lactamases that hydrolyze several broad-spectrum cephalosporins and may be effective in cases in which neither of its two components is active.